Spieth, Herman T.

Proceedings of the Hawaiian Entomological Society

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Vol. XIII, No. 1, April 1979 143Courtship Behavior of Samoaia attenuataand S. leonensis1Herman T. Spieth2The drosophiloid genus Samoaia Malloch, comprised of seven species, is restricted to the Samoa Islands. Wheeler and Kambysellis (1966) observe that "aconsiderable amount of local evolution has taken place" on the Samoa Islandssince 30 of the 48 drosophiloid species found on the islands appear to be endemic.Excluding the seven Samoaia species, the remaining 23 endemics all belong tovarious genera that have representatives in other parts of the world. It thus appearsthat the ancestral Samoaia population arose on the Samoa Islands and that its descendants have undergone at least seven speciation events but none has been able tomigrate to other areas.The Samoaia species are difficult to maintain in the laboratory, but by using special culture media (Wheeler and Clayton, 1965) two of the species, S. attenuataand S. leonensis, were maintained for a period of time at the University of Texas.The courtship behaviors of these species were studied and are as follow:CourtshipsSamoaia attenuata Wheeler and KambysellisThe male orients upon a nearby female and moves to her rear, positioning himself closely behind the tip of her abdomen. He then lifts and extends his forelegsover the female's abdomen. The extended legs each form an arc resulting in the tipsof the tarsi being directed inward. The wing vanes of both sexes of the Samoaiaflies when in repose are not held horizontal, as occurs in other Drosophila, butrather are "roof shaped" with the costal margins strikingly depressed. Further, thewing-tips are depressed and thus the flies appear humpbacked. Having extendedand arced his forelegs, the male then begins to vibrate the distal segments of hisfore tarsi rapidly up and down in small amplitude movements, and at the same timethrusts the tips of the tarsi under the sides of the female's abdomen. This movementis possible because of the laterally downward slanting position of the female's wingvanes. The male, after successfully thrusting his foreleg tarsi under the female'sabdomen appears to be gently "hugging" the female's abdomen and wings. Simultaneously as he vibrates his tarsi under and against the venter of the female's abdomen, he very slowly curls the tip of his abdomen down and forwards and under histhorax, resulting in the abdominal tip being directed forward. A receptive femalethen extends the tip of her abdomen rearward and thrusts it against the male'sgenitalia, thus enabling intromission to occur. At the start of copulation, the maleretains his "hugging" grasp of the female and the venter of his thorax rests on thedorsal surface of her wing vanes. Almost invariably the copulating male quicklyenters a cataleptic trance and often falls off the female, especially if she makes anymovement. The genitalic union, however, is maintained and the female may rarelydrag the inert male behind her. At the termination of copulation, the genitalia arequickly disengaged and the male immediately recovers from his trance and jumpsto his feet.Supported in part by PHS Research Grant #GM-22221.department of Zoology, University of California, Davis, CA 95616.144 Proceedings, Hawaiian Entomological SocietyA non-receptive female kicks with her hind feet, decamps, or flies away when amale attempts to embrace her with his forelegs.Eight copulations were observed. The minimum copulatory time was 4'25", themaximum 9'28" and the mean duration 6'37".Samoaia leonensis Wheeler and KambysellisThe male orients and positions himself behind the female in the same manner asdoes the attenuata male. He then raises, straightens, and extends both of hisforelegs over the female's abdomen and repeatedly strikes sharply downwardagainst the dorsal surfaces of the female's wing vanes, resulting in his tarsi and thedistal portion of the tibia sliding outward across the sloping female wing vanes.After repeatedly striking the female, he curls the tip of his abdomen under andforward, and attempts to achieve intromission. If the female refuses to extend herabdominal tip rearward and allow intromission, but does not decamp or fly away,then the male will relax his abdomen to the normal position and engages in a boutof body trembling. He rapidly moves his body, especially the wings and abdomen,up and down so that the abdominal tip and wing tips appear to be vibrating. At thesame time he repeatedly opens and closes the genitalic claspers. After a bout ofbody trembling, he returns to foreleg striking and attempts intromission. The receptive female thrusts the tip of her abdomen rearward against the male's genitaliaand allows intromission.Mounted on top of the female's wings, the copulating male does not enter acatalyptic trance, and usually will periodically claw the female's wing surfaceswith his fore tarsi. At the end of copulation he turns and quickly dismounts.Non-receptive females may kick, decamp, or fly away—but usually they engagein wing scissoring by repeatedly extending both wings sharply out and back about10°-15° from the resting position. This action is effective in causing the courtingmale to desist courtship and turn away from the female.Four copulations were observed and two were timed. The duration of these twowas 38 and 49 seconds, respectively.DiscussionThe founders of the leonensis stock were collected in a tiny remnant of disturbedlowland forest near Leone, Tutuila Island. The attenuata stock was acquired from adense jungle type forest on Upolu Island at an elevation of about 650 m (2,000 ft.).Wheeler (personal communication) notes that the adult flies are extremely difficultto locate in the field, that they prefer shaded localities with low light intensities andhigh moisture levels. The flies sit on the damp soil or vegetable litter. When disturbed, they move by a quick hopping flight, remaining close to the substrate. Theytherefore are difficult to collect by netting. Wheeler collected some individuals bysweeping over mushrooms, but the larval substrates are unknown.Wheeler and Kambysellis (1966), on the basis of the external genitalia, dividedthe seven Samoaia species into two groups, i.e. (1) attenuata, comma, and hirta,and (2) leonensis, nuda, and ocellaria, with the seventh species mallochi as a divergent member of the attenuata group. Thus the two species attenuata andleonensis belong to different groups. The overall pattern of courtship of attenuataand leonensis is similar but they do show interesting dissimilarities, i.e. leonensishas an extremely short couplatory period, one of the shortest known, and the maleVol. XIII, No. 1, April 197 145uses its extended forelegs merely to strike against the wings. In comparison attenuate wraps its forelegs around the female's abdomen and wings, a movementunknown for any other drosophiloid, but has a copulatory period that is moderatein duration when compared to other species.The unique configuration of the wings of Samoaia species results in the wingvanes forming a type of pseudocarapace which completely covers the abdomen.When the individual is sitting or walking, the anterior margins and the tips of thewing wanes are in close proximity to the substrate—so close that often the ;ourtingmale experiences difficulty in thrusting the tip of his abdomen under the wings ofthe female. The typical behavior exhibited by other drosophiloids is that the receptive female allows the mounting male either (1) to push her wings upward and apartwith his head and/or forelegs, or (2) she herself spreads her wing vanes outwardand upward (Spieth, 1952,1966). The result is that the copulating male's body restsupon the dorsum of the female's abdomen. Neither female of the Samoaia speciesstudied spreads or allows her wings to be spread, and the copulating male restsupon the dorsal surface of the female's wing vanes. The Samoaia flies tend to beimmobile during copulation and the male of the attenuata also enters a catalyptictrance. The flies are predominantly dark brown to brownish black in overall coloration , and have patterned wings. This assemblage of characteristics, i. e., structural,pigmentary and behavioral that the Samoaia flies possess, appear to be adaptationsfor dwelling on or close to a tropical forest floor and also probably serve as crypticadaptations that protect against the inevitable predators that are found in such anarea. Finally, it appears that development of the unique conformation of the wingshas served as the selective force that molded the courtship behavior of the males.Literature CitedSpieth, H.T. 1952. Mating behavior within the genus Drosophila. Bull. American Museum Nat. Hist.99:395-474.Spieth, H.T. 1966. Courtship behavior of endemic Hawaiian Drosophila. University Texas Publ.6615:245-313.Wheeler, M.R., and F.E. Clayton. 1965. A new Drosophila culture technique. Drosophila InformationService 40:98.Wheeler, M.R., and M.P. Kambysellis. 1966. Notes on the Drosophilidae (Diptera) of Samoa. University Texas Publ. 6615:533-565.

Courtship Behavior of Samoaia attenuate and S. leonensis

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Courtship Behavior of Samoaia attenuate and S. leonensis

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