Turkish Journal of Zoology
http://journals.tubitak.gov.tr/zoology/
Research Article
Turk J Zool
(2016) 40: 720-728
© TÜBİTAK
doi:10.3906/zoo-1510-19
Mandibular morphology and dietary preferences in two pygmy mole
crickets of the genus Xya (Orthoptera: Tridactylidae)
Kateřina KUŘAVOVÁ*, Petr KOČÁREK
Polyneoptera Research Group, Department of Biology and Ecology, Faculty of Science, University of Ostrava, Slezská Ostrava,
Czech Republic
Received: 07.10.2015
Accepted/Published Online: 10.03.2016
Final Version: 24.10.2016
Abstract: Pygmy mole crickets (Tridactylidae) are an interesting but little-known group of Orthoptera; their food biology and dietary
preferences have not been adequately studied. Our study focused on the diet composition and associated functional morphology of
the mandibles in two European species: Xya pfaendleri Harz, 1970 and X. variegata (Latreille, 1809). Based on postmortem gut content
analyses, the studied species feed mainly on detritus (decaying organic particles), representing 91% and 97% of the diet in X. pfaendleri
and X. variegata, respectively. The mandibular structures are associated with the processing of such food. The incisor area is equipped
with three robust and sharp teeth, and the molar area consists of a small molar ridge forming a trituration area with parallel molar
slats. The size of the molar ridge is determined by sex and the side (left/right) of the mandibles: females have larger molar ridges than
males and left molar ridges are larger than the right. Food preferences as well as measured parameters of the mandibles were similar in
both studied species due to the similar pattern of their biology. The narrow food niche indicates a low diversity of consumed food and
uniformity of food intake.
Key words: Detritophagy, detritovory, food biology, mouthparts, exoskeleton
1. Introduction
Although the food strategies of many Orthoptera species are
relatively well known, the food strategy in certain species has
only been estimated based on the structure of the mouthparts,
habitat preferences, or feeding behavior observed in only a
few individuals. Species of the suborder Caelifera are mostly
herbivorous (Acridomorpha) and only rarely detrito-/
bryophagous (Tetrigoidea). Species of Tridactylidae (pygmy
mole crickets) are probably omnivorous, although detailed
analyses of their food preferences have not been performed.
The consumption of a specific diet is associated with the
shape and structure of the mandibles; four types of mandibles
(graminivorous, forbivorous, herbivorous, and bryovorous
types) have been identified in orthopterans (Isely, 1944;
Kaufmann, 1965; Uvarov, 1977; Aguirre et al., 1987; Smith and
Capinera, 2005). A correlation between mouthpart structure
and the specific diet of orthopterans has been reported for only
a few species, i.e. Locusta migratoria (L., 1758); Schistocerca
gregaria (Forskål, 1775); Melanoplus desultorius Rehn, 1907;
Trimerotropis pallidipennis (Burmeister, 1838); Opeia obscura
(Thomas, 1872); and Tetrix tenuicornis (Sahlberg, 1891)
(Gangwere, 1960, 1961, 1965; Patterson, 1983, 1984; Kang et
al., 1999; Kuřavová et al., 2014).
*Correspondence: [email protected]
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The food biology and dietary preferences of pygmy mole
crickets (Caelifera: Tridactylidae) are poorly understood
due to their small body size (up to 5 mm) and hidden
way of life. Pygmy mole crickets build horizontal galleries
beneath the surface of wet damp substrates, usually on
the banks of ponds or rivers, and they are considered to
be algae-feeders or omnivorous insects (Blackith, 1987;
Deyrup and Eisner, 1996).
Our study focused on determining whether the pygmy
mole crickets Xya pfaendleri Harz, 1970 and X. variegata
(Latreille, 1809) are dietary specialists or omnivorous
detritus feeders. This study was based on analyses of
postmortem gut contents and the mandibular structures
associated with food processing. The aims of the study were
to describe and compare the structure of the mandibles in
the two pygmy mole crickets, evaluate food preferences of
pygmy mole crickets, and assess interspecific differences in
these highly similar and related species.
2. Materials and methods
2.1. Study organisms
The pygmy mole crickets Xya pfaendleri and Xya variegata
(Orthoptera: Tridactylidae) are small (4–6 mm total body
KUŘAVOVÁ and KOČÁREK / Turk J Zool
length), shiny, dark or black and cryptic-colored insects.
They are characterized by stout and enlarged femurs of the
hind legs that are adapted for leaping. They can jump as
far as 1 m (Burrows and Picker, 2010; Burrows and Sutton,
2012). Pygmy mole crickets are riparian and thermohygrophilous, and they have a Ponto-Mediterranean
distribution (Zechner et al., 1999; Berg et al., 2000;
Kočárek et al., 2005; Baur et al., 2006; Münsch et al., 2013).
They live in shallow burrows, which they build in sandy
soil alongside bodies of water, including streams, ponds, or
sand quarries with permanent or temporal water (Blackith
and Blackith, 1979; Münsch et al., 2013).
2.2. Collection and treatment of samples
Specimens of Xya pfaendleri and X. variegata were
collected from two localities in the southeastern part of the
Czech Republic. Specimens of X. pfaendleri were collected
on 12 August 2013 in a littoral zone (sand substrate) of
a small pond (~25 m2) in the area surrounding Lanžhot
(48°40′34″N, 16°55′25″E) at approximately 200 m a.s.l.
Specimens of X. variegata were collected on 4 June 2013
at a sand quarry in Mušlov near Mikulov (48°47′30″N,
16°41′11″E). This locality has an area of 20 m2 and an
elevation of 250 m a.s.l. The specimens were collected
by sweeping and were immediately stored in 70%
alcohol for later analysis of the alimentary tract contents
and morphology of the mandibles. A total of 24 adult
specimens (6 males and 6 females of X. pfaendleri, 6 males
and 6 females of X. variegata) were used for the analysis.
In the laboratory, both mandibles of each specimen
were dissected, examined, and documented by scanning
electron microscopy (SEM; JEOL JSM-6610LV, Tokyo,
Japan) (SEI, 12 kV, WD 41 mm, SS30, 130–1500×
magnification). The mandibles were sputter-coated with
gold prior to SEM (Automatic Sputter Coater: JEOL JFC1300, Tokyo, Japan). The parameters of both mandibles
(length of the mandible, determined as the distance
between the mandibular junction and the peak of the first
incisor; surface of the molar ridge; number of slats) were
measured using the software SEM JEOL/EO version 1.0.
The alimentary tract contents of each specimen were
permanently mounted in Hoyer’s solution (Anderson,
1954) on a glass microscope slide. The slides were
observed using a light microscope (Olympus CX41RF,
Hamburg, Germany) at 400× magnification and were then
photographed using a camera (Canon EOS 1100D, Tokyo,
Japan) mounted on the microscope. The entire alimentary
tract preparation from each specimen was photographed.
These photographs were evaluated using BaDra 2.0
image analysis software (BaDra, 2011). The software was
used to determine the proportions of food components
(proportion of detritus + plant cellular matter + other
particles = 1):
(%)
(1)
Proportions of diet = ∑ pi
where pi is the proportion in the alimentary tract content,
and i is the type of diet component as either detritus
(unidentified decaying organic particles), plant cellular
matter (lower plants, algae), or other particles (fragments
of arthropod cuticles, mineral particles).
The proportions of the components of the diet were
evaluated and were used to calculate the food niche
breadth based on the Gini–Simpson index (Gini, 1912, in
Di Russo et al., 2014):
Gini–Simpson index = 1 – ∑pi2
(2)
where pi is proportion of the ith component in the
alimentary tract content.
2.3. Statistical analysis
The measured parameters of the mandibles (distance
between the mandibular junction and the first incisor,
surface of the molar ridges, and number of molar slats) are
presented as the means ± SD (Table 1). According to the
Table 1. Parameters measured on the left and right mandibles of Xya pfaendleri and Xya variegata. LM - length of the mandible (distance
between the mandibular junction and the peak of the first incisor); SMR - surface of the molar ridge; NS - number of slats.
Species
Left mandible
Right mandible
LM [µm]
SMR [µm ]
NS
LM[µm]
SMR [µm2]
NS
Male
506.9 ± 34.9
8767 ± 703
12.5 ± 0.8
520.3 ± 7.5
8186 ± 495
12.5 ± 0.8
Female
676.0 ± 25.9
16204 ± 678
12.8 ± 0.8
612.9 ± 1.0
11720 ± 319
12.2 ± 0.5
Male
570.9 ± 0.7
14180 ± 346
13.0 ± 0.3
530.1 ± 0.6
13453 ± 402
13.0 ± 0.3
Female
714.7 ± 29.3
21804 ± 1153
13.8 ± 0.6
653.2 ± 13.9
17730 ± 438
12.8 ± 0.6
2
X. pfaendleri
X. variegata
A total of 24 specimens of X. pfaendleri and X. variegata were measured. The values (means ± SD) are in micrometers.
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Shapiro–Wilk normality test, all data were not normally
distributed. The effects of the side of the mandible, sex,
and species on the measured parameters of the mandibles
of X. pfaendleri and X. variegata were evaluated using
general linear models (gamma distribution function).
Analysis of variance (ANOVA) was used to determine the
effects of the left/right mandible, sex, and species on the
measured parameters of the mandibles of X. pfaendleri and
X. variegata.
The alimentary data were evaluated using general
linear models (Gaussian distribution function). The effects
of sex, species, and the interaction of sex versus species on
the proportion of food components in the alimentary tracts
were tested using multifactorial ANOVA (general linear
model with a Gaussian distribution function). The effects
of sex, species, and the interaction of sex versus species
on the food niche breadth were tested using multifactorial
ANOVA (general linear model with a Gaussian distribution
function). All analyses were performed using the R Ver.
2 statistical program (R Development Core Team, 2003).
The significance level was α = 0.05 throughout.
3. Results
3.1. Mandible structures of pygmy mole crickets
The mandibles of pygmy mole crickets of the genus Xya
have an asymmetrical shape (Figure 1). The left mandibles
are longer than the right mandibles (the average difference
was approximately 1% in X. pfaendleri and 8% in X.
variegata), although the difference was significant only for
X. variegata (Table 2). The lengths of the left mandibles
are greater than the lengths of the right mandibles, with
the exception of males of X. pfaendleri and X. variegata.
Both the right and left mandibles are bigger in females
than in males (Table 2; Figure 2). The general shapes of the
mandibles are similar in both species: the left mandibles
are elongated and narrower and the right mandibles are
shorter and wider. Each of the mandibles consists of
two functional parts: the incisor area and the molar area
(Figure 1). The incisor area is equipped with three robust
and sharp teeth. The molar area consists of a small molar
ridge, which is strongly concave on both mandibles. The
size of the molar ridge is affected by sex and the side (left/
Figure 1. Dorsal and ventral views of the left and right mandibles of Xya pfaendleri and X. variegata. The microstructures of the mandibles
were documented by scanning electron microscopy (SEM; JEOL JSM-6610LV): SEI, 12 kV, WD 50 mm, SS30, 65× magnification.
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Table 2. ANOVA results for the mandibular parameters in X. pfaendleri and X. variegata as affected by independent variables, including
mandible side, sex, and studied species. LM - length of the mandible, distance between the molar junction and the peak of the first
incisor; SMR - surface of the molar ridge; NS - number of slats on the molar ridge.
Independent
variable
Left/right side
X. pfaendleri
X. variegata
Sex
X. pfaendleri
X. variegata
Between species
Left mandible
Right mandible
LM
SMR
NS
F-value
P-value
F-value
P-value
F-value
P-value
2.6
28.1
0.11
<0.001*
23.4
24.3
<0.001*
<0.001*
0.75
2.82
0.39
0.10
73.4
191.7
<0.001*
<0.001*
109.6
149.3
<0.001*
<0.001*
0.75
1.25
0.39
0.11
1.9
1.1
0.17
0.29
10.3
40.7
<0.01*
<0.001*
4.1
3.2
0.05
0.08
A total of 24 specimens of pygmy mole crickets were analyzed. *P < 0.05.
Figure 2. Comparison of the measured parameters of the mandibles in females (gray boxes) and males (white boxes) of Xya pfaendleri
and X. variegata (Tridactylidae). a, d: length of the mandible (distance between the mandibular junction and the peak of the first
incisor); b, e: surface of the molar ridge; c, f: number of molar slats. The box plots show the median (50th percentile) and 25th and 75th
quartiles; error bars show the 10th and 90th percentiles. Filled symbols indicate outliers.
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3.2. Dietary preferences
The digestive tracts of the two species of pygmy mole
crickets from the genus Xya contained mainly detritus,
defined as unidentified decomposed organic matter, which
accounted for 91% and 97% of the consumed diet in X.
pfaendleri and X. variegata, respectively (Figure 4). Algae
formed a minor component and body parts of invertebrates
(fragments of cuticles, and setae) were rare, accounting for
9% and 3% of the total diet in the alimentary tract in X.
pfaendleri and X. variegata, respectively. Angiosperms,
fungal spores, and mosses were not found. The ratios of
detritus/plant cellular matter/other particles were 29:1:3
and 19:1:0 in females and 47:1:1 and 1:0:0 in males of X.
right) of the mandibles: females have larger molar ridges
than males, and the left molar ridges are larger than the
right (Table 2). The molar ridge forms a trituration area
with parallel molar slats. The molar slats contain several
parallel teeth. These teeth are short and regularly aligned.
There was an average of 12 molar slats per molar ridge, and
the number of molar slats was not significantly different
between the sides of the mandibles or between females
and males (Table 2). Differences in measured parameters
between the studied species of pygmy mole crickets were
significant only with respect to the parameter surface of
the molar ridge, with X. variegata having a larger molar
ridge than X. pfaendleri (Table 3; Figure 3).
Table 3. ANOVA results for the composition of detritus, plant cellular matter, and other particles in Xya pfaendleri and X. variegata as
affected by independent variables, including sex and species.
Independent
variable
Detritus
F-value
Sex
Species
Sex × species
5.52
2.74
0.73
Cellular matter
Other particles
Gini–Simpson index
P-value
F-value
P-value
F-value
P-value
F-value
P-value
0.03*
0.11
0.40
2.74
0.05
2.06
0.11
0.82
0.16
2.47
2.47
2.47
0.13
0.13
0.13
6.06
0.12
0.40
0.02*
0.12
0.53
A total of 24 specimens of pygmy mole crickets were analyzed. An asterisk indicates significant differences (P < 0.05) between types of
diet.
Figure 3. Differences in the surfaces of the (a) left and the (b) right mandibles in Xya pfaendleri and X. variegata. The box plots show
the median (50th percentile) and 25th and 75th quartiles; error bars show the 10th and 90th percentiles.
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Figure 4. The proportion of detritus in the diet of females (F) and males (M) in (a) Xya pfaendleri and (b) X. variegata as determined
based on the postmortem gut content analyses. The box plots show the median (50th percentile) and 25th and 75th quartiles; error bars
show the 10th and 90th percentiles. Open circles indicate outliers.
pfaendleri and X. variegata, respectively. The proportion of
detritus in the diet differed in males and females, although
the proportions of cellular matter and other particles
were similar (Table 3; Figure 4). The diet niche breadth
according to the Gini–Simpson index was wider in females
than in males and was similar between species (Table 3;
Figure 5). The maximum average index was 0.1, where
values less than 0.5 indicate a low variability of diet and
consumption of only a single food resource.
Figure 5. The food niche breadths in males (M) and females (F) of (a) Xya pfaendleri and (b) X. variegata (Orthoptera: Tridactylidae).
The food niche breadth was cumulatively evaluated based on the Gini-Simpson index. The box plots show the median (50th percentile)
and the 25th and 75th quartiles; error bars show the 10th and 90th percentiles.
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KUŘAVOVÁ and KOČÁREK / Turk J Zool
4. Discussion
We evaluated the dietary preferences and morphological
structure of the mandibles of two pygmy mole crickets from
the genus Xya (Orthoptera: Tridactylidae). The dominant
component of the diet was detritus, which we defined as
noncellular decomposing organic matter. Differences in
the morphological structures of the mandibles of the two
species were minimal to nonexistent. The mandibles are
asymmetrical in both species; the left and right mandibles
differ in size and partly in shape. Uniformity in the diet
is associated with the functional morphology of the
mandibles as well as similar patterns of the biology and
habitat preferences of the studied pygmy mole crickets.
Asymmetry of body structures is a rare characteristic
of animals that have evolved independently many times
(Palmer, 2009). In Orthoptera, the fixed asymmetry is
associated with mouthparts, side of forewing, and shape
of genitalia (Huber et al., 2007; Palmer, 2009). Pygmy
mole crickets have asymmetrical mandibles, with the
left mandible being larger than the right, which is based
on the use of the mandibles during food intake. Other
studied Caelifera (grasshoppers and groundhoppers) also
show asymmetry in mandible size, with the left mandible
always overlapping the right (Snodgrass, 1935; Isely, 1944;
Uvarov, 1977; Bernays et al., 1991; Chapman, 1995; Köhler
et al., 2000 for grasshoppers, and Kuřavová et al., 2014 for
groundhoppers). The mandibular asymmetry is important
for the correct mutual contact of mandibles during
biting to assure that the cusps intermesh with each other
(O’Brien, 1984).
Adaptation to food intake has led to behavioral and
morphological specialization of mouthparts (Snodgrass,
1935). Grasshoppers (Caelifera: Acridomorpha) are often
phytophagous (Fry et al., 1978), and the adaptations of
their mandibular structures are associated with different
types of food and different species of ecological groups of
plants, grasses, forbs, flowers, and seeds (Bernays, 1998;
Isely, 1944; Patterson, 1984). Blackith (1987) suggested
that many species of the basal orthopteran families, such as
the Tridactylidae and Tetrigidae, are mainly algae feeders
(lower-plant feeders), and the mouthparts are adapted to
this food source. We found that the mandible structures of
the pygmy mole crickets are reduced in comparison with
related groundhoppers of the Tetrigidae (Kuřavová et al.,
2014). The incisor area of the groundhoppers consists of four
incisors, and the molar area bears the central teeth and a
molar ridge with parallel molar slats (Kuřavová et al., 2014).
Pygmy mole crickets have only three incisors and a molar
ridge with only a few molar slats. The basic features (shape of
the incisor teeth and teeth on the molar ridge) are similar to
those of Tetrigidae due to their evolutionary, morphological,
and functional association. The incisors of both the left and
right mandibles overlap and work as a pair of scissors, and
726
the molar area acts as a grinder to fragment food into small
pieces. The mandibular areas of groundhoppers have similar
functions (Kuřavová et al., 2014).
The measured parameters (the distance between the
mandible junction and the peak of the incisor, the surface of
the molar ridge, and the number of slats) were significantly
different between sexes, between sides of the mandibles,
and between studied species. Females of Caelifera typically
have a larger body size than males (Zechner et al., 1999;
Kočárek et al., 2005; Bidau et al., 2013); therefore, the
lengths of the mandibles and the molar ridges’ surface are
considerably larger in females than in males. The number
of molar slats does not differ between sexes (on average 12.8
± 0.3), and these structures are uniform among individuals
of both species and between species. Some tropical species
of groundhoppers (Discotettiginae, Cladonotinae, and
Scelimeninae) show different numbers of slats on the molar
ridge (from 9 to 19 slats), although differences between
sexes have not been reported (Kuřavová et al., unpublished
data). In summary, the measured parameters were not
significant between the studied pygmy mole crickets; X.
pfaendleri and X. variegata have almost the same features
regarding the morphological structures of the mandibles,
likely due to similar dietary and behavioral patterns.
The aspects of food strategies, such as food
selection, food preferences, feeding patterns, and diet
specialization, have been studied by many authors in a
variety of grasshoppers (e.g., Williams, 1954; Mulkern,
1967; Otte and Joern, 1976; Bernays and Chapman,
1977; Uvarov, 1977; Joern, 1979, 1983, 1985; Bernays
and Bright, 1993). The food biology of groundhoppers,
as detrito-/bryophagous insects, has been studied only
in a few species (for references, see Kuřavová et al., 2014;
Kuřavová and Kočárek, 2015). The dietary preferences
of pygmy mole crickets (Tridactylidae) have not been
studied in detail. Only Deyrup and Eisner (1996) recorded
filamentous algae in the gut of the pygmy mole cricket
Neotridactylus archboldi Deyrup & Eisner, 1996. Blackith
(1987) also suggested that pygmy mole crickets are algal
feeders. Our analysis of the alimentary tract contents
of pygmy mole crickets showed that the species of the
genus Xya are mainly detritophagous, and cellular matter,
such as algae, is consumed only incidentally together
with detritus. Detritus is present in large amounts in the
pygmy mole cricket habitat and is easily accessible inside
their underground corridors. We confirmed that the
pygmy mole crickets do not search for specific matter
(e.g., lichens, mosses, or plants) as diet but utilize the
diet present in their burrows and their surroundings.
The determined dietary uniformity could reflect their
habitat preferences, especially their subterranean lifestyle.
Pygmy mole crickets are forced to stay in subterranean
corridors to protect themselves from predators, such as
KUŘAVOVÁ and KOČÁREK / Turk J Zool
hymenopteran Tachytes spp. (Hymenoptera, Sphecidae,
and Larrinae) that dig beneath the surface of the ground
to capture pygmy mole crickets (Kurczewski, 1966).
The studied species of pygmy mole crickets had similar
diet breadths. The species showed food niche breadth
values of up to 0.4, which corresponds with a narrow
food spectrum. For comparison, the groundhopper Tetrix
tenuicornis (Tetrigidae) showed a maximum food niche
breadth value of 0.6 (Kuřavová and Kočárek, 2015). Males
of X. pfaendleri and X. variegata exhibited a significantly
higher uniformity of diet than females, likely because
females require a more nutritionally balanced diet for egg
production. X. pfaendleri and X. variegata have almost the
same dietary preferences, and the food biology of both is
related to their habitat preferences. The studied species
have similar demands on habitat quality and occupy the
same areas, sometimes in a syntopic manner (Holuša and
Kočárek, 1999; Gavlas et al., 2007).
Patterson (1983) compared the structure of grasshopper
mandibles with their niche variation. The niche variation
hypothesis predicts greater overall mandibular variability
in forms having the broadest trophic niches. Comparisons
have shown that groundhoppers of the family Tetrigidae
(Kuřavová et al., 2014; Kuřavová and Kočárek, 2015) and
pygmy mole crickets of the family Tridactylidae have
similar patterns of mandibular variation and dietary
preferences. The Tetrigidae have more complex mandible
structures and more heterogeneous food intake than the
Tridactylidae. This association of the morphology and
dietary preference in each group supports Patterson’s
niche variation hypothesis.
Acknowledgments
This study was supported by Institutional Research Support
grants from the University of Ostrava (SGS24/PřF/2014,
SGS22/PřF/2015), Project CZ.1.05/2.1.00/03.0100 (IET),
which was financed by the Structural Funds of the
European Union, and by Project LO1208 of the National
Feasibility Program I of the Czech Republic.
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