Multiple Origins of a Major Novelty: Moveable Abdominal Lobes in Male Sepsid Flies (Diptera: Epsidae), and the Question of Developmental Constraints
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EVOLUTION & DEVELOPMENT 3:3, 206–222 (2001) Multiple origins of a major novelty: moveable abdominal lobes in male sepsid flies (Diptera: epsidae), and the question of developmental constraints William G. Eberhard Smithsonian Tropical Research Institute, and Escuela de Biología, Universidad de Costa Rica, Ciudad Universitaria, Costa Rica Correspondence (email: [email protected]) SUMMARY Contrary to the impression given by their ex- productive decisions on particular types of stimuli will tend to treme scarcity among extant species of flies, moveable pro- favor male abilities to elaborate such stimuli, sexual selection cesses on the abdomen are apparently of relatively simple by female choice may sometimes result in sustained selection developmental origin, and they have evolved multiple times in for certain types of innovations in males. The lack of moveable males of the small family Sepsidae. They are used to stimulate appendages in most dipterans may be due not to develop- the female during copulation in two groups, where they are mental constraints, but to lack of selective advantages. probably independently derived. Because female cuing of re- INTRODUCTION able abdominal processes, the early stages of acquisition of new abdominal appendages (“lateral lobes”) are relatively Phylogenetic uniformity in a trait is sometimes thought to common in males of the family Sepsidae (Fig. 1). This study imply the existence of developmental constraints against al- shows, using comparative morphology and direct observa- ternative designs of that trait, and major innovations are tions of behavior, that the lateral lobes in two closely related sometimes said to require substantial reorganization of the species are independently derived, and that in both cases phenotype (e.g., Müller and Wagner 1991). Constraint argu- these structures are used during sexual interactions and are ments of these sort run the risk of being tautological—what- probably under sexual selection by cryptic female choice. ever has not changed must have been constrained, and what- The complex musculature associated with similar structures ever has changed must not have been constrained. The basic in some other sepsids suggests that they are also moveable, claim of constraint is potentially falsifiable, however, by and that they may also have arisen independently under sim- finding that repeated changes in a trait have occurred in an ilar selection. otherwise unremarkable subgroup of a larger group in which the trait is otherwise uniform. This is the situation docu- mented here with respect to moveable abdominal append- Specialized male morphology in sepsid flies ages in flies. Sepsidae is a small family of flies of worldwide distribution Although abdominal appendages were present in the an- that includes about 240 species (Pont 1979). Adult males cestors of adult insects (e.g., Snodgrass 1935; Birket-Smith have various sexually dimorphic traits that are often spe- 1984), all except those associated with the genitalia were lost cies-specific in form (Hennig 1949; Pont 1979; Steyskal early in insect evolution. Subsequent evolution of new ab- 1987a), including a modified fourth abdominal sternite, the dominal appendages or moveable processes has occurred subject of this study. In all sepsid genera in which the posi- only rarely (e.g., Snodgrass 1935; Chapman 1999; Yen et al. tions of copulating pairs have been determined, the male’s 2000). In Diptera, aside from the dramatic and repeated fis- fourth sternite touches or is very close to contacting the fe- sions of genitalic sclerites (reviewed by Wood 1991; Sinclair male during copulation, including Meroplius and Themira et al. 1994; Cumming et al. 1995), moveable abdominal pro- (ϭCheligaster) (Sˇ ulc 1928) (Fig. 1), Sepsidimorpha sp. (Fig. cesses are very rare (McAlpine et al. 1987). Thus, it might be 2), Sepsis (Parker 1972; Schulz 1999; Eberhard unpub. ob- thought that such structures are developmentally difficult to servation of S. neocynipsea), Archisepsis (Eberhard and evolve. I will argue that this is not the case. Although adults Pereira 1996), and Microsepsis (Eberhard 2001 in press). This of most families of flies completely lack nongenitalic move- contact probably often occurs between the posterior portion © BLACKWELL SCIENCE, INC. 206 Eberhard Evolutionary novelties in sepsid flies 207 Fig. 1. Schematic view of the long setae on the abdominal lobes of Themira (ϭ Cheligaster) leachi, with the setae of the copulating male (white) touching the female (stippled) (after Sˇ ulc 1928), and the relationships of Sepsidae (a member of the superfamily Sciomyzoidea) and other groups of higher Diptera (Schizophora) (after McAlpine 1989). of the male sternite and the female’s proctiger, which is Linn. and Nemopoda nitidula (Fallen), collected in Germany and pre- flexed dorsally to expose her vulva and permit intromission served in 70% ethanol were made from preparations mounted in Eu- (e.g., Fig. 2; see also Eberhard and Pereira 1996 on Archisep- parol on microscope slides, using a camera lucida. sis; Eberhard 2001 in press on Microsepsis). The nominal relations of Palaeosepsis sp. and P. nigricoxa are not clear. Silva (1993) argued that the monotypic genus Pseudopalae- osepsis should be included in her newly erected genus Archisepsis. Archisepsis, in turn, has classically been included in Palaeosepsis MATERIALS AND METHODS (senus latu) (Duda 1925, 1926; also Meier 1995). The two species thus belong to Palaeosepsis sensu latu, but the phylogenetic rela- Copulating pairs of Palaeosepsis sp. (an apparently undescribed tions among the species in Palaeosepsis sens. lat. remain to be de- species near P. chauliobrechma Silva) were observed in the field termined. near San Antonio de Escazu, San José Province, Costa Rica (el. 1400m). Each pair was aspirated into a plastic tube after copulation had begun, and induced to walk into a clear glass vial where the RESULTS flies were observed using a 2ϫ headband magnifier and 10ϫ and 20ϫ hand lenses. The bristles of the male’s fourth sternite were Morphology and movement of modified sternites clearly visible. Palaeosepsis sp. In addition, copulation behavior of both P. sp. and Pseudopalae- The fourth abdominal sternite of P. sp. is larger than the pre- osepsis nigricoxa Ozerov was observed in captivity under a dissect- ceding sternites and has a brush of very long setae at each ing microscope, using flies raised in captivity from cow dung (P. sp.), or from howler monkey dung (Alouatta palliata) on Barro Colorado posterior lateral corner (Figs. 3 and 4). The fifth sternite is Island, Panama (P. nigricoxa). Copulating pairs of P. sp. were frozen small (Fig. 4) and is hidden in an inward fold of the abdom- with ethyl chloride spray and fixed in 70% ethyl alcohol or FAA prior inal cuticle. The base of each brush is heavily sclerotized and to being prepared for examination with a scanning electron micro- more or less circular (Fig. 4). When at rest, the long setae of scope. Drawings of specimens of these species and of Themira putris the brushes project posteriorly (Fig. 3). The cuticle of the 208 EVOLUTION & DEVELOPMENT Vol. 3, No. 3, May–June 2001 Fig. 2. Copulating pair of Sepsidimorpha sp. (A) The male (left) grasps the lateral portions of the female’s fifth abdominal sternite with his surstyli. (B) The thick setae on the male’s fourth abdom- inal sternite press forcefully against the female’s tergite. fourth sternite just medial to the base of each brush is thin and membranous (Figs. 3 and 4). The central portion of this sternite is, in contrast, thicker and more rigid. Internally, the central portion is invaginated to form a keel to which the cen- tral ends of a large bundle of muscle fibers are attached (Fig. 4). The other ends of these muscle fibers are attached to the base of the brush. Thus, the contraction of these muscles probably produces the ventral tapping movements of the se- tae that occur during copulation (below). A second, smaller band of muscle fibers runs anteriorly from the base of the brush to near the anterior margin of tergite 3 (Fig. 4). Con- traction of these muscle fibers presumably moves the brush Fig. 3. Scanning electron micrographs of Palaeosepsis sp. (A). Ven- anteriorly, causing the setae to project ventrally from the tral view showing positions of male surstyli and fourth sternite brushes at rest. (B) Lateral view of male abdomen (right) with male’s abdomen. brush directed ventrally and contacting female abdomen during The male tapped the female’s abdomen rapidly with his copulation. (C) Close-up ventral view of base of brush at rest, show- brushes in all copulating pairs of P. sp. (four in the field, four ing thin, flexible portion of sternite proximal to base of brush. Eberhard Evolutionary novelties in sepsid flies 209 Fig. 4. Ventral view of fourth (sparse stippling) and fifth (dense stippling) abdominal sternites of Palaeosepsis sp. and Pseudopalaeosepsis nigricoxa (above), and dorsal view of sternites and associated muscles of the same species. 210 EVOLUTION & DEVELOPMENT Vol. 3, No. 3, May–June 2001 in captivity). The brushes were flexed anteriorly, and the se- Themira putris tae fanned out to assume a cupped form (Fig. 3B), and then The strong and complex muscles associated with the lateral the brushes vibrated in an approximately dorsoventral direc- lobes of T. putris (Fig. 5) indicate that the lobes in this spe- tion an estimated 3–10 times/sec, so that the setae tapped the cies are also moveable. Ventral projection of the long setae sides of the female’s abdomen (Fig. 3C). In the most com- in T. putris is probably produced, at least in part, by contrac- plete observation of a copulation, which lasted 104 min, the tion of the medium-strong band of muscles running dorsally brushes beat continuously on the female for at least 12 min from the anterior margin of the sternite to tergite 4 (Fig. 5). soon after copulation began, then became quiet.