Eur. J. Entomol. 103: 743–750, 2006 ISSN 1210-5759 Seasonal wing dimorphism and life cycle of the mole cricket Gryllotalpa orientalis (Orthoptera: Gryllotalpidae) CHIHIRO ENDO Department of Zoology, Graduate School of Science, Kyoto University, Sakyo, Kyoto 606-8502, Japan; e-mail [email protected] Key words. Gryllotalpidae, mole cricket, wing dimorphism, flightlessness, life cycle, voltinism, seasonal morph, Gryllotalpa orientalis Abstract. Control of seasonal wing dimorphism in the oriental mole cricket Gryllotalpa orientalis Brumeister (1839) from a wetland habitat in western Japan is described. The long-winged (LW) morph appeared from mid-June to September, whereas the short- winged (SW) morph appeared from September to mid-June. Individuals overwintered in either the adult or juvenile stage. The sea- sonal shift in wing morphology was linked to the overwintering stage. Individuals that hatched in May became SW adults in September–October and then overwintered, whereas those that hatched in June and July overwintered as juveniles and became LW adults in June of the following year. The life cycle of both morphs was univoltine. Reproductive benefits and constraints of each wing morph of G. orientalis are compared. INTRODUCTION cricket Scapteriscus abbreviatus Scudder is a flightless In many pterygote insects, wing polymorphism, i.e., the species, whereas the northern mole cricket Neocurtilla presence of macropterous (long-winged) individuals and hexadactyla Party shows geographic variation in flight- obligate flightless (wingless or with reduced wings) indi- less morph (Hayslip, 1943; Semlitsch, 1986; Capinera et viduals, has been classified as a type of dispersal poly- al., 2005); nonetheless, flightlessness in mole crickets has morphism (Harrison, 1980; Roff, 1986; Zera & Denno, received little attention. Light traps and sound traps that 1997). When the population density is low and competi- play back the conspecific male song (Walker, 1982) are tion for food and mates is not severe, individuals do not effective methods for collecting mole crickets; however, need to disperse and short-winged (SW) or wingless they only attract flying mole crickets. Thus, the basic life morphs would be favoured. In contrast, when the popula- cycle of mole crickets will only be elucidated by per- tion density is high and the competition is severe, the forming seasonal soil excavations. long-winged (LW) morph, which can disperse to a better Seasonal wing dimorphism in a local population of the habitat, would be favoured. oriental mole cricket Gryllotalpa orientalis (Orthoptera: Seasonal wing polymorphism has been found in organ- Gryllotalpidae) is presented. The seasonal emergence pat- isms with multivoltine life cycles, and the favoured tern and life cycle of each morph of this species is docu- morph differs between the first and second or later gen- mented and the reproductive benefits and constraints of erations (e.g., Harada, 1996; Van Dyck & Wiklund, 2002; each wing morph compared. Questions regarding whether Olvido et al., 2003). In seasonal polymorphism, the selec- the seasonal shift in wing morph is generally related to tion pressure varies seasonally, and the favoured morph is the voltinism and whether this pattern of alternating wing fixed for each season. When two or more morphs are pre- length with the life cycle occurs in other species of mole sent simultaneously, each morph simultaneously or alter- cricket are considered. nately confer advantage. Thus, it is important to deter- MATERIAL AND METHODS mine how wing polymorphism is maintained in the life Study species cycles to understand why it is maintained. Although wing polymorphism is found in many cricket The oriental mole cricket Gryllotalpa orientalis Brumeister species (Masaki & Walker, 1987), wing polymorphism in (Orthoptera: Gryllotalpidae) is widely distributed across Japan, China, the Korean Peninsula, and Taiwan. Populations in Japan mole crickets has not previously been reported. Mole were long regarded as G. africana, but are now assigned to G. crickets are adapted to life underground; thus, direct orientalis (Townsend, 1983). Juveniles moult eight times during observation is difficult and details of their ecology remain development (Sakurai et al., 1960). As juveniles do not develop unknown in most species. Males call from burrows at wing buds before the seventh stage (instar), the wing morph of night to attract mates and conspecific females fly to the early instars cannot be determined in the field. Seventh instars burrows of males (Ulagaraj, 1975; Forrest, 1983). have small wing buds, whereas eighth instars have large wing Females fly to search effectively for mates and to move to buds. Adults have complete wings and the sexes can be distin- new habitats for oviposition or feeding. Males can also guished by the presence of a stridulating organ and pattern of fly and are attracted to the songs of conspecific males, as venation in the forewings. well as females (Walker, 1982). The short-winged mole 743 Fig. 1. Long winged (LW) morph and short winged (SW) morph. a – LW male, b –LW female, c –SW male, d – SW female. A black arrow indicates abdomen and a white arrow indicates tips of hindwing. Study site was placed under the light to capture the attracted insects. The The study site was a wetland area in the Hyogo Prefectural light trap was set for about 10 h from after sunset to dawn. Homeland for the Oriental White Stork (35°35´N, 134°51´E), Sound traps were constructed according to Walker (1982). A Toyo-oka, western Japan. This wetland area was located in a speaker (Panasonic, RP-SP30K) was connected to a portable CD valley and was surrounded by woodlands consisting of tem- player (FISHER, Z-ACDP1[S]). The speaker was sealed in perate deciduous forest and broadleaf evergreen forest. Many waterproof wrapping and placed in a plastic bucket (45 cm in paddy fields were located outside the wetland. The area receives diameter, 20 cm deep) filled with water. The CD player was snow from December to February. placed in a plastic case outside the bucket. A series of male calls from the same population recorded in May 2002 at Toyo-oka Sampling methods was used as the sound source. The sound was played for five h Mole crickets were sampled regularly for approximately four beginning after sunset, which corresponds to the natural calling days each month from March to November in 2002 and 2003 time of males. The plastic buckets trap only flying insects and from March to October in 2004. On each sampling day, sev- because insects could not climb the outer walls. Trapped mole eral 50 × 50 cm quadrats were placed in the wetland and the soil crickets were collected the following morning. In total, the light within each quadrat excavated to a depth of approximately 20 trap was active for 18 nights in 2003 and 19 nights in 2004, and cm to collect all mole crickets. The mean area excavated each the sound traps were active for 19 nights in 2003 and 28 nights month was 3.32 (± 2.41 SD) m2. Areas were selected for exca- in 2004. vation based on the presence of signs of tunnelling or feeding by For mole crickets collected from soil excavations and traps, mole crickets. For areas that were too small to fit the standard the pronotum width and forewing and hindwing length were quadrat, the area was measured after excavation. Mole crickets measured to the nearest 0.01 mm using calipers. Pronotum oviposit a clutch of eggs in an egg chamber. When an egg width is an index of body size. According to Walker and Siv- chamber was found during soil excavation, the number of eggs inski (1986), the ratio of hindwing (HW) to forewing (FW) was counted. length is used to discriminate among wing morphs. Several To collect flying adults, a light trap and sound traps were females collected each month were dissected to determine deployed for three to four nights each month from June to whether the ovaries were mature and whether there was sperm October 2003 and from April to October 2004. One light trap in the spermatheca. The wet mass of sperm in the spermatheca was set beside the wetland and four to five sound traps were set was measured using an electric balance (Sartorius BP210D). at intervals of about 50 m along the Kamatani River. The light Presence of corpus luteum in the ovaries was regarded as evi- trap consisted of an ultraviolet fluorescent light (15 W, 300–400 dence of oviposition. The remaining crickets were released in nm wavelength) reflected on to white cloth (100 × 200 cm) at a the wetland after measurement. height of 100 cm. A funnel (100 cm in diameter) connected to a plastic bucket (25 cm in diameter, 40 cm deep) containing water 744 Fig. 2. Relationship between HW/FW ratio and pronotum width of males and females in 2003 (a, b) and 2004 (c, d) (black circle: SW, white circle: LW). Data analysis < 0.0001; Fig. 2). In 2002, the sample size was too small Adult wing ratio (HW/FW) was compared in each sex using to perform the analysis. analysis of covariance (ANCOVA) with wing morph as the Wing morph and life cycle factor and pronotum size as the covariate. Juvenile wing ratio was compared between individuals in spring and autumn using Seasonal changes in the size distribution of mole ANCOVA with season as the factor and pronotum size as the crickets in 2002 and 2003 are shown in Fig. 3; this same covariate for seventh and eighth instars. One-way ANOVA was trend was observed in 2004. Egg chambers with eggs used to compare the number of eggs found in egg chambers were discovered mainly from April to June and in small among months. A Mann-Whitney U-test was used to detect dif- numbers in July, but few egg chambers were found in ferences between wing morphs in the number of eggs per egg June 2003.