DOI: 10.1111/j.1570-7458.2008.00772.x
Blackwell Publishing Ltd

Variation in the geometry of foreleg claws in
sympatric giant water bug species: an adaptive
trait for catching prey?
Shin-ya Ohba1*, Haruki Tatsuta2† & Fusao Nakasuji1‡
1

Laboratory of Insect Ecology, Graduate School of Environmental Science, Okayama University, Tsushima, Okayama 7008530, Japan, and 2Laboratory of Ecological Risk Assessment, National Institute for Environmental Studies, Tsukuba, Ibaraki
305-8506, Japan
Accepted: 5 August 2008

Key words: Allometry, claw development, predatory behaviour, Kirkaldyia (=Lethocerus) deyrolli,
Appasus japonicus, Belostomatidae, Heteroptera

Abstract

When giant water bugs (Heteroptera: Belostomatidae) encounter prey animals that are larger thanthey are themselves, they first hook the claw of their raptorial legs onto the animal, and then use all
their legs to pin it. The claws of the raptorial legs in giant water bugs play an important role in catchinglarger prey, but the relationship between the claws, body lengths of predators, and prey size has notbeen fully investigated. To elucidate the functioning of claws in catching prey, we investigated prey
body size relative to predator size in nymphs of two sympatric belostomatid giant water bug species,
the vertebrate eater Kirkaldyia (=Lethocerus) deyrolli Vuillefroy and the invertebrate eater
Appasus japonicus Vuillefroy, captured in rice fields. The younger nymphs of K. deyrolli caught preysthat were larger than themselves, whereas those of A. japonicus caught preys that were smaller.
Younger nymphs of K. deyrolli had claws that were curved more sharply than those of A. japonicus.
The more curved claws of younger nymphs of K. deyrolli probably hook more easily onto larger
vertebrates and thus this shape represents an adaptation for acquiring such prey.

Introduction
Differences in size and shape of the feeding apparatus
among sympatric species are considered to have evolved
under the context of prey selection (Houston & Shine,1993). Competition for food resources induced by niche
overlap may promote divergence in function of traits thatare used for gathering food, eventually leading to ecologicalcharacter displacement (McPhail, 1992; Schluter, 1994).
For instance, in the larvae of water-scavenger beetles, themandibles of insectivorous species are symmetric, but
those of snail-eating species are asymmetric (Inoda et al.,2003). Thus, knowledge of the feeding apparatus aids in*Correspondence and present address: Shin-ya Ohba, Department of
Vector Ecology and Environment, Institute of Tropical Medicine,
Nagasaki University, Sakamoto 1-12-4, Nagasaki 852-8523, Japan.
E-mail: oobah8ag@yahoo.co.jp†
Present address: Laboratory of Subtropical Zoology, Graduate
School of Agriculture, University of the Ryukyus, Nishihara,
Okinawa, 903-0213, Japan.
‡
Present address: Sanyo 2-14-4, Akaiwa, Okayama 709-0827, Japan.

understanding how these body parts evolve under resourcecompetition.
Here, we focus on the relationship between the shape ofthe feeding apparatus and feeding behaviour in two
sympatric Belostomatidae (Heteroptera) species, Kirkaldyia(= Lethocerus) deyrolli Vuillefroy (Lethocerinae) and Appasus
japonicus Vuillefroy (Belostomatinae). The diets of these
two species differ in Japanese rice fields (Ohba & Nakasuji,2006). Nymphs of A. japonicus eat invertebrates such assnails and aquatic insects, whereas nymphs of K. deyrolli
mainly feed on vertebrates such as tadpoles and fish (Ohba& Nakasuji, 2006; Ohba et al., 2008). Unlike K. deyrolli,
A. japonicus does not digest vertebrates well, because, in
general, the saliva of Belostomatinae does not have enzymes
capable of digesting protein-rich meals (Swart et al., 2006).
Belostomatid bugs use all their legs to catch animals larger
than they are, possibly to ensure success (Smith, 1997; Ohba
et al., 2006, Figure 1B). When bugs encounter a prey that islarger than they are, they first hook the claws of their raptorial
legs onto the animal, and then use all their legs to pin it(Figure 1B). Consequently, species that feed on prey larger than© 2008 The Authors Entomologia Experimentalis et Applicata 129: 223–227, 2008
Journal compilation © 2008 The Netherlands Entomological Society

223

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Ohba et al.

Figure 1 Scheme of predatory behaviour in
Belostomatidae. Photos indicate the first
instar nymph of Kirkaldyia deyrolli
capturing a tadpole that is (A) smaller or(B) larger than itself. Bar = 1.0 mm.

themselves may need more sharply curved claws that hookeasily onto the prey. Thus, it is expected that the geometry ofclaws will differ by food habits in the two sympatric species.
The primary goal of the present study was to reveal
whether the shape of curved claws on raptorial legs was
associated with prey body size relative to predator size inthe two belostomatid species. For that purpose, we coupledfield observation with a laboratory experiment. We thendiscuss interspecific differences in the functioning of thefeeding apparatus in the context of prey size.

Materials and methods
Study sites and field observations

To investigate prey body size relative to predator size, we
conducted a field census at 1- to 3-days intervals in rice
fields at Uekidani (35°1′N, 134°19′E) in Hyogo from Juneto mid-August in 2004 and 2005, and at Chuoh (34°58′N,
133°58′E) in Okayama, Honshu, Japan, from June to
mid-August in 2004 and 2005. Census plots were set up in
ditches along the ridge of the rice fields. The plot areas were
118 m2 in Uekidani and 100 m2 in Chuoh.
We searched the ridges around the rice fields for belostomatids, using a flashlight every 2 h between 20:00 and
01:00 hours. Belostomatids are primarily nocturnal, andthey ambush prey at the water surface; flashlights do notinterfere with their foraging behaviour (S Ohba, unpubl.).
We identified dietary items to be those captured individuals
into which the bug inserted its proboscis. After recording
the species of predator and prey, we captured them. We
measured the length from top (head or proboscis) to tip ofabdomen as total body length. The total body lengths ofboth predator (K. deyrolli or A. japonicus) and prey, orshell lengths in the case of snails, were measured as indicesof body size with calipers. Wilcoxon’s signed-rank test wasused to compare body lengths of predators and their preyanimals at each nymphal stage.
Younger nymphs (first to third instar) of K. deyrollicaught preys, mainly tadpoles, that were larger thanthemselves (see Results). To clarify the approximate size of
captured prey by K. deyrolli compared to the size of available(i.e., captured and non-captured) prey individuals, wemeasured available and captured tadpole size in each instar
nymph. A D-frame dipnet (28 cm in width, 3 mm mesh)
was used for sweeping. The dipnet was slowly pulled along
the paddy bottom so as to collect tadpoles. Tadpole total
length was measured and classified into six categories: <5,
5–10, 11–15, 16–20, 21–25, and >25 mm. After measurement,
the tadpole was released into the paddy. This work wasdone weekly during the survey periods. The median of
tadpole total length in present and captured nymphs ineach instar was compared using a Mann–Whitney U-test.
Rearing experiments

To compare the growth patterns of curvature of the claw
on the forelegs between the two species, rearing experiments were conducted. Two male–female pairs collectedfrom rice fields in May 2004 were released in an aquarium(60 × 45 cm for K. deyrolli and 30 × 20 cm for A. japonicus,
both 10 cm deep) and females were allowed to lay eggs.
Because hatched nymphs of K. deyrolli are often cannibalistic, individual hatchlings were isolated in a cage
(10 × 10 cm, 20 cm high, 3 mm mesh size) and placed in
an aquarium (10 cm water). Nymphs were reared at
25.0 °C with a photoperiod of L16:D8 until moulting toadults (n = 43). The nymphs were provided with various
animals including tadpoles [Hyla japonica Günther;
Gosner stage 35–42 (Gosner, 1960)] and Odonata nymphs(Platycnemididae: Copera annulata Selys; Lestidae: Lestes
spp., 10–20 mm in total length; Libellulidae: Orthetrum

Claw variation in nymphal stages in Belostomatidae 225

albistylum speciosum Uhler, Sympetrum frequens Selys,
and Sympetrum infuscatum Selys; and Aeshnidae: Anax
parthenope julius Brauer, Anax nigrofasciatus nigrofasciatus
Oguma, and Gynacantha japonica Bartenef, 20–40 mm).
Prey density was kept constant in each cage (six individuals
per container for the first–second instars and 10 individuals
per container for the third–fifth instars). Because claw
developmental patterns were almost identical irrespectiveof food items (Ohba et al., 2006), the data from three preytreatments (tadpoles, Odonata nymphs, and tadpole–
Odonata nymph mixture) were pooled. The dead bodies ofprey were removed daily and replaced with live prey.
Hatched nymphs of A. japonicus were reared individuallyin Petri dishes (9 cm diameter, 1.5 cm high) at 25.0 °C witha photoperiod of L16:D8; a piece of floating net cloth wasused as a perching site (n = 47). The nymphs were
provided with various prey, namely, aquatic isopods(Asellus spec., 2–10 mm in total length) and aquatic snails(Lymnaeidae and Physidae, 2–10 mm in shell length). The
prey density in each cage was kept constant [five aquatic
isopods (Asellus spec.) and five aquatic snails per container
for the first–second instars, along with 10 aquatic isopods
and 10 aquatic snails per container for the third–fifth
instars]. The dead bodies of prey were removed daily andreplaced with live prey.
Measurements of morphological characteristics

Exuviae of each instar of K. deyrolli and A. japonicus werecollected from the rearing cage or Petri dish and preservedin 70% ethanol. Moulting and the preservation process do
not change the claw’s shape (S Ohba, unpubl.). The
curvature of the claw on the right foreleg from the dorsalside of the exuviae was measured as follows. First, a line
was drawn from the claw tip (A) to the base of the claw (B)to give the claw-arc (Type 1 landmark, see Bookstein, 1991;
Figure 2). Second, the farthest part from claw-arc on the
inner claw was pointed as C (Type 2 landmark, Bookstein,1991). Two lines were drawn from C to both A and B, andthe angle of ACB was measured as the curvature of theclaw. Claws of the same individual were measured from thefirst to the fifth instar.
To evaluate differences in angles of the curvature ofclaws, we performed repeated-measures one-way analysis
of variance (ANOVA), with species as the betweensubject factor and nymphal stages (first–fifth instar) as
the within-subject factor. Mauchly’s test for sphericity
indicated a significant violation of the assumption in all
characteristics (P<0.05). For this reason, significance levels
for within-subject effects of the claw angle of curvature
were determined using the Greenhouse–Geisser correction(G-G ε). Scheffé’s tests were carried out to assess differencesamong nymphal stages when significant effects were detected.

Figure 2 Measurement of the claw angle of a Kirkaldyia deyrolli
nymph. A, claw tip; B, base of the claw; C, the furthest part fromthe claw-arc, AB, on the inner claw.

All statistical tests were performed using SPSS for Windows
version 11.0 (SPSS, 2001).

Results
Comparison of predator and prey size

The bodies of A. japonicus were significantly larger thanthose of their prey, except when the predator was the first
instar (Figure 3). In contrast, the body lengths of the first,
second, and third instars of K. deyrolli were significantly
smaller than those of the prey. Fourth instars of K. deyrolli
were the same size as their prey and fifth instars were larger
than the prey (Figure 3).
Frequency distribution of medians of each body size
range of available and captured tadpoles is shown in
Figure 4. Mean total body length of tadpoles captured
by K. deyrolli was significantly larger than that of tadpole
present in all nymphal stages (Mann–Whitney U-test:
P<0.001; Figure 4).
Variation in claw angle

The repeated-measures one-way ANOVA revealed that
effects for species, nymphal stage, and species*nymphal

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Ohba et al.

Figure 3 Body lengths of Kirkaldyia deyrolliand Appasus japonicus predators (first–fifth
instar) and their captured prey in a rice
paddy field. Box plots are based on mediansand quartiles. Numbers in parentheses
show sample size. Asterisks denotesignificantly different sizes of the predator
and its prey (Wilcoxon’s signed-rank test:
P<0.05).

stage interaction were all significant, indicating different
developmental patterns for the angles of claws [species:
F1,8 = 787.7, P<0.001; nymphal stage: F3.5,305.3 = 104.7,
P<0.001 (after Greenhouse–Geisser correction);
species*nymphal stage: F3.5,305.3 = 203.2, P<0.001 (afterGreenhouse–Geisser correction), G–G ε = 0.867 for
log-transformed data]. One-way ANOVA for eachbelostomatid species revealed that the claw angles differedsignificantly among nymphal stages (K. deyrolli: F4,210 =
286.8, P<0.001; A. japonicus: F4,230 = 6.1, P<0.001). In
K. deyrolli, claws became less curved as nymphs grew andnymphs tended to catch smaller prey than their body size.
What is more, the claw angle of the first-instar nymph wasconsiderably smaller than that of other instars (Figure 5).
In contrast, the claw angle of the first instar of A. japonicus

Figure 4 Comparison of medians of total size of available andcaptured tadpoles in each of five instars of Kirkaldyia deyrolli.
Numbers in parentheses show sample size. The difference
between available and captured tadpoles was significant in all
instars (Mann–Whitney U-test: P<0.001).

was significantly larger than that of the fifth instar, withthose of the second, third, and fourth instars being
intermediate (P<0.05).

Discussion
It is frequently seen that predators catch preys that aresmaller than they are themselves (Pearson & Mury, 1979;
Brodie & Formanowicz, 1983; Ganeshaiah & Belavadi,
1986; Nentwig & Wissel, 1986; Warren & Lawton, 1987).
Prey size increases as predator nymphs grow (Cloarec,
1992; Perez-Goodwyn, 2001). The present study revealed
that these general tendencies applied to the case of thenymphs of the invertebrate eater A. japonicus. Nymphs ofthe vertebrate eater K. deyrolli, however, did not conformto this pattern (Figure 3). In particular, observations in thefield demonstrated that the first, second, and third instarsof K. deyrolli caught preys, mainly tadpoles, that werelarger than they were themselves.
The eggs of anurans, such as H. japonica, Rhacophorus
schlegelii (Günther), and Rana nigromaculata (Hallowell),
species that are principal dietary items for K. deyrolli hatch
in rice fields in mid-May. Consequently, tadpoles growlarge enough when the hatchlings of K. deyrolli occur from
June to July (Figure 4). Under these feeding circumstances,
curvier claws of K. deyrolli might be expedient to hooklarger prey. Younger nymphs of K. deyrolli possessing
sharply curved claws hook more easily onto tadpoles andother preys that are larger than the predator itself and cling
to prey with all legs (Figure 1B; Smith, 1997, Ohba et al.,2006). The same feeding behaviour was observed in anotherexperiment in the laboratory (S Ohba et al., unpubl.). On
the other hand, the invertebrate eater A. japonicus mainly
use the forelegs to capture prey smaller than their own
body size (Figures 1A and 3) and they hardly employ the
behaviour of claw hooking and clinging using all legs asClaw variation in nymphal stages in Belostomatidae 227

Figure 5 Changes in the claw angle in
Kirkaldyia deyrolli and Appasus japonicus infive nymphal instars. Data indicate
means ± SD. Different letters indicate
significant differences between nymphal
stages in a species (Scheffé’s test: P<0.05).

shown in Figure 1B. Thus, geometric differences in foreleg
claws between the two species may be relevant to the mannerof catching prey.
Conspicuous differences in the geometry of curved
claws between the two species provide circumstantial
evidence that the diets of these two species differ inJapanese rice fields. According to Smith (1997), members of
the genus Lethocerus, including Kirkaldyia, are vertebrate
specialists. He argued that selective regimes to facilitate thehandling of progressively larger aquatic vertebrate prey
tend to expedite the evolution of size and shape of the claws(Smith, 1997). The present study strongly suggests thatK. deyrolli nymphs preferred larger tadpoles among
available ones (Figure 4). Our findings of catching larger preyin the nymphs of K. deyrolli are in line with Smith’s (1997)
evolutionary scenario. To reveal the evolutionary process
in Belastomatidae, phylogenetic information as well as further
comparative data of feeding behaviour are necessary.

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