Zuolngzcal Journal of the Linnean Society (1987), 89: 171-201. With 9 figures
The accoutrements of spiders’ eggs
(Araneae)with an exploration of their
functional importance
W. F. HUMPHREYS
Department of Biogeography and Ecology, Western Australian Museum., Francis Street,
Perth, W.A. 6000, Australia
Received Nouember 1985, accepted for publication March 1986
Analysis is presented of the characteristics of spheres coating the chorion surface of thi: eggs from 41
species belonging 10 15 families of spiders. Examples of eggs from the arachnid orders Opiliones,
Amblypygi and Uropygi are illustrated for the first time. T h e spheres are unique, among arachnids,
to the Araneae. Their form and size class distributions overlap considerably between families and
the spheres are present in all 22 families of Araneae that have been examined; they represent a very
conservative morphological feature in the evolution of the Araneae. Evidence of spectalized regions
of the egg surface (micropyllar areas and plastrons) was found only in an opilionid. As spiders’ eggs
are laid in silk egg sacs, mostly in clutches, the possible effects of these characteristics are discussed
from evidence in the literature and from a functional theoretic stance.
KEY WORDS:-Eggs
Amblypygi
-
~
egg sacs
-
morphology
function
-
-
Arachnida
-
Araneae
Opiliones
~
Uropygi.
CONTENTS
Introduction . .
Methods . . .
Rcsults
. . .
Discussion.
. .
Acknowledgements
References.
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Appendix.
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171
172
173
185
196
196
185
INTRODUCTION
The surface of spiders’ eggs has a granular structure (Ehn, 1963:i which results
from a coating of spheres in the size range c. 0.1 to 11 pm diameter (Grim &
Slobodchikoff, 1978, 1982; Humphreys, 1983a). The spheres account for the
‘bloom’ seen on surface when spiders’ eggs are examined at low magnification.
This coating by spheres is located where the complex ‘shell’ structiire is found in
most terrestrial oviparous animals (insects, Hinton, 1981 ; snails, ‘Tompa, 1976;
reptiles, Packard, Packard & Boardman, 1982; birds, Tullett, 1984). It is
supposed that, together with the silk egg sac into which spiders place their eggs,
+
0024-4082/87/02017 1 3 I 003.00/0
5
171
01987 T h e Linnean
Society of London
172
W. F. HUMPHREYS
the spheres somehow assume the functions ascribed to the ‘shell’ of, and found
necessary in, other terrestrial oviparous animals (Humphreys, 1983a).
This paper analyses the sphere coating of c. 41 species of spiders from 15
families of Araneae and extends to 22 the number of families that have been
examined (Grim & Slobodchikoff, 1978, 1982; Humphreys, 1983a). Together
these papers cover two of the three suborders of Araneae, 11 of 18 superfamilies,
21 of 86 families and 53 genera (c. 2%). In addition the egg surfaces of some
other orders of arachnids are examined, either from the literature or de novo, to
determine the taxonomic limits of the superficial coating by spheres
characteristic of eggs of the Araenae. Available data on the surface of arachnid
eggs is collated and, to gain some perspective, the possible effects of the
microsphere coating and of batch deposition of eggs are examined.
METHODS
The material used in this study came mainly from museum collections which
had been preserved dry or in alcohol ( Q M refers to material from the
Queensland Museum and KS to Australian Museum catalogue numbers) but
some eggs were collected freshly after laying. Eggs were air-dried or freeze-dried,
mounted on stubs, using double sided adhesive tape or aerosol contact adhesive,
or on planchets with silver paint, before splutter coating with gold. The material
was examined using either a JEOL JSM 35C or a Philips 501B scanning
electron microscope. The nominal magnification of the instruments was
calibrated using electroformed mesh (Polaron Equipment, Watford, England).
Size class frequency analysis was conducted, following the algorithms of Sokal &
Rohlf (198l ) , on measurements taken pseudorandomly from electron
micrographs.
Classification of the Araneae a t the familial level is uncertain and I follow
that of Levi (1982) with the addition of the Hexathelidae and Miturgidae. Data
on clutch, egg and spider size were extracted from published sources (Bristowe,
1939; Kaston, 1948; Locket & Millidge, 1951, 1953) for 155 species of Araneae,
using mid-range values wherever a range of size was given. The relationships
between body size and the other two variables were analysed separately by
regression analysis. Logarithmic transformation of the variables was used in the
final analyses. When sufficient data were available the families were analysed
separately and the slopes and intercepts compared by ANOVA; the
displacement of the regression lines being compared using an a posteriori multiple
comparison test (the GT2 procedure). To extend the range of families
considered all the data were corrected to the covariate of body size (or egg
diameter in the case of the analysis of the egg/clutch size relationship) and the
scaled data subject to one-way ANOVA and SNK aposteriori multiple
comparison tests. All statistical procedures followed the algorithms in Sokal &
Rohlf (1981).
The statistics of the sphere size and distribution (Grim & Slobodchikoff, 1982;
Humphreys, 1983a, this paper) for each species were treated as samples and
arranged for multiple discriminant analysis (MDA) by families and
superfamilies (sensu Levi, 1982), and by the egg sac classification of Austin
(1985). MDA followed the algorithms of Davies (1971).
173
SPIDERS’ EGGS
RESULTS
Clutch size
Clutch size is directly related to spider size (Table 1; Fig. 1A:. Nine families
showed significant regressions of clutch size on body size and the slopes of these
regressions d o not differ significantly (Table 1). Comparison of the regression
lines for the common slope shows that the Theridiidae are significantly displaced
above all families save the Thomisidae and the Araneidae (Table 1). T h e
regressions for web building and non-web building families have parallel slopes
but web building spiders generally have larger clutch sizes than non-web
building spiders (Table 2). Note, however, that the Agelenidae have small
clutch sizes and that they represent only 10% of the data for web building
species (Table 1).
Table 1. Regression statistics describing the relationship between spider length
(I,, mm) and clutch size (C) in nine families of spiders. They are arranged in
ascending rank order of the adjusted mean C values for the common slope of
1.601. Common letters in the second last column denote values which do not
differ at a = 0.05. W = Web building and NW = non-web building spiders.
The regression for the families combined includes, in addition, species from the
Atypidae, Dysderidae, Gonoridae, Pholcidae, Uloboridae, Linyphiidae,
Mimetidae, Tetragnathidae, Theridiosomatidae, Nesticidae, Pisauridae,
Microphantidae, Micryphantidae and Segestuidae. U T = untransformed
~~
~
Family
Regression equation log C =
.N
r2
P*
All families
Agelenidae W
Salticidae
NW
Dictynidae
Clubionidae NW
Gnaphosidae NW
Lycosidae
NW
Thomisidae NW
Aranridae
W
Theridiidae W
1.44 (log L ) f0.601
1.99 (log L)--0.167
1.75 (log L ) f0.112
1.31 (log L ) f0.530
1.48 (log L ) f0.416
1.80 (log L ) f0.147
1.25 (log L ) f0.654
1.81 (log L ) f0.451
2.12 (log L ) f0.144
1.86 (log L ) f0.620
154
6
11
4
9
0.56
0.99
0.82
0.90
0.53
0.82
0.66
0.42
0.54
0.60
<O.OOOl
<O.OOOl
<O.OOOl
0.049
0.027
10
31
10
19
22
ANOVA ~ I o p e s ~ - - F ,=~ 1.069,
, ~ ~ ~ P = 0.390; intercepts-Fs8,,12
*Probability of slope differing from zero.
adj. XC U T adj. XC
1.562a
1.573a
1.650a
1.658a
1.663a
1.675a
1.954ab
1.974ab
2.120b
t0.0001
t0.0001
0.042
<O.OOOI
<0.0001
= 9.335,
36
37
45
45
46
47
90
94
131
P < 0.0001.
Table 2. Regression statistics describing the relationship between spider length ( L ,
mm) and clutch size (C)for web building and non-web building spiders. The families
considered are those denoted by W and NW in Table 1
Group
Regression equation log C =
Jv
1.2
P*
Web building
Non-web building
1.436 (log L ) +0.783
1.380 (log L ) f0.551
47
71
0.56
0.60
<0.0001
<0.000 1
= 0.064, P = 0.801; intercepts F,
ANOVA: slopes F,
*Probability of slope differing from zero.
= 28.763,
P < 0.0001.
W. F. HUMPHREYS
174
A
3-
2 -
I .
1
0.5
0
2.5
.
2.0
-
1.0
C
3.0
1.5
1.0
0.5
1.5
5
6
8
7
I
I
SPIDERS’ EGGS
175
Table 3. Regression statistics describing the relationship between spider length
( L , mm) and egg diameter (D,mm) in three families of spiders. They are
arranged in ascending rank order of the adjusted mean D values fix the common
slope of 1.601. Common letters in the second last column denote values which
do not differ a t u = 0.05. W = Web building and NW = non-web building
spiders. The regression for the families combined includes, in addition, species
from the Dysderidae, Pholcidae, Uloboridae, Araneidae, Mimetidae,
Tetragnathidae, Theridiosomatidae, Nesticidae, Agelenidae, Gnaphosidae,
Pisauridae, Thomisidae, Salticidae, Microphantidae and Micryphantidae.
U T = untransformed
Family
Regression equation log
All families
Theridiidae W
Clubionidae NW
Lycosidae
NW
0.427
0.388
0.573
0.285
D
JV
=
(log L ) -0.402
(log L ) -0.425
(log L ) -0.523
(log L ) -0.249
85
15
5
25
ANOVA for the three families only: slopes-Fs
P*
T2
<0.0001
<0.0001
0.0024
<0.0001
0.72
0.71
0.97
0.71
2,39 = 2.678,
adj. XD U T adj. XD
-0.1180a
-0.0567ab
-00184b
P = 0.081; intercepi s-F,
2,41
0.761
0.877
0.959
= 13.109,
P <O.OOOl.
*Probability of slope differing from zero.
Egg size
Egg size is directly related to spider size (Table 3, Fig. 2). ‘Three families
show significant regressions of egg size on spider size and they have parallel
slopes (Table 3). The Theridiidae have smaller eggs than do the Lycosidae but
there is no separation in egg size between web and non-web building species
(Table 3). Analysis of data pooled from families with the most similar habits
shows that the combined salticid and thomisid group has smaller eggs than the
other combinations considered (Table 4).
Ranking all the size-scaled data shows that there is considerable difference in
the family ranking for egg size and the reversed ranking for clutch size (i.e.
assuming that the two could be inversely related). I n addition the variation
between families is similar for both egg size and for clutch size (Table 5). Hence
1.2
-0
-
-E x
’G
1.0
D
0
0.
0.
u 08.
I
W
-
06
-
0
.
0.5
1.0
Figure 2. The relationship between spider size (body length mm) and egg diameter (mm x 10) in
three families of Araneae. 1 = Clubionidae
2 = Lycosidae (u),5 = Theridiidae ( 0 ) .The
lines are fitted to the commom slope.
(m),
176
W. F. HUMPHREYS
Table 4. Regression statistics describing the relationship between spider length
( L , mm) and egg diameter (D,mm) in three groups of spiders. They are
arranged in ascending rank order of the adjusted mean D values for the
common slope. Common letters in the second last column denote values which
do not differ at LY = 0.05. U T = untransformed
Regression equation log D
Family
=
Thomisdae+Salticidae
0.331 (log L ) -0.258
0.286 (log L ) -0.249
Lycosidae Pisauridae
Gnaphosidae+ Clubionidae 0.523 (log L) -0.477
N
r2
12
25
10
+
ANOVA: slopes-F, 2,4, = 2.84, P = 0.070; intercepts-Fs
*Probability of slope differing from zero.
2,43 =
P*
adj irD U T adj. i D
0.43 <0.0001
0.71 <0.0001
0.89 <0.0001
-0.0432
-0.0101a
-0.0083a
0.905
0.977
0.981
3.26, P = 0.048
there are real differences in clutch size and in egg size between families when the
spider size is taken into account.
Relationshz$ between clutch size and egg size
Regression of egg diameter ( D , mm) on clutch size (C) shows a direct
relationship (Fig. 3; log D = 0.129 (log C)-0.283); (t, = 4.306 with 82 d.f.,
P < 0.001). Scaling the clutch size data to the mean egg diameter (0.903 mm)
and conducting analysis of variance on the eight families with sufficient data
1.817, P=O. 10).
shows no significant differences between families (Fs7.62=
Table 5. Spider families arranged in ascending rank order of egg size (mm) and
descending rank order of clutch size. Asterisks denote families included in
ANOVA. Common letters denote families which do not differ significantly at
LY = 0.05. The data were scaled to the mean spider size for each data set,
respectively 6.80 mm and 6.58 mm using the regression equations for the entire
data sets (Tables 1 and 2)
Family
Rank
A
Pisauridae
1
*Theridiidae
2
3
* Araneidae
Anyphaenidae
*Clubionidae
Agelenidae
*Gnaphosidae
*Lycosidae
*Thomisidae
Pholcidae
*Salticidae
4
5
6
7
8
9
10
11
Egg
diameter
(scaled)
0.772
0.777a
0.806a
0.867
0.89Oab
0.906
0.943ab
0.954b
0.96 I b
1.035
1.112b
N
2
Family
Pisauridae
8 *Theridiidae
15 *Araneidae
*Thomisidae
2 Linyphiidae
5 *Lycosidae
3 *Gnaphosidae
5 *Clubionidae
23 Dyctinidae
6
Agelenidae
2 *Salticidae
6 Dysderidae
Rank
ofA
1
3
2
9
8
7
5
-
6
11
-
Clutch
size
(scaled)
N
173
114a
93ab
85ab
60
46ab
45abc
43bc
41
37
35c
28
4
22
19
10
4
31
10
9
4
6
11
5
Egg diameter: Bartlett’s test, x 2 = 5.632, P = 0.466. ANOVA: F, 6,61= 8.399, P < 0.0001. Variation among
groups = 45%.
Clutch size: Bartlett’s test, x 2 = 12.039, P = 0.061. ANOVA: Fs6,105= 10.564, P < 0.0001; Variation
among groups = 38%.
177
SPIDERS’ EGGS
.
0.5
0
0.5
I .o
1.5
I
2.0
2.5
3.0
3.5
lop (clutch size)
-
1.5
EtIIlIIl
3
lo9 [clutch s i z e )
Figure 3. The relationship between clutch size and egg diameter (mm x 10) in the data extracted
from the literature.
Indeed further analysis by regression of the two families with adequate samples
sizes showed a significant regression of egg diameter on clutch size for Lycosidae
(log D = 2.72 (log C) 1.777; 1, = 3.32 with 22 d.f., P = 0.0032) but not for the
Theridiidae (1, = 1.37 with 14 d.f., P = 0.20).
+
Agglutination
I n the data extracted from the literature, 10 of 18 families of Labidognatha
had non-agglutinate eggs and one had only agglutinate eggs (Table 6). The
degree of agglutination was unrelated to spider or egg size but was correlated
with clutch size (Table 6).
Appearance of the egg surfaces
The surface of eggs that have been stored in alcohol are generally convoluted
(Fig. 4A), while fungal hyphae often appear on the surface of dry-stored
material (Fig. 4B-D). Most of the material examined showed the smooth
178
W. F. HUMPHREYS
Figure 4. Scanning electron micrographs of the eggs of various spiders. A, Low power view of the
surface of Trochosa tristicula (Lycosidae) egg showing the deep folding of the chorion of this alcohol
preserved specimen; QM; scale bar = 100pm. B, Entire egg of Archemorus simsoni (Araneidae)
showing fungal hyphae over the surface of the hatched egg later stored dry; KS8089; scale
bar = 100 pm. C , Wide variation in sphere size, typical of Araneidae, on the surface A . rimsoni egg;
note the split spheres and fungal hyphae; KS8089; scale bar = 20 pm. D, Surface view of egg of
A . simsoni showing irregular splitting of spheres, fungal hyphae and partially collapsed fungal
spores; note the darker band on the sphere in the upper right quadrant; KS8089; scale
bar = 10 pm. E, Egg ‘shell’ of hatched egg, subsequently stored dry, of Dinopsis sp. (Dinopidae)
showing intense folding of the chorion; note that along the fold lines the attached spheres are small
and vary little in size, while the superficial spheres are larger and, in places, several layers deep;
SPIDERS’ EGGS
179
Table 6. The distribution of agglutinate, semi-agglutinate and non-agglutinate
eggs in spider families of the Labidognatha. Data extracted from .Kaston (1948).
A = Agglutinate; sA = semi-agglutinate; nA = non-agglutinate
~~
Family
Uloboridae
Theridiidae
Nesticidae
Linyphiidae
Araneidae
Mimetidae
Tetragnathidae
Theridiosomatidae
Agelenidae
Total species
Percentage
Correlation matrix:
Spider length
Number of eggs
Egg diameter
Agglutination
nA
sA
A
%nA
1
0
0
100
5
2
1
2
1
0
1
2
1
0
0
1
0
0
0
1
0
0
0
5
0
1
0
1
83
100
I00
25
100
0
100
25
I
2
3
4
1
1.000
2
0.614*
1.000
Family
nA
sA
A
%nA
Amaurohiidae
Dictynidae
Anyphaenidae
Gnaphosidae
Chhionidae
Lycosidae
Pisauridae
Thomisidae
Salticidae
2
0
2
4
5
16
0
1
0
0
1
1
0
0
0
0
0
0
100
3
0.758*
0.254*
1.000
1
0
0
6
7
58
81
1
0
7
10
0
0
7
10
0
100
100
83
94
100
86
100
4
0.156
0.496*
0.07 1
1.000
*denotes P < 0.05.
spheres (Fig. 4D-H) seen elsewhere (Humphreys, 1983a) and some showed
beading of the spheres (Figs. 4F, H) and split spheres (Fig. 4D1, K, L) which
have been reported previously (Humphreys, 1983a). Some spheres examined
appeared grossly eroded (Figs 4J, K, 5A, B). T h e spheres were found adhering
to hymenopteran parasitoids (Fig. 5C, D) that had emerged from the eggs.
Size class distribution of the spheres
The eggs of all species of Araneae examined had a surface coating of spherical
objects in the size range 0.3-16.3 pm and with a range of median values
between 0.64-3.53 pm (Table 7; Figs 6-8). Significant skewness (g,) was
exhibited by 63% of the analyses, of which 19% were negatively skewed.
Significant kurtosis (g,) was exhibited by 54% of the analyses of which one was
negative (Table 7).
KS8103; scale bar = 20 pm. F, Egg surface of Procambridgea sp. (Stiphidiidae) which has only
attached spheres showing some heading hetwecn the spheres; KS8087; scale bar = 5 pm. G ,
Kcgular spheres with small ‘buds‘ on the egg surface of Poecilopachys bispinosa (Araneidae); sphere
statistics (Table 6) are presented including and excluding these ‘buds’; KS8122; scale bar = 2 pm.
H, Spheres on Procambridgea sp. egg showing heading between spheres of all sizes; KS8087; scale
bar = 2 pm. I, Shallow pitting on the surface of the larger spheres of Gradiingula sp.
(Gradungulidae) eggs; KS8132; scale bar = 2 pm. J, Beading between, and deep sculpturing on,
the surface of Arbanitis uarzabilis (Ctenizidae) eggs; Q M ; scale bar = 2 pm. K, sculpturing of the
surface spheres on egg of Mimetus sp. (Mimctidae); note the regular cratering (‘buds,’detached? cf.
Fig. 4G) and the two deep trenches in the large entire sphere; KS8144; scale tlar = 5 pm. L,
Spheres on egg of fsopoda sp. (Heteropodidae); it seems to show remnants of large spheres which
have fractured and lost the distal part, the fracture face is similar in appearance to the fractures in
Fig. 4D above and to pl. li in Humphreys (1983a); OM; scale bar = 5 pm.
180
W. F. HUMPHREYS
Figure 5. Scanning electron micrographs of the eggs of various arachnids. A, Massive pitting in the
spheres of Zsopoda sp. (Heteropodidae) eggs; QM; scale bar = 2 pm. B, Massive pitting in spheres of
Pholcus litoralis (Pholcidae) eggs; their appearance is similar to fractures caused by the expansion of
gas bubbles trapped in minerals; KS8143; scale bar = 2 pm. C, MastisoPrOctllsformidabilis (Uropygi:
Theliphonidae) showing a granular covering on part of the egg surface next to a crack in the
chorion; scale bar = 10 pm. D, A wasp (Hymenoptera: Proctotrupoidea) on the egg surface of
P o e c z ~ o ~ ~ o mspeciosm
zs~5
(Thomisidae); these wasps are egg parasitoids; KS8121; scale bar = 500 pm.
E, Detail of wasp in D above showing egg spheres adhering to a bristle on the wasp’s leg; scale
bar = 2 pm. F, Phryniscus bacillifer (Uropygi) showing ‘sharks teeth’ projecting from a coarse matrix;
scale bar = 10 pm. G , Egg of Dicranopalpus ramasus (Opiliones) showing a crescent shaped region of
fine papillae; scale bar = 100 pm. H, Main egg surface of G (above) showing deep open reticulated
network; scale bar = 5 pm. I, Amblypygid egg (Tarantulidae) showing the low papillae which
SPIDERS' EGGS
181
Sphere diameter and density
Regression of log mean sphere density on log mean sphere diameter for the
data in Table 7 and those published elsewhere (Grim & Slobodchikoff, 1982;
Humphreys, 1983a) has a slope of - 1.76 (s.E. = 0.071, .N = 68, P < 0.0001)
which differs from a slope of -2 (t, = 3.36, with 66 d.f., P = 0.001) but not
from the slope of - 1.87 (t, = 1.54, P = 0.13) found previously (Grim &
Slobodchikoff, 1982; Humphreys, 1983a).
Uniuariate analysis o f the distributional data
T o determine which distributional statistics are most useful in separating the
data, those presented here and elsewhere (Humphreys, 1983a) were pooled by
species into family groups with similar habits (Main, 1976), each species being
treated as a sample, and the analysis conducted on the statistics shown in
Table 7.
The statistics used, except g 2 , showed significant difference by ANOVA but
only the mean, median and density showed separation of groups from each
other on the multiple comparison tests (Table 8). The skewness and kurtosis of
the size class frequency distributions, the range of sphere size and their
dispersion ( s 2 / j 7 ) are not useful in separating groups of spiders with similar
habits. Examination of Table 8 shows that mean sphere size is large in the
Orthognatha and the mainly aerial web-building spiders, that Pholcidae have
low median sphere size and that the sphere density is high in mainly vagrant
spiders.
Multivariate anahsis of the sphere statistics
No significant separation of the superfamilies was obtained when the data
were classified according to egg sac type, nor was separation of families obtained
within the well represented superfamily Araneoidea. Significant separation of
the data was obtained when the data were pooled into superfamilies
(xz = 83.1, P < 0.0001) and two significant vectors were extracted from the
data which accounted for 62% (x:* = 41.4, P < 0.0001) and 23lyO (x;,, = 22.3,
P = 0.014) of the variance extracted. The separation mostly rtsults from the
location of the Orthognatha and the Araneoidea (Fig. 9) and there is
considerable overlap between the other superfamilies.
Egg surface o f other arachnid orders
Surface views of previously undescribed eggs of species in the arachnid orders
Opiliones, Amblypygi and Uropygi are shown in Fig. 5. These, together with
illustrations in the literature (Appendix, Table A l ) show that the coating by
spheres of the egg surface is unique to the Araneae (Appendix, Table A2). Brief
descriptions of the surface characteristics are given in Appendix Table A l . As
with eggs of Araneae, evidence of specialized micropyllar areas of plastrons,
cover part or the surface; scale bar = 20 pm. J, Edge of crescent in G (above) showing the fine
papillae within the crescent and the deep coarse network covering the general egg surface; scale
bar = 20 pm. K, Broken edge of network in G (above) showing the depth of the covering and the
smooth surface of the chorion below; scale bar = 10 bm. L, Amblypygid egg ('Tarantulidae)
showing the smooth surface on parts of the chorion; scale bar = 10 bm.
100
100
100
100
100
100
67
54
100
100
100
100
57
100
82
94
I00
100
138
100
100
Namirea planip s Raven
Atrax sp.
Steafoda sp.
Arachnura higginsii (L. Koch 1871)
Araneus sp.
Archemorus curfulis Simon 1903
Archemorus simroni Simon 1893
Argiope aefherea Keyserling 1856
Celaenia atkinsoni (Cambridge 1879)
Celaenia kingbergii Thorell 1868
Cyclosa sp.
DicrastichusJurcatus (Cambridge 1877)
Dicrostichus magnifius Rainbow 1897
Gasferacantha minaw Thorell 1859
Neasconella sp.
Ordgarius ?monsfrosus Keyserling 1886
Phonognatha graeji (Keyserling 1865)
Poecilopachys bispinosa (Keyserling 1865)
Poecilopachys bispinosa
Pal@ illepidus C. L. Koch 1843
Caramides so.
Dipluridae
Hexathelidae
Theridiidae
Agelenidae
Araneidae
N
Species
Family
~~
2.98
3.44
1.33
1.32
2.08
1.72
3.55
1.16
1.39
2.79
2.21
1.93
1.76
2.51
1.89
1.77
1.20
1.57
1.23
1.89
2.37
X
0.10
0.14
0.07
0.03
0.05
0.08
0.36
0.02
0.05
0.05
0.05
0.03
0.05
0.08
0.03
0.04
0.07
0.03
0.05
0.03
0.13
S.E.i(
15
20
31
14
20
56
16
48
14
53
22
32
31
51
21
25
47
82
14
33
19
(Yo)
cvs
Diameter
0.14NS
0.958
0.61**
1.363
1.76***
0.675
0.17NS
0.277
0.525 -0.42NS
1.12***
0.812
2.85***
2.906
0.160 -0.61NS
1.06***
0.460
0.523 -0.55*
5.45***
0.492
0.295 -0.90***
0.356 -0.23NS
0.780
3.97***
0.264
0.24NS
0.361 -0.lONS
0.667
2.18***
0.249 -0.60*
0.590 -0.66**
0.264 -0.16NS
1.265
1.12***
Sl
0.44NS
0.54NS
2.59***
-0.21NS
-0.16NS
1.31**
9.20***
3.73***
2.92***
-0.16NS
42.03***
2.97***
-0.28NS
16.88***
1.47**
-0.06NS
5.01***
0.90NS
-1.14**
0.74NS
1.84***
sz
Distribution
0.98
0.89
0.66
0.79
0.91
0.53
1.18
0.54
0.63
1.35
1.41
0.90
1.02
1.95
1.09
1.02
0.55
0.76
0.20
1.07
0.52
Min.
(1)
6.28
7.33
3.62
2.05
3.23
4.49
16.27
1.68
3.45
3.84
6.20
2.85
2.54
6.93
2.66
2.85
4.11
2.16
2.16
2.55
7.14
2.94
3.33
1.10
1.29
2.16
1.61
2.74
1.16
1.31
2.86
2.13
1.97
1.81
2.33
1.90
1.79
0.96
1.62
1.49
1.87
2.11
37
38
15
40
54
27
10
54
24
61
15
55
52
8
52
42
12
61
66
54
24
Max. Median 3- 1
(2)
(3) 2-1
Size range (pm)
~
0.22
0.26
0.38
0.38
0.26
0.56
0.18
-
0.09
0.07
0.84
0.45
0.16
0.20
0.16
~
0.47
0.45
~
0.08
10-lzm-l
-
0.85
0.71
0.45
0.46
0.29
0.57
0.70
0.39
0.58
0.61
0.59
0.82
1.08
0.52
-
0.75
0.82
-
0.74
s2X-’
Density
Table 7. Statistics describing the size, size class distribution, density and dispersion of the spheres on the surface of spider eggs and
other sources
I
= not
significant.
Procambridgea sp.
Geolycosa godeffroyi (L. Koch 1865)
Pardosa nigropalpis Emerton 1885
Trochosa tristicula
Micromerys sp.
Pholcus litoralis L. Koch 1867
Physocyclus sp.
Psilochorus sp.
Phyarachne sp. nov.
Poecilothomisus speciosus (Thorell 188I )
Euryattus albescens (Keyserling 1865)
Mimetus sp.
Mimetus sp.
Menneus angullatus (L. Koch 1878)
Dinopsis sp.
Dinopsis sp.
Arbanitis uariabilis
Isopoda sp.
Isopodaflauida L. Koch 1875
Miturga sp.
Gradungula sp.
Widder et al. 1978; fig. 1A
Mullins & Keil 1980; fig. 1
Packard et al., 1982; fig. 3B
Thies 1979; fig. 1
* P < 0.05, * * P < 0.01, ***P < 0.001, NS
Miturgidae
Gradungulidae
Albumen microspheres
Urate spherules
Anolas carolinensis eggs
Polylactic acid
microcapsules
Heteropodidae
Ctenizidae
Dinopidae
Mimetidae
Salticidae
Thornisidae
Pholcidae
Lycosidae
Stiphidiidae
87
51
100
100
100
100
60
LOO
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
0.98
0.91
0.66
2.19
0.85
1.19
0.75
1.09
0.85
0.70
1.01
1.30
2.16
1.47
1.35
1.67
3.63
1.58
1.48
1.24
1.15
0.63
1.69
2.11
76.1
0.04
0.01
0.02
0.04
0.05
0.05
0.01
0.03
0.01
0.01
0.03
0.09
0.09
0.04
0.04
0.07
0.11
0.07
0.08
0.03
0.08
0.03
0.13
0.17
3.0
33
16
31
20
60
44
19
28
14
20
32
67
41
24
27
43
31
42
57
22
66
38
78
56
39
0.121
0.142
0.325
0.874
0.890
0.356
0.366
0.722
1.117
0.669
0.837
0.268
0.761
0.238
1.314
1.180
30.30
0.309
0.327
0.149
0.204
0.436
0.510
0.524
0. I44
1.84***
1.08***
0.02NS
-0.09NS
3.54***
I .54***
1.15*
0.21NS
8.08***
2.97***
9.38***
3.03***
-0.57*
2.50***
6.92***
2.20***
-0.42NS
0.26NS
0.02NS
0.38NS
0.17NS
-0.03NS
2.41***
7.09***
4.39***
2.12***
0.06NS -0.45NS
-0.12NS
-0.09NS
0.90***
0.56NS
0.78**
2.03***
0.85* * *
0.27***
].lo***
0.54NS
-0.15NS
-0.OlNS
3.17*** 12.60** *
1.12***
0.67NS
2.92*** 10.48***
1.23***
4.47NS
-0.OINS
-0.74NS
0.42
0.43
0.29
0.98
0.44
0.49
0.30
0.26
0.46
0.41
0.29
0.49
0.94
0.70
0.47
0.50
1.13
0.56
0.30
0.54
0.45
0.29
0.28
0.45
20
2.22
1.31
1.46
3.63
2.94
3.58
1.20
2.60
1.15
1.12
2.05
5.21
6.77
2.35
2.28
4.16
7.60
3.44
3.95
1.89
5.27
1.33
8.31
5.65
157
0.86
0.90
0.64
2.19
0.72
1.06
0.75
1.04
0.85
0.69
1.04
I .03
1.95
I .47
1.34
1.50
3.53
1.49
1.13
1.24
0.92
0.56
1.37
I .95
74.5
26
14
29
29
10
24
53
30
46
11
18
50
33
57
39
43
I1
17
47
48
35
37
32
23
52
~
~
~
-
~
0.51
4.05
-
0.58
0.31
0.47
0.51
0.42
0.83
0.49
0.66
0.67
0.67
0.46
0.71
0.68
0.55
0.43
0.65
0.66
1.34
~
-
0.66
0.60
~
0.65
0.89
1.74
0.13
0.71
0.53
1.18
0.44
1.15
1.82
0.51
0.27
0.16
0.50
0.58
0.32
0.09
0.33
184
vv. r . nuivirnnora
I
I
t
4
u
,
=
m
o
-4
'
1
I
f
P
N
0
0 .N
-ln
m
I
W
U
N
0
o
:
4
t
m
N
o
N
~m
o
SPIDERS' EGGS
185
Table 8. Univariate analysis of distributional statistics of spheres on the surface
of spider eggs. The groups refer to Main's (1976) family groupings (roman
numerals): 1 = Orthognatha Groups I and 11. 4 to 9 = Labidognatha Groups I
to VI; 4 = Pholcidae, 5 = Dinopidae, 7 = Mimetidae, Theridiidae, Linyphiidae
and Araneidae, 8 = Agelenidae, Lycosidae and Pisauridae, 9 == Clubionidae,
Heteropodidae, Salticidae and Thomisidae. Common letters al'ter the group
numbers in each row denote mean values which do not differ at o! = 0.05
0
10
Parameter
Mcan
g'
g2
Minimum
Maximum
Density
S*/X
Median
Groups (upper) and mean values (lower)
Fs
d.f.
P
Among*
8a
0.98
4a
1.01
9a
1.05
5ah
1.37
7ab
1.80
Ib
3.28
7.03
5,62
<0.001
38
la
0.33
5a
0.53
9a
0.76
4a
0.86
8a
1.08
7a
1.76
2.77
5,62
<0.025
15
4
0
7
0.44
1
1.00
6
2.65
5
5.66
3
10.79
2.13
5,62
0.07
10
9a
0.38
8a
0.40
4a
0.41
5a
0.56
7a
0.85
la
1.0
6.51
5,62
<0.001
36
9a
2.12
8a
2.18
5a
2.93
4a
2.98
7a
4.33
la
7.07
3.52
5,62
0.007
20
la
0.09
7a
0.41
5ab
0.48
4ab
0.74
9b
0.97
8b
1.39
9.24
5,62
<0.001
46
5
0.54
7
0.66
I
0.70
9
0.70
8
0.73
4
1.36
1.59
5,62
0.174
6
4
0.89
8a
1.15
5a
1.44
7a
1.84
5.00
3,23
0.008
43
* Percentage variation among means.
which are widely present in insect eggs (Hinton, 1981), was not fclund, with one
exception; eggs of the opilonid Dicrunopalpus rumosus have a sunken crescent
shaped region of anastomosing truncated papillae not overlaid by the larger
complex network present over the remainder of the egg (Fig. 5G, H, J, K ) .
DISCUSSION
All species of Araneae examined had a coating of spheres over the egg surface
and brings to 22 the number of families with this feature. Other than the
Figure 6. Size class frequency distributions of spheres on the egg surface of 20 species of spider
(where N # 100 the frequencies expressed as percentages are included to the right of they-axis). The
ordinate is in micrometres. Column 1 (top to bottom): Archemorus curtutis (Araneidae), A . szmsoni
(Araneidae), A . simsoni expanded scale 0-20 pm, Dicrostichus furcatus (Araneidae), D.magnijcus
(Araneidae). Column 2: Arachnura higginsii (Araneidae), Celaenza kingbergii (Araneidae), Aruneus sp.
(Araneidae), Isopoda sp. (Heteropodidae), I . Jauida (Heteropodidae). Column 3: Micromety sp.
(Pholcidae), Pholcus litoralis (Pholcidae), Psilochorus sp. (Pholcidae), Psilochorus sp. (Pholcidae),
Geolycosa godeffroyi (Lycosidae). Column 4: Pardosa nigropalpis (Lycosidae), Trochosa tristicula
(Lycosidae), Mimetus sp. (Mimetidae), Mimetus sp. (Mimetidae), Sleatodu sp. (Theridiidae).
186
W. F. HUMPHREYS
1
U
m
N
-
1
II
t
0
O
~
N
N
~
W
O
-
1
i
2
m
w
u
~
o
7
d
t
,
+o: E
4
N
m
X
E
~
O
N
W
187
SPIDERS’ EGGS
0
2
4
6
8
1
0
0
60
120
I80 2 4 0 500
Size c l a s s ( wn)
Figure 8. Size class frequency distributions of sphcres on the egg surface of three species of spidcr
and spheres of similar appearance from three other sources (where JV# 100 the frequencies
expressed as percentages are included to the right of they-axis). The ordinate is in micrometres and,
except where shown, range from 0-10 pm. Column 1 (top to bottom): Dinopsis sp. (Dinopidae,
Euryatlus atbesmns (Salticidae), Phonognatha graejl (Araneidae). Column 2: Urate spherules from the
ootheca of a cockroach (Mullins & Keil, 1980; fig. l ) , albumen microspheres (Widder, Senyei &
Scarpelli, 1978; fig. 1a ) , polylactic acid microcapsules (Thies, 1979).
Araneae (Appendix, Table A2), no arachnid order examined showed similar
egg-surface features (Appendix, Table A l ) . This coating by spheres is
apparently unique to the Araneae and a conservative feature in their evolution
and suggests that it is a feature of some importance.
Examination of the data presented here and those previously published
(Humphreys, 1983a) shows the size class frequency distribution can be, within a
family, positively or negatively skewed (Araneidae and Pholcida.e), hence that
attribute of the distributions is not of taxonomic relevance at the family level.
While some families seem to have consistent forms of distribution,, other families
are widely variant between species. For example the mostly small spheres on
lycosid eggs show little variation in size, while those of the Araneidae exhibit
Figure 7. Size class frequency distributions of spheres on the egg surface of 20 specics of spider
(whereN# 100 the frequencies expressed as percentages are included to the right of they-axis). ‘The
ordinate is in micrometres. Column 1 (top to bottom): Poltys illepidus (Araneidaej, Cyclosa sp.
(Araneidae), Miturga sp. (Miturgidae), Poecilopachys bispinosa (Araneidae), P. bispinosa including
‘buds’-see
text. Column 2: Arbanitis uariabilz’s (Ctenizidac), Procambridgea sp. (Stiphidiidae),
Gradungula sp. (Gradungulidac), Neosconella sp. (Araneidae), Ordgarius ?monstrosu!. (Araneidae).
Column 3: Auella angullalus (Dinopidae), Corasoides sp. (Agelenidae), Namirea planipe: (Dipluridac),
Casterocantha minax (Araneidae), Poecilolhornisu speciosus (Thomisidaej. Column 4: .Irgiope aetherea
(Araneidae), Alrax sp. (Hexathelidae), Dinopsis sp. (Dinopidae), Phrynarachne sp. (Thomisidae),
Celaenia atkinsoni (Araneidae).
6
188
W. F. HUMPHREYS
N
..
0
Y
>”
Vector I
Figure 9. Plot of the two significant vectors resulting from MDA on the sphere statistics with the
data grouped into the Orthognatha and the superfamilies (wnsu Levi, 1982) of the Labidognatha.
1 = Orthognatha,
2 = Pholcoidea,
3 = Eresoidea,
4 = Araneoidea,
5 = Lycosoidea,
6 = Thomisoidea. 7 = Philodrornoidea.
considerable variation in both size and frequency distribution. Pholcid eggs
have similar distributions of spheres to those of the Lycosidae but, in addition,
have a group of large spheres. The three families of Orthognatha examined all
have large spheres showing wide variation in size, but they are within the range
of variation shown by the Araneidae.
The attempt to classify the sphere statistics showed that they are unrelated to
the egg sac type and are of little use in discriminating between families and
superfamilies, although the Orthognatha appear separate from the
Labidognatha (Fig. 9). This is in accord with other studies of the spheres on
spider eggs (Grim & Slobodchikoff, 1982; Humphreys, 1983a) which suggested
that their size, frequency distribution and morphology have little taxonomic
relevance at the family level; in this they contrast with the calcified eggs of land
snails (Tompa, 1976) and birds (Board, Tullett & Perrott, 1977) whose
structural diversity is adequate to identify parental animals to the generic and
even the species level from the ultrastructure of the eggs.
Amongst other features, however, spherical objects do appear superficially on
the surface of eggs of other taxa. The eggs of Anolis carolinasis (Reptilia:
Iguanidae) have an open network of calcareous spheres (1-6 pm diam.; read
SPIDERS’ EGGS
189
from Fig. 3b of Packard et al., 1982) on the surface which seem to leave
impressions in the egg membrane. I n some avian eggs the palisade layer of the
shell is overlaid by shell accessory materials consisting of small spheres (1-10 pm
diam.) of organic (domestic hen, Gallus domesticus) or inorganic (flamingo, e.g.
Phoenicopterus roseus; mallee fowl, Le$oa ocellata) composition (Board, 1982), The
spheres coating the cuticle surface of some terrestrial isopods, which are similar
in size though not in surface appearance to those on spiders’ eggs, are, a t least
when small, attached to the cuticle by a stalk and seem to develop in situ
(Hadley & Hendricks, 1985). The common presence of spheres of similar size
lying superficially on the egg surface of both vertebrate and invertebrate taxa
suggests a common function; even in birds eggs, however, which have been
extensively studied, the function and origin of the spheres is unltnown (Board,
1982).
The other orders of arachnids examined have eggs without the coating of
microspheres which is found throughout the Araneae. The other orders have
mostly complex surfaces to their eggs but a granular surface was seen in a
Uropygid and a mite (Trombiculidae); the former may have been degraded by
long storage in alcohol. The eggs of D.ramasus (Opiliones) have a complex
network structure similar to that seen in some insects (Hintmon, 1981) but
sectioned material is required before further comment can be made on the eggs
of the non-Araneae orders.
The range of morphological variation in the surface spheres found during this
study is similar to that previously reported (Grim & Slobodchikciff, 1978, 1985;
Humphreys, 1983a). The eroded nature of some spheres (Figs 45 & 5A) may be
an artefact of long-term storage in alcohol, but the massive pitting was seen also
in spheres from eggs that had been stored dry (Fig. 5B). The presence of the
spheres on parasitoids is to be expected as they sometimes occur on the surface
of spiderlings (Grim & Slobodchikoff, 1978; Humphreys, 1983a) and predatory
mites (Grim, 1980).
The depth of the sphere coating may differ between families; they occur as a
monolayer (Lycosidae, Salticidae, Pisauridae, Stiphidiidae) sometimes with a
sparse coating of larger spheres (Pholcidae, Dinopidae), while some families
have multiple layers of unorganized spheres (Araneidae) .
Spider eggs are sometimes ‘glued’ together in the mass, a characteristic
referred to as agglutination. It results from the viscous nature of the collaterial
fluid which is deposited with them (Kaston, 1948) and which soon dries. In the
data extracted from Kaston (1948) the degree of agglutination is correlated with
clutch size and suggests that the adhesion of the eggs stabilizes lai-ge egg masses.
The substance causing adhesion is probably that illustrated elsewhere
(Humphreys, 1983a, pl. I I h ) but sectioned material is needed to show the
relationship between the adhesive substance responsible for agglutination, the
spheres and the matrix substance (Humphreys, 1983a, pl. Ii, IIc, IId)
surrounding them. Most orb-web weaving spiders lay their egg:; upwards and
have agglutinate or semi-agglutinate eggs which prevents them fdling. Vagrant
spiders mostly lay their eggs downwards and do not produce copious quantities
of collaterial fluid; the eggs of lycosids, for example, are loose within the egg sac
(non-agglutinate). Not all spiders producing clutches lay them en masse but may
lay them spaced on the substrate (Richman & Whitcomb, 1981) and protected
by the female within a silk brood sac.
190
W. F. HUMPHREYS
Egg size
The diameter of spiders’ eggs is related directly to body length and reached an
asymptote a t c. 1.3 mm diameter at a body length ofc. 15 mm (Humphreys, 1983a).
The separation of families according to the egg size suggests that reproductive
strategies may be family specific. The large eggs of Lycosidae (Table 3) are
associated with carrying by the female of both the egg sac and the emerged
spiderlings, but Clubionidae also guard their eggs and some Theridiidae even
feed their young (Foelix, 1982). There appears then to be no direct relationship
between egg size and brood care. Among families with similar habits (Table 4)
there is similarly a mix of parental care in families with different sized eggs.
Salticidae and Clubionidae guard their eggs in a nest and both the Pisauridae
and Lycosidae carry their egg sacs and care for the spiderlings.
Clutch size
Enders (1976) analysed data on clutch size of spiders gleaned from two of the
sources used here. Several problems exist in this analysis mainly relating to
statistical procedures: the comparison of regression intercepts when the slopes
were not equal and the use of inappropriate pairwise tests to compare the
regressions for each family. The present analysis confirms Enders’ (1976) finding
that web building spiders generally have greater clutch size than non-web
building spiders. The families, however, with the largest (Theridiidae) and
smallest (Agelenidae) clutch sizes are both web builders, albeit webs of different
types. In addition the slopes of the regressions are parallel and do not exhibit
the divergence shown by Enders (1976: fig. 2). The nature of the data base
make it inappropriate to comment on particular species which may be outliers;
the original data are non-standardized observations reported simply as a number
or a range so the values may be unrepresentative of any one species. For
example, comparison of clutch size of eight species of Pardosa showed distinct
differences in clutch sizes, on both absolute and weight scaled data, between
species and within species from different localities and was influenced by food
availability (Kessler, 1973). Pardosa spp. respond to food shortage mainly by
producing smaller clutches and show only slight variation in egg size (Kessler,
1971).
Energy content of eggs
As found generally, the energy density of spider eggs is greater than in adults
and shows significant variation between species (Anderson, 1978). The variation
in energy density is probably related to the tiMe taken by spiderlings to
establish themselves as independent beings, as the metabolic costs of this stage is
high compared with that for development in the egg (Anderson, 1978) and to
the wide variation in the time taken to become free living (Anderson, 1978;
Humphreys 1983b). Reproductive effort calculated using female mass specific
clutch size (Enders, 1976) does not correlate with reproductive effort, cakulated
as the female mass specific energy content of the clutch (Anderson, 1978).
Indeed there are inadequate data on the number and size of clutches, the
metabolic rates of adults, eggs and spiderlings, and on egg sizes and their energy
content, to make general statements about variation in reproductive effort in
spiders, either within or between species.
SPIDERS' EGGS
191
Egg number and clutch volume
Spider length ( L ) accounts for 56% of the variance in the number of eggs (C )
in an egg sac (C = 1.83L' 43; F, ,,53 = 194, P < 0.0001); the scaling factor of 1.43
is c. half that expected (C cc L 3 ) if the number of eggs is related to spider weight
(W). The relationship between spider length and egg diameter (D)is
asymptotic at an egg diameter of c. 1.3 mm at a body length of c. 17 mm
(Humphreys, 1983a). By convolving the two equations the expected volume of
eggs per clutch (V) can be calculated; Y = 7.18269,r2 = 0.997 and the power
function is different from 3 (t, = 3.01, P < 0.01). The above relationship is
slightly sigmoidal and the scaling factor is maximal (c. 3.0) at a spider length of
c. 6 mm; this results from the clear asymptote in the relationship between spider
length and egg diameter so the peak in the scaling factor is unlikely to be an
artefact of the data.
scales to length ( L ) as Wcc L3 and respiration ( R ) scales to mass
As mass
as R cc M'"l5?hen respiration scales to length as R cc L"'"; hence it seems that
although the volume of eggs produced per clutch is not scaled directly to spider
weight or their metabolic rate, it is much more closely coupled to these factors
than is egg number (scaling factor 1.44, Table 1). As there is rather small
variation in the caloric density of spider eggs (Anderson, 1978), the volume of
eggs produced is a rough measure of parental investment per clutch. Hence for
spiders generally there is a tendancy for the volume of eggs to be maximized
rather than the number per clutch, at least until the asymptotic egg diameter of
c. 1.3 mm is reached at a spider length of c. 17 mm; only spiders larger than this
increase disproportionately the number of eggs.
The efect of spheres
The minimum size of spheres recorded here and elsewhere is c. 0.2 pm while
the mean size is >0.6 pm; these sizes are much greater than the mean free path
of oxygen a t 23°C (0.1 pm; Hinton, 1970), hence the diffusion of gases will not
be significantly impeded by the spheres per se. I n considering the possible
function of spheres it is pertinent to consider the influence of sphere size and
packing density on the effective surface area of the egg. Some species have more
than one layer of spheres (Humphreys, 1983a) but I have no estimate of the
mean number of layers involved. In three dimensions the maximum packing
density of spheres is about 74% (the absolute value remains to be proven;
Sloane, 1984).
Consider two-dimensional packing of spheres on the egg surhce. Let SC be
the ratio of the combined sphere surface area to the chorion surface area.
Assume the egg surface is smooth, as it appears under SEM, and that the
spheres are of equal size and perfectly spherical; they have respective diameters
of D,and D,.The surface area of the egg and the spheres are respectively no:and
7
1
0
:
and the projected sphere surface area is n(DJ2)'. The maximum number of
sphere circular areas which can be packed on the chorion surface is then:
M
=n
.D,'
. 1/ 7 ~. (D,/2)2,
times the packing density ( k ; limits 0 , l ) . The ratio of the surface area of all
spheres to the chorion surface area is then:
SC = M * n . 0 : - I / n . 0,'.
K,
= 4k.
192
W. F. HUMPHREYS
Hence the effective surface area of the spheres is not influenced by the size of the
spheres but only by the density a t which they are packed (namely k). This
conclusion is true also for the porosity of the sphere layer (Scheidegger, 1974).
More generally the layer of spheres on the surface of the eggs have a porosity
and surface area independent of the sphere size but dependent on packing
density and the number of layers of spheres.
The maximum packing density on a hexagonal lattice is c. 0.91 (SC c. 3.6)
and on a checker board lattice is c. 0.79 (SC c. 3.1) (Sloane, 1984). The mean
value of k for the spheres on spiders’ eggs is c. 0.5 (SC = 2.0; Humphreys, 1983a:
fig. 3). O n average, then, the spheres have double the surface area of the
chorion but as only half a sphere is on the outside, the spheres do not increase
the effective surface area of the egg. Clearly the sphere size is unimportant and
potentially interesting differences between taxa should be sought in the packing
density and number of layers of spheres.
Variation in egg sacs and parental behaviour
All Araneae lay eggs within a silk sac which may contain few (one in Telema
tenella and three in Peckhamiapicata; Kaston, 1948) to more than 1000 eggs; up to
1200 in Argiope aurantia (Kaston, 1948) and 2500 in Cupiennius salei (Melchers,
1963). Egg sacs range from scanty net-like structures (Pholcus and Scytodes) to
thick leathery structures (Celaenia spp.) which may be coated externally with
additional substances (see Humphreys, 1983a). The wall of the egg sac may
consist of one to three layers of flocculent or non-flocculent silk of varying
thickness (50 pm to 15 mm) and density (Austin, 1985). T h e structure of the egg
sac probably determines which scelionids parasitize the eggs because different
scelionid genera utilize spiders according to the structure of the egg sac rather
than the spider’s taxonomic affinities (Austin, 1985).
Similarly there is a wide range of maternal care in spiders. Nephila clavipes
abandons the egg sac shortly after its completion and pays it no further
attention (Christenson & Wenzl, 1980). Some produce eggs within the
protection of a maternal silk nest (e.g. some Salicidae and Clubionidae) from
which the female guards the eggs. The egg sac may be carried by the female
(Lycosidae, Pisauridae, Scytodidae, Eresidae, Psechridae, Pholcidae and some
Heteropodidae and Ctenidae; Levi, 1982) and she may assist the spiderlings to
emerge by tearing open the egg sac (Lycosidae; Foelix, 1982). Some species also
guard the emerged brood until it disperses; pisaurids move the egg sac onto
vegetation, where they spin a nursery web onto which the emerging young
gather, while lycosid spiderlings are carried on the back of the female for a few
days to several months (Humphreys, 1983b). About 20 species of spider are
known to actively feed their young (Foelix, 1982) and some species use
communal webs. Communal compared to solitary Philoponella oweni
(Uloboridae) produced the same number of egg sacs but more eggs per egg sac,
however, due to greater egg sac parasitism by wasps, spiderling survival in the
communal group was the same as in the solitary group (Smith, 1982).
Christenson & Wenzl (1980) analysed the function of the egg sac per se of
N . clavipes, which spends several months overwintering without parental care.
Prior to hatching the factors important to egg survival were; the position of the
egg sac on the tip of a twig, the silk loop about the connecting branch, the leaf
canopy above the egg sac and the silk surrounding the eggs. Changes in these
SPIDERS EGGS
193
conditions variously resulted in the eggs rotting, being invaded by fungus or
predated upon. After hatching, but prior to dispersal, only the integrity of the
silk loop around the twig was important.
There is a clear relationship between parental behaviour and the number of
egg sacs produced. Spiders with 1-year life cycles that do not guard their eggs
produce two to three egg sacs, while those guarding them produce only one egg
sac. Spiders with a 16-month life cycle which do not guard their egg sacs
produce five to 10 egg sacs, while those guarding them produce two to five egg
sacs (Bonnet, 1927).
Why egg masses?
Why do spiders lay the eggs of one clutch in one sac? As with other taxa that
have been addressed (Pechenik, 1979) it is assumed to have a protective
function, although from what they are being protected is uncertain. Egg laying
in masses is a prerequisite for maternal guarding behaviour arid other brood
care. Female Clubiona robusta guarded their egg sacs and effectively prevented
most egg predation, although in the absence of this protection the egg sac alone
was insufficient to reduce predation (Austin, 1982). Austin (1985) has
convincingly argued that the diversity of egg sacs is a n adaptive response to
biotic mortality factors (predators and egg parasites); indeed egg sacs of diverse
types are produced by different species in the same physical environment. Burch
(1979) reported that, for Araneus diadematus, more eggs hatched when isolated
from the egg mass than when part of it but that more spiderlings emerged from
the egg masses than from isolated eggs; the results were contradictory, however,
and only the pooled data support Burch’s conclusions. A number of studies have
shown, and theory suggests, that other adaptive forces may be involved; these
will be considered below.
I n the absence of maternal care there are a number of differences between a
mass of eggs and the same number of independent eggs:
1. Egg masses have less surface area exposed to the environment than the
same eggs deposited singly and may thus be better protected from predators and
parasites. As discussed above, the egg sac of C. robusta does not alone protect
eggs from predation or alone prevent egg parasitism by Cerotobaeus masneri
(Scelionidae), although it is limited by the size (and hence food intake; see
above) and shape of the egg mass because the ovipositor cannot reach eggs in
the centre (Bradoo, 1972; Austin, 1984). Scelionids with short ovipositors
burrow through the egg sac wall to parasitize the eggs. These adaptations are
related to the density and thickness of the egg sac wall (Austin, 1984). I n
addition, when exposed to radiant heat, the equilibrium temperature of objects
with similar characteristics is directly related to their size. Hence the
characteristic batch deposition by spiders of their eggs could have thermal
significance in species which expose their eggs to radiant heat (Humphreys,
1978, in press); this applies especially to the Lycosidae.
2. Compact masses require less material for a given amount of protection of
embryos than do eggs laid singly and so influences the allocation of resources
and the parental investment per egg. Spider egg sacs may be complex and
substantial, requiring large investment of both time (more than 12 h to
construct) and silk (Forster & Forster, 1973; Main, 1976). Egg sac size is
194
W. F. HUMPHREYS
directly related to spider size but not necessarily to the number of eggs deposited
(Foelix, 1982); because the same female generally lays progressively fewer eggs
in subsequent clutches (Bonnet, 1927; Melchers, 1963; Kessler, 1973; Yoshikura,
Shiroto & Kondo, 1977), the cost of protecting each egg is probably greater in
later clutches.
3. Synchrony of development resulting from a common environment and the
behavioural synchrony of spiderling emergence from the egg sac may, through
saturation, result in lower parasitism and/or predation.
4.Water loss per egg should be reduced from large egg masses due to both the
effect of relative surface area and to high interstitial humidity. The removal of
the silk cocoon from eggs of Florina bucculenta (Linyphiidae) does, indeed, reduce
the survival time a t low humidity (32% R.H.) by about half (Schaefer, 1976)
but it is unclear how much of this effect results from the egg sac alone. Assuming
that the interstices within a spherical egg mass are saturated with water vapour,
it can be shown, by algebra, that the surface evaporative area of a number of
single eggs, relative to the same eggs in an egg mass, is:
(Egg sac diam./egg diam.) x packing density.
Under given environmental conditions water loss is proportional to surface area,
hence a considerable reduction in water loss may be expected to result from
their mass deposition. For eggs of 0.9 mm diameter at a packing density of 0.5
the water loss from isolated eggs, compared with the same number of eggs en
masse are, for egg sac diameters of 5, 10 and 15 mm, respectively 2.8, 5.6 and 8.3
fold greater. This relationship should be considered in conjunction with the
surface area to volume ratio (SVR) due to variation in egg size alone. The SVR
varies by about 3.7 in the range of egg size considered elsewhere (Humphreys,
1983a) but eggs exist up to 2.5 mm diameter Uocqut, 1985; Atrophonysia
intertidalis) increasing the range of SVR to 5.9; this latter species lays eggs
intertidally (Jocquk, 1985) and they have microspheres of c. 3 pm diameter (B.
Cutler, pers. comm.).
5. Protection from harsh physical environment. Austin & Anderson (1978)
could not demonstrate this for Nephila edulis but the egg sac of other Nephila
species enhanced survival of both the eggs and the juveniles (Christenson &
Wenzel, 1980). Intact egg sacs of F. bucculentu, by their contained air, protected
eggs immersed in water from freezing; eggs without the protection of the egg sac
were killed by freezing (Schaefer, 1976). The egg sac, however, did not effect
the resistance of eggs exposed in air to prolonged periods of cold ( - 15°C;
Schaefer, 1976). The packaging of eggs within sacs facilitates the incubation of
eggs (lycosids; Humphreys, 1978) and the movement of eggs to a cooler location
when subject to over-heating ( 7heridion saxatile; Nerrgaard, 1956). The maternal
nests of C. robusta, which have thick opaque walls, protect the contained spider
and her young from waterlogging (Austin, 1982). Egg sacs of Agelenopsis aperta
are impermeable to water and the eggs survive seasonal flooding which destroys
the adults (Riechart, 1982). I n addition, the spheres themselves may be
hydrophobic and protect the egg mass from waterlogging (see discussion in
Humphreys, 1983a).
6. Exchange of respiratory gases may be impeded in large masses. The egg sac
of a spider, the surface spheres and the chorion have a number of possible
functions and could offer a resistance network to gaseous diffusion (Humphreys,
SPIDERS’ EGGS
195
198%). The form and size of egg masses of marine invertebrates (gastropods and
polychaetes) does influence development by constraining oxygen supply to the
centre of the egg mass (Chaffee & Strathmann, 1984). Large eggs (1 mm diam.)
are preadapted to life in large clutches (10 mm diam.) of adherent eggs and
require little head of pressure for adequate ventilation (Strathmann & Chaffee,
1984). As some spiders (e.g. large lycosids) have eggs and egg masses about this
size it is pertinent to consider whether the constraints are applicable to them.
The diffusion constant of oxygen in air is five orders of magnitude larger than in
water (Goldstein, 1977) and three orders of magnitude greater than the
difference between air and water in any other relevant coefficient (see equations
3 and 8 in Strathmann & Chaffee, 1984; data for water, Goldstein, 1977; and
for air, Chemical Rubber Company, 1974).
I t is apparent therefore that without the surface spheres, development is
unlikely to be impeded by insufficient gaseous diffusion around the eggs in an
egg mass. However this is based on a porosity value (P) of between 0.259 and
0.875 being the loosest and tightest packing of equal sized spheres; for well
shaken spheres, P = 0.36-0.43 (Scheidegger, 1974). The surface spheres on the
eggs will, for P = 0.4, reduce the porosity by 60% to P = 0.16. ‘The conclusion
remains, that diffusion alone should be more than adequate to supply oxygen to
the eggs of a spider egg mass if the egg sac itself is not a serious diffusion barrier.
Humphreys (1983a) speculated that the spheres could result by separation
within a biphasic system. Possible processes for the formation of the spheres is
suggested by the striking similarity in the appearance of the spheres on spider
eggs and those produced by a number of physical processes, among them being
the crystalization of salicylic acid from a triphasis solution (Kawashima,
Okumura & Takenaka, 1982) and the products of microencapsulation
technology which can produce spheres similar in appearance, size and size class
frequency distribution to those on the surface of spider eggs (fig. 1 in Thies,
1979). Such microcapsules can be generated using a wide range of systems: 1,
polymer phase separation; 2, interfacial polymerization; 3, spray drying; 4, air
suspension encapsulation; 5, engulfing processes; 6, drying-in -liquid process
(Thies, 1979). Methods 1 and 2 employ cross-linking; the spheres of spiders’ eggs
are sulphur rich while the matrix contained no detectable sulphur under EDAX
scans (Humphreys, 1983a) and suggests that stability may result from disulphide cross-bridging. Methods 3, 4 and 5 produce spheres much greater in
size than those on spiders’ eggs while Method 1 produces spheres in the same
size range as those of spiders’ eggs.
Blunt anastomosing projections, similar in surface appearance to those seen on
some of the non-Araneae orders of arachnids are found also on the egg surfaces
of parasitic cestodes; there the globule substance, due to its frequent fusion, was
considered to have been deposited in a plastic state and hardening in situ
(Leithbridge, 1976).
How eggs breath
A spider egg contains, at least until emergence from the egg sac, all that is
needed for development except that it requires oxygen (Anderson, 1978) and
must dispose of carbon dioxide (Humphreys, 1973). Like a bird’s egg
(Diamond, 1982) the design is probably one of compromise; it must be strong
196
W. F. HUMPHREYS
enough to withstand packing in the egg sac but allow escape of the young, it must
allow transport of oxygen and carbon dioxide but not dry out, and it must have
sufficient energy reserves to last until the spiderling starts feeding. Insect eggs
have complex ‘shells’ and often specialized aeropylar areas and plastrons to
permit gaseous exchange (Hinton, 1981) but such structures have not been
found on spider eggs and so gaseous exchange probably occurs across the entire
surface of the chorion. Spider eggs are small and their large surface to volume
ratio should pose problems for water balance. Water loss could be reduced by
location of the eggs in microenvironments of suitable humidity, by their being
laid in masses and by the structure of the egg sac. The resistance of spider eggs
to drying has not been studied but it may be high as some are difficult to freezedry. Since oxygen, carbon dioxide and water follow identical diffusion paths,
measurement of the conductance to one (KO,, KCO, or IcH,O) permits the
calculation of the other two conductances (Diamond, 1982). Our understanding
of the structure of spider eggs and the significance of mass laying and egg sacs
must await such measurements.
ACKNOWLEDGEMENTS
I thank the following for kindly providing me with arachnid eggs of known
origin; D. G . Brown (Bath), V. Davies (Queensland Museum), M. R. Gray
(Australian Museum), M. S. Harvey (CSIRO, Canberra), P. D. Hillyard
(British Museum, Natural History), L. E. Koch (Western Australian Museum)
and H. W. Levi (Museum of Comparative Zoology, Harvard). The electron
optic equipment used for this work was provided by the Science and
Engineering Research Council (at Bath University) and the School of
Veterinary Science, Murdoch University.
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Species
Nature of egg surface
I.
".."111"""
,,."La,
.."...~~,
L.".ALYLC
-
Scorpiones: scorpions. Ova develop in the lumen of the ovariuterus (Levi, 1982)
* UROPYGI: whipscorpions. Eggs develop in sac attached to gonopore; female carries young (Levi, 1982)
Rough matrix with craters and emergent 'sharks
Phryniscus bacillqeer
teeth'
Rough granules in places covered by smooth
Mastigoproctus formidabilis
matrix
SCHIZOMIDA: micro whip scorpions. Eggs, glued together, are attached to female gonopore (Levi, 1982)
*AMBLYPYGI: tailless whip scorpions. Eggs carried on underside of female (Levi, 1982) in egg sac;
prenymph carried by female (Levi, 1982)
Charontidae
C h a m grayi
Rough 'crumbly' matrix
Heterophrynus sp.
No-orientated fibres over smooth surface
Tarantulidae
Wavy surface with small crenulations
Unidentified; Rio Ulna, Honduras
MCZ
Unidentified; Yucatan, Mexico MCZ
Smoothly bubbled surface
Truncated concave papillae and 'sharks teeth'
Unidentified; Holquin, Cuba MCZ
PALPIGRADA: no data on eggs (Levi, 1982)
RICINULEI: one or two eggs carried by female (Levi, 1982)
* PSEUDOSCORPIONES: pseudoscorpions. Most commonly eggs laid in brood sac attached to female, who
also nourishes the brood (Weygoldt, 1969). Comments refer to the surface of brood sacs
Garypidae
&nsphronus magnus
Smooth fine crenulations, small area of globules
SOLIFUGAE: wind scorpions. Eggs deposited en masse under ground and loosely stuck together with 'mucus'
(Muma, 1966)
Fremnhniae,I.,mnnnn~.r
P,,,c,
..,:,l-1..
,
.
.
"
,
,
,
I
-..-""+- -.p&,;!!ae
..
ayucc"
(diam.
Fremnha?idaP
8"-1.3 pm).
Spaced round papillae (0.7 pm)
E. palpisetulosus
Low nodular papillae (diam. 0.5 pm)
E. nodularis
Distinct fine nodular papillae (0.8 pm)
Erernorhax magnus
Spaced truncate papillae ( I .2 pm)
Therobates bilobalus
Galeodidae
Galeodes sp.
Minute pits
Order and family
Hingston (1925)
Muma (1966)t
This paper
This paper
This paper
This paper
This paper
This paper
This paper
This paper
Reference
Table A l , The egg laying habits and egg characteristics of arachnid orders other than Araneae. * = de nouo information on
egg surfaces. MCZ is unidentified material from the Museum of Comparative Zoology, Harvard. Except for the Opiliones,
all the material presented here de nouo is from old alcohol stored specimens and preservation artefacts may be expected.
APPENDIX
-
u2
10
Species
Cheyletiella yasguri
Neotrombicula zachvatkini
Pergamascus uiator
P. quisquiliarum
P. manicatellus
P. monticola
Paragamasus sp.
Holoparasitus sp.
Parasitus lunulatus
Cheyletiellidae
Trom biculidae
Parasitidae
TDiameters estimated from photomicrographs in the paper.
~~
Sarcoptes scabiei
Sarcoptidae
}
Odiellus spinosus
ACARI: ticks and mites. Eggs laid mostly in small clusters
Bdella humida
Bdellidae
Biscirus thori
B. arenarius
Spinibdellas cronini
Cyta latirosfris
c. longiseta
Cheyletus eruditus
Cheyletidae
Tyrophagus putrescentiae
Acaridae
DicranopaQus ramasus
*OPILIONES: harvestmen. Eggs deposited en masse in soil
Order
Sleeve-like folds with a constant arrangement
within a species. Produced by epithelial glands
of posterior uterus
Smooth with sparse low ridges
Studded with irregular mounds; no aeropyles
and no special micropyle
Irregular mamelons 0.4-1.6 pm packed on
surface. No micropyle but openings (aeropyle?)
present. Fibrillose in dense matrix bounded by
laminae. Former two affected by protease.
Irregular tubercules
Granular
Long fine twisted projections
Twisted membrane
Circular projections
Irregular globules
Fine irregular projections
Complex deep network; crescent ‘micropyllar’
area
Rough cracked pavement
Nature of egg surface
Table A l . Continued
3
Witalinski (1977)
Marchiondo (1981)
Daniel et al. (1973)
Mazzini & Baiocchi (1983)
Mumcuoglu et al. (1973)
Callaini & Mazzini (1984)
Wallace & Mahon (1972)
This paper
This paper
Reference
2
m
0
N
SPIDERS’ EGGS
20 1
Table A2. Families of Araneae for which data are available on the form of the
egg surface. Electron micrographs have been taken of all the families listed.
* Denotes published electron micrographs
ARANEAE
ORTHOGNATHA
Mcricobothriidae
Dipluridae
Hexathelidae
Ctenizidae
LABIDOGNATHA
Gradungulidae
Pholcidae
Uloboridae
Dinopidae
Theridiidae
Linyphiidae
Araneidae
Mimetidae
Agelenidae
Clubionidae
Miturgidae
Stiphidiidae
Lycosidae
Pisau rid ae
Thomisidae
Heteropodidae
Philodromidae
Salticidae
Grim & Slobodchikoff (1982)*
This paper
This paper
This paper*
This paper*
Grim & Slobodchikoff (1982)*; Humphreys (1983)*; This paper*
Grim & Slobodchikoff (1982j*
This paper*
Grim & Slobodchikoff (1978*, 1982*; Humphreys (1983;~);This paper
Humphreys (1983a)*
Grim & Slobodchikoff (l982j*; Humphreys (1983a)*; This paper*
This paper*
Grim & Slobodchikoff (1982)*; Humphreys (1983a); This papcr
Grim & Slobodchikoff (1982)*
This paper
This paper*
Grim & Slobodchikoff (1982j*; Humphreys (1983aj*; This paper*
Humphreys (1983a)*
Grim & Slobodchikoff (1978)*; This paper*
This paper*
Grim & Slobodchikoff (1982)*
Grim & Slobodchikoff (1982)*; Hurnphreys (1983aj*
REFERENCES
CALLAINI, G. & MAZZINI, M., 1984. Fine structure of the egg shell of Tyrophagus putrescentzae (Schrank)
(Acarina: Acaridaej. Acarologia, 25: 359-364.
DANIEL, M., SIXL, W., SIMONOVA, V. & WALTINGER, H., 1973. Scanning micrographs of the chigger
Neotrombicula zachuatkini (Schluger 1948): spermatophores and eggs. Folia Parasitologica, Prazue, 20: 273-274.
HINGSTON, R. W. G., 1925. Nature at the Desert’s E d p . London: Weatherby.
MARCHIONDO, A. A., 1981. Scanning electron microscopy of the egg of Cheyletiellaycsguri Smiley (Acari:
Cheyletiellidaej with a description of the egg burster. International Journal of Insrct Morpholology and
Embryology, 10: 167-1 7 I .
MAZZINI, M. & BAIOCCHI, R., 1983. Fine morphology of the egg-shell of Sarcoptes scabiei (L.) (Acarina:
Sarcoptidaej. International Journal of Parasitology, 13: 4 6 9 4 7 3 .
MUMA, M. M., 1966. Egg deposition and incubation for Eremabates durangonus with notes on the eggs of other
species of Eremobatidae (Arachnida: Solpugida) . Florida Entomologist, 49: 23-3 1.
MUMCUOGLU, Y . , GUGGENHEIM, R. & HENNING, L., 1973. Raster Elektonenmikroskopische
untersurhungen von Cheyletus eruditus (Schrank, 1781 j (Acarina, Cheyletidae). Acarologia, 15: 644-648.
WALLACE, M. M . H . & MAHON, J. A., 1972. The taxonomy and biology of Australian Bdellidae (Acarij.
I. Subfamilies Bdellinae, Spinibdellidae and Cytinae. Acarologia, 14: 544-580.
WEYGOLDT, P . , 1969. T h e Biolou of Pseudoscorpions. Harvard University Press: Cambridge, Massachusetts.
WITALINSKI, W., 1977. Scanning microscopy investigations of egg surface of some mesostigmatic Acari.
Pedobiologia, 17: 97-101.