The Relation between Aggressive and Sexual Behavior and Allometry in Palaeosepsis dentatiformis Flies (Diptera: Sepsidae) Author(s): William G. Eberhard Source: Journal of the Kansas Entomological Society, Vol. 75, No. 4 (Oct., 2002), pp. 317-332 Published by: Allen Press on behalf of Kansas (Central States) Entomological Society Stable URL: http://www.jstor.org/stable/25481792 . Accessed: 09/09/2011 22:03 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. Kansas (Central States) Entomological Society and Allen Press are collaborating with JSTOR to digitize, preserve and extend access to Journal of the Kansas Entomological Society. http://www.jstor.org JOURNAL OF THE KANSAS ENTOMOLOGICAL SOCIETY 75(4), 2002, pp. 317-332 The Relation between Aggressive and Sexual Behavior and Allometry in Palaeosepsis dentatiformis Flies (Diptera: Sepsidae) William G. Eberhard Smithsonian Tropical Research Institute, and Escuela de Biologia, Universidad de Costa Rica, Ciudad Universitaria, Costa Rica abstract: Male Palaeosepsis dentatiformis (Duda) (Diptera: Sepsidae) fought near dung pats that were used as feeding and mating sites by males and females. Males stilted and displayed their sexually dimorphic wing spots during stereotyped threat displays; they fought using rapid pulling and slamming movements of their front legs, and head butting. Males performed several apparent courtship movements during copulation, including tapping the female with their sternal brushes and genitalic squeezing. Allometric slopes of body parts used in aggression (wing spots) were steeper than control values, as is typical of weapons in other species. The slope of sternal brush length was lower, as in the genitalia of this and many other species. key words: sexual selection, sepsid flies, allometry Males of many species of sepsid flies congregate near feeding and oviposition sites such as dung and carrion (Pont, 1979). Typically, males are more abundant, and they mount and ride females as they oviposit, then mate with them when they are finished (summary in Schulz, 1999). The females of some species also mate at other stages of their reproductive cycles (Schulz, 1999; Eberhard, 2000). Males of many sepsids bear a variety of secondary sexual characters, including modifications of the form of the front and hind legs and ab dominal sternites, and markings on the middle legs and wings (Pont, 1979). It is not clear for most of these traits whether they are used in male-male battles or to court females. Aggressive interactions among male sepsids are common (Parker, 1972a, b;Ward, 1983; Zerbe, 1993; Schulz, 1999), and fights of male Sepsis punctum Fabricius can last up to 5 min (Zerbe, 1993). Larger males of S. punctum and S. cynipsea Linn, occupy sites closer to the dung and have greater access to females (Ward, 1983; Zerbe, 1993) (see however, McLachlan and Allen, 1987). Details of fights between males have never been described. Sepsid males are typically said not to court females (e.g., Parker, 1972a), but detailed ob servations of Microsepsis spp. documented apparent courtship both prior to and during copulation (Eberhard, 2001a, b). Precopulatory courtship may also occur in Themira (Man gan, 1976) and Archisepsis (Eberhard, 2001c, in press a). Thus the functional significance of different sexually dimorphic traits in sepsids is only poorly understood. The flies' small size and rapid movements make it difficult to be cer tain on that reports based only naked eye observations (as has been true for nearly all pre vious studies, especially those in the field) were sufficiently complete to resolve fine be havioral details. Theoretical considerations and empirical patterns (e.g., Petrie, 1988, 1992; Green, 1992; Pomiankowski and M0ller, 1995; Eberhard et al, 1998) predict that threat devices and weapons used inmale-male battles will have unusually high allometric slopes (larger males will have disproportionately large structures). In contrast, contact courtship devices that do not depend on male strength to be effective, such as genitalia, are expected to have unusually low allometric slopes (larger males will have disproportionately small structures). This study describes behavioral observations of Palaeosepsis dentatiformis (Duda) in the field and in captivity that clarify the probable functions of several secondary sexual characters in males of this species. It also examines the relations between the static al lometries of these traits and their functions. Field observations of sepsids are often com 318 JOURNAL OF THE KANSAS ENTOMOLOGICAL SOCIETY plicated by the co-occurrence of several species that cannot be reliably distinguished (es pecially females) (e.g., Parker, 1972a; Ward, 1983; Schulz, 1999). This study takes ad vantage of the combination of the relatively large size of P. dentatiformis (about 5-7 mm long) and their distinctive, sexually dimorphic wing spots, which allowed easy identifica tion of both the sex and species of undisturbed individuals in the field, and which also fa cilitated videotaping their behavior. The only previous publication on the behavior of P. dentatiformis described the behavior and morphology of the male sternal lobes, which are used as copulatory courtship devices (Eberhard, 200Id). Materials and Methods Behavior was observed near cow dung in the field near San Antonio de Escazu (San Jose Province, Costa Rica, el. approximately 1400 m) late in the dry season during April 2000. The dung was near the edge of a forested stream in a cool, steep-sided ravine which or received neither early morning late afternoon sun. In approximately five years of inter mittent study of sepsid flies in this area, this was the only time and place I ever saw more than a single individual of this large and distinctive species (they were absent the follow ing year). Aggressive and pre-copulatory courtship were taped with a Sony Digital Handy cam (DCR-VX1000) video camera equipped with +5 closeup lenses, using exposure times varying between 1/200 and 1/500 sec. The high resolution digital reproduction and the fast shutter speeds made it possible to decipher even relatively rapid movements. Drawings are based on prints of images imported into a computer with a Genius Pro II series videocap ture card. Structures not clearly visible in the images were omitted from the drawings. Cop ulation behavior of five pairs was also observed in the field after aspirating the mating pair into a small glass vial, using a hand-held lOx magnifier combined with a 2x headband magnifier. Observations in captivity involved offspring of field-collected females confined with cow dung. On emergence as adults, the flies were separated by sex during the first day. were a Pairs of virgins placed in small petri dishes under dissecting microscope. Copulat ing females did not remain on the roof when the petri dish was turned upside down, and ventral views were obtained by placing a mirror under the petri dish. Pairs to be observed in the SEM were frozen with ethyl chloride spray, and then air-dried rather than fixed, to allow materials on the flies' surfaces to remain undisturbed (Eberhard, 2001b). A previous study (Eberhard and Pereira, 1996) showed that the positions of the genitalic surstyli are not altered by freezing with ethyl chloride. a Measurements were made under dissecting microscope using specimens preserved in ethanol. The following straight line dimensions of body parts were measured: head width (maximum distance between lateral margins of the eyes in dorsal view); thorax length (maximum length in dorsal view when the scutellum just covered posterior edge of tho rax); wing length (from the junction of the R2+3 and R4+5 veins to the distal tip of the wing); wing spot (the innermost discoloration of the wing membrane to the distal end of the R4+5 vein); lengths of front, middle and hind tibia (maximum lengths in posterior view); widths of abdominal sternite 2 and the sexually dimorphic sternite 4 (maximum widths in ventral view); length of setae on sternal lobe (maximum length of longest setae in ventral view); and length of hypandrium (maximum length from tip of surstylus to dorsal surface of hy pandrium in lateral view). All means are followed by ? one standard deviation. Regression analyses were performed using both least mean squares (LMS) and reduced major axis (RMA) techniques, as there is uncertainty regarding which ismore appropriate (Green, 1999; Eberhard et al, 1999). The results of both are reported in Table 1, but the VOLUME 75, ISSUE 4 319 two techniques gave qualitatively identical results, and only LMS results are cited in the text. Results fighting: Males were present in larger numbers than females, both on the dung and in leaf litter nearby. The maximum numbers of flies seen on and near dung (<20 cm from the edge) on three days were 119 males and 39 females. Males were more likely to be in leaf litter rather than on the dung when compared with females (84.6% of 119 vs. 56.3% of 39; = = Chi2 =10.1, d.f. 1, P 0.0015). Nearly all males and females that were on the dung were feeding, repeatedly extending their mouthparts to touch its surface. Females never oviposited into the dung, and I saw no riding pairs, such as those that occur inmany other sepsids (Parker, 1972 a, b; Schulz, 1999). Both males and females were more common on dung that was several days old than on fresh dung (for instance, at 10:20 on 2 April, there were 54 flies on and near an old dung pat, and 0 on a fresh pat of approximately the same size about 1m away).