SYSTEMATICS

Variation in the Shape of Genital Appendages Along a Transect
Through Sympatric and Allopatric Areas of Two Brachypterous
Grasshoppers, Parapodisma setouchiensis and Parapodisma subastris(Orthoptera: Podisminae)
YASUSHI KAWAKAMI1 AND HARUKI TATSUTA2

Ann. Entomol. Soc. Am. 103(3): 327Ð331 (2010); DOI: 10.1603/AN09074

ABSTRACT Divergence in genital traits between closely related species is occasionally greater insympatry than in allopatry, possibly because directional selection facilitates differentiation so as toprevent unÞt hybridization. Here, we report for the Þrst time that the shape of a functional genital
structure, cercus, differs markedly between sympatrically and allopatrically occurring individuals in
two brachypterous grasshopper species, Parapodisma setouchiensis Inoue 1979 and Parapodisma
subastris Huang 1983 (Orthoptera: Podisminae). Although in areas of allopatry the cerci were straightin both species, in areas of sympatry the cerci of P. setouchiensis had an almost orthogonal bend.
Furthermore, the angles of curvature of the cerci of P. setouchiensis varied continuously along atransect through sympatric and allopatric areas. We also observed copulation behavior in pairs andfound that the apical part of the cerci was inserted into the space between the posterior edges of theseventh abdominal sternite and the seventh abdominal tergite of the female. We suggest that theorthogonally bent cerci allows the male to obtain a Þrm grip on the femaleÕs abdomen and proposethat the conspicuous variation in the cerci of P. setouchiensis contributes to the strength ofreinforcement of a premating isolation system.
KEY WORDS Parapodisma, geographic variation, cercus, character displacement

When an incipient allopatric species expands its range
before the establishment of prezygotic isolation mechanisms, selection acting against maladaptive hybrids oragainst costly interspeciÞc matings may facilitate indirect selection pressures that favor trait divergenceor assortative mating (Lukhtanov et al. 2005). In particular, phenotypic divergence tends to be greater for
traits that are useful for species recognition or reproduction, such as the features of the genitalia. For
instance, in closely related beetle species, the range ofoverall body size showed considerable overlap in thesympatric area, whereas there was no overlap of genital size in any sympatric location (Kawano 2002,
2003). This phenomenon may be a consequence ofreproductive character displacement resulting fromdirectional selection so as to prevent unnecessary cost
before reproduction.
The genus Parapodisma (Orthoptera: Acrididae:
Podisminae) is endemic to Japan and East Asia andconsists of 13 species. Among all the species of Parapodisma, Parapodisma setouchiensis Inoue 1979 is themost widely distributed (from Aomori Prefecture in
northernmost Honshu to Yakushima Island in south1 Corresponding author: Tottori Prefectural Museum, Higashimachi 2-124, Tottori 680-0011, Japan (e-mail: kawakamiy@pref.
tottori.jp).
2 Laboratory of Subtropical Zoology, Graduate School of Agriculture, University of the Ryukyus, Okinawa 903-0213, Japan.

ern Japan; Fig. 1). Two species of the genus, P. setouchiensis and Parapodisma subastris Huang 1983, aredistributed sympatrically and allopatrically in the
Kinki district in western Japan (Fig. 1). These species
differ little in external morphology and ecologicaltraits, such as habitat, bionomics, and feeding habits,
but they have distinctively different internal genitalia(Kanô 1983, 1985; Kawakami 2002). So far, there havebeen no reports of interspeciÞc hybridization between these two species in the areas of sympatricoccurrence (Kanô 1985, Tominaga et al. 1996,
Kawakami 1999), suggesting the presence of reproductive isolation mechanisms in these species.
P. setouchiensis is polytypic with respect to malecharacters and is classiÞed into four geographic forms
based on the shapes of genital parts, namely, Basic,
Yamato, Tanba, and Mt. Hyonosen forms. This leads tosome controversy with respect to the taxonomic statusof these forms (Kawakami 1999, Kawakami andHoshikawa 2006, Tominaga and Ito 2006). One ofthe diagnostic characters that is used to distinguish thefour forms is the shape of the cerci in males. The
cercus bends orthogonally in the Tanba form, whereas
it is nearly straight in other forms as well as in females(Kawakami 1999). The bent cercus is presumed to bean apomorphic trait (Kawakami 1999) and intergrades
between the Tanba and Mt. Hyonosen forms found at
the eastern boundary of the latterÕs distribution range.

0013-8746/10/0327Ð0331$04.00/0 䉷 2010 Entomological Society of America

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Fig. 1. Map showing the distributions of P. setouchiensis and P. subastris in Japan (top left) and the locations of our samplingsites. Representative shapes of left cerci of males (lateral view) in each population of the two species are also shown. The
region indicated by the solid line is the range where P. setouchiensis and P. subastris occur sympatrically. Angles of the bent
cerci of individuals collected from sites 22Ð25 (solid squares) were measured on the basis of the Þgures reported by Kanô(1983, 1986) and Tominaga and Kanô (1989).

In contrast to P. setouchiensis, P. subastris males do notshow such a polymorphism, and the cerci are almost
straight (Kanô 1985; Tatsuta et al. 2001).
To explore the taxonomic status of each form in P.
setouchiensis, we investigated the pattern of variationin the shape of the male cerci of P. setouchiensis and P.
subastris collected from transect areas of sympatricand allopatric occurrence. We also observed featuresof genital coupling in a pair of P. setouchiensis andconsidered the possible function of the male cerci.

Materials and Methods
Sampling Sites. During 1995Ð2002, 114 males in total, of which 91 belonged to P. setouchiensis and 23 toP. subastris, were collected from 21 sites in western
Honshu; these sites included areas of both allopatricand sympatric occurrence (Fig. 1). The samples werepooled because annual ßuctuation does not signiÞcantly affect the shape of male cerci (Kawakami 1999).
Most of the samples were preserved in 80% ethanol,

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Table 1.

KAWAKAMI AND TATSUTA: VARIATION IN SHAPE OF CERCI IN Parapodisma SPECIES

329

Sampling locations and arithmetic means and CVs of the angles of curvature of the cerci of P. subastris and P. setouchiensis

No.

Locality

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25

Mt. Sanbe, Shimane
Oki Islands, Shimane
Saihaku-cho, Tottori
Tohaku-cho, Tottori
Kamogawa-cho, Okayama
Misasa-cho, Tottori
Ningyô Pass, Okayama
Mt. Nagi, Okayama
Shibiki Pass, Hyogo
Mt. Hyonosen, Tottori
Kan-nabe Valley, Hyogo
Yabu-cho, Hyogo
Izushi-cho, Hyogo
Tantô-cho, Hyogo
Aogaki-cho, Hyogo
Daimon, Fukuchiyama-shi, Kyoto
Nagao, Fukuchiyama-shi, Kyoto
Ôe-cho, Kyoto
Futasegawa Stream, Kyoto
Miwa-cho, Kyoto
Miyama-cho, Kyoto
Mt. Ponpon, Osaka
Aoyama-cho, Mie
Kuriyama-cho, Tochigi
Hieki-son, Fukushima

Altitude(m)

Datecollected

Allopatric or
sympatric

325

13-VIII-1996
9-VIII-2000
14-VIII-1995
29-VIII-1995
9-VIII-1995
24-VII-1995
10-VIII-1995
17-VIII-1995
23-VIII-1995
14-IX-1996
25-VIII-1995
15-VII-2001
7-VIII-1995
16-VII-2001
19-VII-1995
15-VII-2001
13-VIII-2002
31-VII-1995
13-VIII-2002
20-VII-1995
16-VII-2001
4-VIII-1977
23-VII-1981

AllopatricAllopatricAllopatricAllopatricAllopatricAllopatricAllopatricAllopatricAllopatricAllopatricSympatric
Sympatric
Sympatric
Sympatric
Sympatric
Sympatric
Sympatric
Sympatric
Sympatric
Sympatric
Sympatric
Sympatric
AllopatricAllopatricAllopatric

320
490
240
180
600
500
470
750
480
90
140
230
130
80
90
50
170
160
160

14-VIII-1980

P. setouchiensis
males
N

Mean

1
2
5
7
2
6
5
7
5
5
4
6
5

137.0
139.0
144.2
139.4
143.0
138.8
135.2
131.4
129.0
121.2
106.0
104.7
94.2

CV

4.1
5.2
5.5
5.5
4.0
7.7
6.4
5.2

8
4

94.3
81.3

6.5
7.8

7
7
5

80.3
90.6
79.0

5.3
8.4
3.6

1
1
1
1

78.0
138.0
147.0
140.0

P. subastris males
N

Mean

CV

1

136.0

4

140.3

3.8

6
4

133.7
134.0

4.0
2.7

6

134.3

4.0

2

135.0

3.2
4.0

N, number of individuals. CV, coefÞcient of variation in populations where more than three males were collected. Data for populations 22,
23, 24, and 25 were obtained from Þgures reported by Tominaga and Kanô (1989) (p. 7, Þg. 18), Kanô (1983) (p. 40, Þg. 3), Kanô (1986) (p.
19, Þg. 25), and Kanô (1983) (p. 40, Þg. 26), respectively.

and some were preserved as dried and pinned specimens. Details of sample collection are summarized inTable 1.
Measurements of Genital Appendages. The specimens were dissected, and the left cercus of each male
was drawn in lateral view using a camera lucida and abinocular microscope; the angle between the distal
protuberance and the basal part was measured using
a protractor (see Fig. 2 for details). Additional data
were obtained from the Þgures provided by Kanô(1983, 1986) and Tominaga and Kanô (1989). The
angles of curvature in the cerci of sympatrically oc-

Fig. 2. Measurement of the angle of the bent cerci of males of to the two Parapodisma species. The degree of curvatureof the cerci was represented by the angle between the inner margin of the protuberance and that of the basal part.

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Fig. 3. The angles of bent cerci of P. setouchiensis (closed circles) and P. subastris (open squares) males in areas where
they occur allopatrically and sympatrically. The locality numbers, corresponding to those shown in Table 1 and Fig. 1, areapproximately aligned from west (left) to east (right). Each population is represented by the mean (circle and square forP. setouchiensis and P. subastris, respectively) ⫾ 1 SD (bar).

curring individuals were compared with those of allopatrically occurring individuals (Fig. 1) by using the
WatsonÐWilliams procedure, which tests whether themean angles of cerci are equal for each of the two
species (Zar 1999).
Observation of Genital Coupling. We observed the
copulation behavior of P. setouchiensis in the laboratory. Nymphs were collected from the following areas:
Daimon, Fukuchiyama-shi, Kyoto prefecture, 80-m altitude, on 29 June 1997 and 6 and 18 July 1997 (site 16
in Table 1 and Fig. 1) and Ohmi, Miwa-cho, Kyoto
prefecture, 160-m altitude, 29 June 1997 (site 20).
Nymphs were reared separately in plastic cages (100by 150 by 150 mm) to prevent mating. Individuals
were provided daily with leaves of herbaceous andshrubby plants such as Petasites japonicus (Siebold &Zucc.) Maxim., Hydrangea macrophylla L., and Artemisia princeps Pampan. We used 5- to 7-d-old adultgrasshoppers for observations of mating.
Results and Discussion
The angles of curvature of the cerci of P. setouchiensis and P. subastris males are listed in Table 1 andFig. 3, and representative shapes of the cerci of the two
species are illustrated in Fig. 1. The angles of the bent

Fig. 4. Features of genital coupling in P. setouchiensis (view from the right side). Male cerci holding the edge of a femaleÕs
seventh abdominal sternite. The apical part of the cercus cannot be seen because it was inserted into the space inside theseventh abdominal sternite. Refer to the text for further clariÞcation. (Online Þgure in color.)

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KAWAKAMI AND TATSUTA: VARIATION IN SHAPE OF CERCI IN Parapodisma SPECIES

cerci ranged between 134.0 and 140.3⬚ in P. subastris
males, whereas those of in P. setouchiensis males
showed greater variation (78.0 Ð147.0⬚). The range ofcoefÞcients of variation of the angles was wider in P.
setouchiensis (range, 3.2Ð 8.4%) than in P. subastris(range, 2.7Ð 4.0%; Table 1). A conspicuous and highly
signiÞcant difference was observed in the angles ofcurvature of the cerci between P. setouchiensis males
obtained from areas of sympatric occurrence andthose obtained from areas of allopatric occurrence(F1,93 ⫽ 2398.97; P ⬍ 0.0001). The angles of curvaturein the cerci of P. setouchiensis varied continuously
rather than discretely among site 1Ð22, suggesting that
there is no reason to partition individuals in corresponding areas into different species.
Figure 4 shows an illustration of genital coupling.
The male cerci were observed to be Þxed and immobilized at the femaleÕs seventh abdominal sternite,holding the edge of the femaleÕs abdomen from boththe sides during copulation. The apical part of each
cercus was inserted into the space between the posterior edges of the seventh abdominal sternite and theseventh abdominal tergite of the female. In P. setouchiensis, the orthogonally bent cerci had a Þrm grip on
the femaleÕs abdomen and the bent tip could be inserted inside the seventh abdominal sternite as a hook
(Fig. 4), suggesting that the highly bent form of thecerci helps in strengthening the attachment and providing a Þrm grip. In a related group of species in North
America, the genus Melanoplus, male cerci may help in
obtaining access to the genital chambers of females bypressing against a ßap of the ventral surface of thefemale and pinching a side of the oviduct to expose thegenital chamber (Sirot 2003). This physical amalgamation for better coupling of genital chambers might be
the primary function of male cerci.
To understand how variations in the shape of thecerci in P. setouchiensis have evolved, it should Þrst be
demonstrated whether cerci work against unÞt hybrids, because they are known to do for the copulatory
piece of carabid beetles (Sota and Kubota 1998). Provided the cerci function as a recognition system forpartners, cercus variation in P. setouchiensis is probably due to the reinforcement of a premating isolation
system as a result of directional selection against maladaptive hybridization during expansion of the species. If this were the case, it would be expected that
individuals of P. setouchiensis from a sympatric area
would recognize P. subastris, thus allowing hybridization to be prevented, whereas those from an allopatricarea would not discriminate between species. Thishypothesis could be tested by pairing individuals from
sympatric and allopatric areas.

331

Acknowledgments
We are grateful to K. Hoshikawa and N. Tsurusaki for
guidance throughout this study. We also thank K. Reinhardt,
Y. Kanô, O. Tominaga, A. Ichikawa, and G. Ito for suggestions
and comments.

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