ZOOLOGIST, 9:103-111 (1969).
Control of Drag-Line Spinning in Certain Spiders
RONALD S. WILSON
Department of Zoology, Bristol University, Bristol 8, England
SYNOPSIS. Araneomorph spiders from many different families show some regional
differentiation of the duct which carries the drag-line silk, but only in the orb-web
spiders is there a well-defined control valve.
This valve, and its associated muscles, is described for Araneus diadematus (Argiopidae), an ecribellate spider, and is compared with that found in Uloborus octonarius
(Uloboridae), a cribellate spider. It is suggested that the remarkable similarity
between the valves in these two groups implies evolutionary convergence.
Some evidence is presented which suggests that variations in body pressure are used
to control the drag-line spinning, at least in the more primitive Araneomorphs.
The drag-line plays a very important
part in the life of a spider, ranging from a
simple lifeline in the Salticids to a fundamental part of the complex web in the
Argiopids. Its function is sometimes less
clear in the more primitive spiders, for
instance in the Gnaphosids and Clubionids
which are typically nocturnal hunters, but
even with these it may be a safety device.
Spiders are a very varied group, and one
might, therefore, expect to find equally
varied uses for the drag-line in the different families, and even between the species
within a single family.
It is clear from studies of the silk glands
and their arrangement that there is an increase in complexity from the more primitive hunting spiders to the more advanced
web-spinning spiders, culminating in the
Argiopidae which build orb-webs and possess six different kinds of silk gland, (Bristowe, 1958; Millot, 1949; Savory, 1952). It
is interesting, therefore, to find that, even
in the most primitive Araneomorph spiders, certain glands are already marked out
for the production of drag-line silk. These
glands are mostly of the type known as
"ampullaceal" (Fig. 1), which have a tubular portion in which the silk is secreted as
a liquid, followed by an ampulla or reservoir where the silk is stored, followed in
turn by a thin and sometimes looped duct
to a spigot on one of the anterior spinnerets. In the duct lies the control valve, a
thickened region of the cuticular lining of
the duct, which will be described later in
more detail.
There are three pairs of spinnerets in
most spiders, and the principal drag-line
spigots are found on the anterior spinnerets (Fig. 2).
The general form of the drag-line spigot
is a long tapering cone of flexible cuticle
set in a sclerotized base, and located
towards the inside edge of the anterior
spinneret, within an area usually bounded
by hard cuticle, which is connected to the
main flexor muscle of the spinneret (Fig.
3).
This arrangement allows the drag-line
spigot to pivot independently of the other
spinning and to be retracted into the spinneret by the flexor muscle.
Adult spiders of many families possess
two glands, ducts, and spigots per anterior
spinneret, both fully functional, and this is
also true of juvenile Araneus and other
members of the Argyopidae. However, in
Araneus the more dorsal of each pair degenerates at the last molt in both male and
female, being represented in the adult by
a small cuticular knob.
This study is principally concerned with
the arrangement and operation of the
drag-line spinning apparatus in the two
main families of orb-web spiders, namely
the Argyopidae and the Uloboridae, represented by Araneus diadematus and Uloborus octonarius, respectively. These families belong to separate divisions of the
103
104
RONALD S. WILSON
secretory
part
loop in duct
dragline spigot
anterior spinneret
reservoir
(ampulla)
mm
FIG. 1. Drawing of the major anipullaccal gland oC Araneus diadematus.
filiform gland
aciniform gland spools spigot
piriform gland spools
major ampullaceal
gland spigot
lobed gland spigots
aciniform gland spools
cylindrical gland spigots
minor ampullaceal gland
spigot
anal
papilla
500
h
colulus
posterior
anterior spinneret
FIG. 2. Drawing to show the arrangement of the
spinnerets and their spinning tubes (spigots and
median spinneret
spools) in Araneus diadematus.
spinneret
105
DRAG-LINE SPINNING IN SPIDERS
piriform gland
spools
major ampul laceal
gland spigot
terminal
sclerite
flexible
cuticle
intermediate
sclerite
lyriform
organ
principal
apodeme
50
spigot
depressor muscle
FTC. 3. Drawing of the tip of the right anterior
spinneret of Araneus diadematus, showing how the
Araneomorph spiders, namely the Ecribellatae and Cribellatae, and are reputed to
have developed independently. I must particularly thank Mr. Eberhardt of the Harvard Biological Laboratories for his kindness in procuring preserved specimens of
Uloborus for me; unfortunately, I have not
yet been able to study living specimens.
In addition, the drag-line spinning apparatus of spiders from 16 Araneomorph
families has been examined, and, although
there is not space to present the results in
detail, some of the more interesting points
will be mentioned later.
spinneret
muscle
flexor
spigot o£ the major ainpullaceal
mounted on the terminal sclerite.
gland
STRUCTURE OF THE DRAG-LINE SPINNING
APPARATUS
The structure of the drag-line spinning
apparatus, described below for Araneus, is
based on previously published work (Wilson, 1962a).
The ampullaceal gland (Fig. 1), has a
duct which loops back on itself before entering the base of the spinneret. The duct
consists of a layer of cells surrounding a
tubular cuticiilar lining through which the
silk passes. Just where the duct enters the
spinneret base, the cuticular lining is swol-
106
RONALD S. WFLSON
piriforrn gland spools
spinneret
extensor
muscle
terminal sclerite
major ampullaceal.
gland spigot
intermediate
sclerite
duct
auct
duct
stabilizer
muscle
spigot
depressor
muscle
dorsolateral
muscles
principal
apodeme
position
of valve
spinneret
flexor
VI muscle
ligament
valve
tensor
muscle
duct levator
muscle fpart)
levator
muscle
B
(part)
.cuticular lining
of duct
FIG. 4. Drawings to show the internal anatomy of
the right anterior spinneret of Araneus diadema~
lus. A, ventral half (from inside); B, dorsal half
(from outside).
Jen to form a control valve, and the cells
surrounding it are modified as tonofibrillae, which connect on one side with the
"valve-tensor" muscle, and on the other
with a ligament which attaches to the inside wall of the spinneret. In addition,
there is a "duct-levator" muscle, which
runs from the duct below the valve to a
point on the dorsal wall of the spinneret;
and there is a "stabilizer" muscle which
runs across the base of the spinneret and
links with the duct and the ligament. The
duct at this point shows a kink or bend, so
that the portion containing the valve lies
along the axis of the valve-tensor muscle.
The main muscles which move the spinneret, as opposed to those operating the
control valve, include a main flexor muscle
attached to the principal apodeme; a spigot-depressor, also attached to the principal apodeme and running to the dorsal
wall of the spinneret; and a main extensor
muscle or muscles running up the ventrolateral wall of the spinneret to connect
with the terminal sclerite of the spinneret
(Fig. 4).
Bearing in mind that the spinneret will
be maintained in an extended position by
the internal body pressure, the contraction
of the flexor will bend the spinneret inwards and will retract the drag-line spigot;
the spigot-depressor will pull the principal apodeme dorsally and thus will rotate
the spigot ventrally about the hinge-point
(illustrated in Fig. 3); and the main extensor will pivot the spinneret and the
terminal sclerite outwards so the spigot
and spools are exposed. During spinning,
the spinnerets have been observed to show
both simple lateral to-and-fro movements
as well as more complex rotary ones.
With respect to the drag-line duct, the
cuticular lining proximal to the valve is
composed of two layers, an outer "striated"
layer, and a thin inner layer of apparently
homogeneous cuticle. At the valve, however, this inner layer is modified to form
the valve lips which comprise a ring of
DRAG-LINE SPINNING IN SPIDERS
loosely knit laminar cuticle surrounding
the lumen; and a new type of cuticle appears, termed "fibrillar," which is interposed between the valve lips and the striated cuticle. This fibrillar cuticle extends
distally from the valve for a short distance,
but thins down and disappears before the
spigot is reached.
The lips of the control valve can be
inflated so as to occlude the lumen of the
duct, and the valve-tensor and duct-levator
muscles act in opposition to each other so
that the valve is compressed longitudinally, thus driving fluid out of the fibrillar
cuticle and inflating the lips (Fig. 5). The
valve opens as the muscles relax (Wilson,
19626).
It was with this information at hand
that a study was made of Uloborus, an
orb-web spinning cribellate spider, and it
was found that there is a remarkable similarity in the structure of the control valve
and its associated muscles between Uloborus and Araneus.
Uloborus possesses a comparable cuticular valve, a valve-tensor muscle and a ductlevator muscle, as well as a spigotdepressor, main flexor and extensor, and
an antero-lateral muscle. Certain differences of detail can be noted, however; for
instance, the spigot-depressor and the antero-lateral muscles run the length of the
spinneret in Uloborus instead of being attached to the spinneret wall as in Araneus;
the control valve lies well up inside the
spinneret instead of being near the base as
in Araneus; and the duct-levator muscle is
attached to the ventral wall of the spinneret instead of to the dorsal wall as in
Araneus (Fig. 6).
It is curious that the two spiders should
have drag-line control valves and muscles
so essentially similar in structure and arrangement, and yet show differences of detail which prevent one assuming that they
are strictly homologous. It seems that a
well-developed control valve is necessary
for orb-web spinning and that this type of
valve has evolved independently in the Argiopids and the Uloborids. Whether this
convergence is due to limited evolutionary
107
potential, or to some special feature associated with silk spinning which favors an
inflatable ring-like valve, is an open question.
A feature of the drag-line spinning apparatus already mentioned is the reduction
of number of drag-line glands, ducts, and
spigots from a double set on each anterior
spinneret to a single set. Prior to the last
molt, Araneus (and Uloborus, judging
from the rudimentary second spigot seen
in the adult) possess a double set of apparatus with control valves in each duct.
The muscles in juvenile Araneus are, however, similar to those of the adult. Since
juveniles spin excellent orb-webs as do the
adults, the explanation for this reduction
must be connected either with the general
increase in size, or with the onset of sexual
maturity.
Size as a factor seems to be ruled out
because one may find adults of smaller
species, for instance Mela sp., whose size
approximates that of a juvenile Araneus
and yet show the reduction. Sexual maturity must, therefore, provide the key to the
problem, and it may be that the spider
sacrifices its duplicate set of drag-line
glands to make room for the developing
gonads. This would be especially important in the orb-web spiders because their
large lobed glands, which secrete sticky
silk, already take up a great deal of room
in the abdomen.
THE CONTROL OF DRAG-LINE SPINNING
New silk is secreted in the gland as a
liquid, and is spun into a thread which
emerges from the spigot. It is well known
that silk threads form because of tension
which aligns the molecules, and not because of exposure to air. The silk thread
will dry out, of course, in the air after it
has been formed, but tension is the formative force. Body pressure cannot of itself
extrude the silk as a thread, but can act on
the soft-walled gland as if it were the
bulb of a pipette, and thus move the liquid
silk down the duct to the tip of the spigot.
Once at the spigot's tip, the silk is at-
108
RONALD S. WILSON
to spigot
tonofibrillar
attachments
silk
tonofibrillar attachments
FIG. 5. Stereodiagrams showing the structure o£
the control valve oE Araneus diadematus in longi-
tudinal section. A, valve closed; B, valve open,
109
DRAG-LINE SPINNING IN SPIDERS
piriform gland spools
spinneret
extensor
muscle
duct
levator
muscle
valve
tensormuscle
major ampullaceal
terminal sclerite
gland spigot
'flexible
intermediate
cuticle
sclerite
spigot
degenerate
depressor
spigot
muscle
duct
principal
apodeme
position
of valve
ligament
dorsolateral
muscle
spinneret
^
flexor
\\ muscle
B
cuticular lining of duct
lOOu
FIG. 6. Drawings to show the internal anatomy o£
the right anterior spinneret of Uloborus octonari-
us. A, ventral half (from inside); B, dorsal half
(from outside).
tached to the substrate, and the spider
drops from its support, thereby allowing
the weight of its body to pull on the silk
and so form the drag-line. It is clear that
at some point along the duct there must be
a transition between liquid silk and silk
thread, and it is feasible that this point lies
normally at the control valve.
Spinning of the drag-line depends on
three factors: (1) the body pressure of the
spider forcing liquid silk up the duct from
the gland; (2) the control valve regulating
the flow of liquid silk; and (3) the tension in the silk thread aligning the molecules. Variation in any of these factors will
result in changes either in rate of drag-line
spinning, or of drag-line thickness. It will
be readily appreciated that the quantity of
silk material emerging at the spigot must
equal the amount of fluid silk moving up
the duct from the gland, if a continuous
thread is to be produced. When the drag-
line is extracted artifically at a very high
rate, the diameter of the silk reduces, the
point where the thread forms (transition
from fluid to thread) moves back along the
duct towards the gland, and at some point
the filament breaks: all these effects being
due to a lack of available liquid silk to
meet the demands of spinning. Evidence
for these statements has been presented
previously (Wilson, 19626).
When a spider suspends itself from a
support on its drag-line, without using its
tarsal claws to grip the silk, the thread
must still be gripped somewhere within the
body of the spider. It is difficult to imagine that this can occur at the spigot tip,
and it seems likely that the control valve is
the operative structure. Although difficult
to prove, it seems reasonable to suggest
that the control valve functions both as a
regulating device for controlling the fluid
silk, and as the functional spinning orifice,
110
RONALD S. WILSON
so that the duct between the valve and
the spigot will normally contain silk
thread, and not liquid silk.
As a falling spider cannot actively regulate the tension in the silk, it follows that
its control of drag-line spinning resides in
its ability either to vary the aperture of the
control valve, or to alter the internal body
pressure. If it is true that the valve is the
functional spinning orifice, then altering
its aperture will affect the thickness and
strength of the thread. On the other hand,
altering the body pressure may affect mobility of the legs, as the two are closely
interdependent. Both of these effects may
therefore be deleterious, but some evidence
can be gleaned by observing the leg positions of a falling spider which indicate that
at least body pressure is involved with
drag-line control.
Spiders dropping on their drag-lines can
be seen to stretch out their legs while
falling, and to curl them in when hanging
stationary. This may be observed with
Araneus, but is more striking when seen in
a spider which does not possess a comparable control valve, such as Segestria florentina. Segestria does not readily let go of a
support on which it is crawling, but I have
observed it controlling its drag-line while
dropping (Fig. 7). This does not rule out
the use of the control valve in those species
which possess them, and indeed if the
valve were to take over this function, it
would leave the legs unaffected, and therefore able to carry on with their delicate
task of building an orb-web.
CONCLUSION
It remains only to mention a few of the
findings with respect to the other, nonorb-web spiders. The following families
and species have been examined:
Dictynidae: Ciniflo ferox
Uloboridae: Uloborus octonarius
Dysderidae: Dysdera crocata, Segestria
florentina
Pholcidae: Pholcus phalangoides
Gnaphosidae: Drassodes lapidosus
Clubionidae: Clubiona phragmitis
Sparassidae: Heteropoda venatoria
FIG. 7. Drawings to show the leg positions adopted
by Segestria florenlina when dropping on the
drag-line. A, while moving; B, while hanging stationary.
Thomisidae: Xysticus ulmi
Salticidae: Marpissa muscosa
Lycosidae: Lycosa sp., Trochosa terricola
Pisauridae: Pisaura mirabilis, Dolom.ed.es
fimbriatus
Agelenidae: Argyroneta aquatica, Tegenaria atrica, Textrix denticulata, Amaurobius terrestris
Theridiidae: Steatoda bipunctata, Theridion sisyphium
Tetragnathidae:
Tetragnatha extensa,
Pachygnatha clerki
Argiopidae: Araneus diadematus, Mela
segmentata, Zygiella x-notata
Linyphiidae: Linyphia triangularis
Of these only Pholcus, Segestria, and
Dysdera have a single spigot without any
trace of a second one; they are the only
three which do not possess some sort of
control valve in the dragline duct; and
only in Uloborus, Drassodes, Heteropoda,
Argyroneta, and all the Theridiids, Tetragnathids, and Argiopids, is there evidence of a reduction from a double to a
single set of drag-line spinning apparatus
in the adult.
There is, however, a difference in size
between the two ducts and spigots of a pair
in all the species with the double set, and
it is invariably the more ventral spigot
which is larger. Thus there is some evi-
DRAG-LINE SPINNING IN SPIDERS
dence of the reduction of the more dorsal
spigot throughout the group.
It must also be emphasized that, although a great many diverse species possess
control valves of a sort, they are frequently only of a simple type, not resembling
the well developed and clear-cut valves of
the Argiopids and their relatives. Only in
the Argiopids, the Tetragnathids, the
Theridiids, and the Uloborids are to be
found both the valve-tensor and the ductlevator muscles; the others frequently have
a tensor, but never a levator, and it is
difficult to see how the simpler valves operate. They may act merely as a point of
high resistance to the flow of silk, while
the actual control of silk spinning relies on
changes in body pressure. It seems, however, that the orb-web spiders require a
more delicate form of control, and that in
111
the two separate evolutionary lines of the
cribellate and ecribellate spiders, respresented by Uloborus and Aranens, respectively, there has been selection for a
fully functional control valve of remarkable similarity.
REFERENCES
Bristowe, W. S. 1958. The world of spiders. New
Naturalist, Collins, London.
Millot, J. 1949. Ordre des Araneides. Chapter in
Traite de Zoologie, vol. 6. P. Grasse, Ed. Masson,
Paris.
Savory, T. H. 1952. The spider's web. Warne,
London.
Wilson, R. S. 1962a. The structure of the dragline
control valves in the garden spider. Quart. J.
Microscop. Sci. 103:549-555.
Wilson, R. S. 19626. The control of dragline spinning in the garden spider. Quart. J. Microscop.
Sci. 104:557-571.