The Relation between Aggressive and Sexual Behavior and Allometry in Palaeosepsis dentatiformis (Diptera: ) 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

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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 . 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 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 (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). Both numbers of flies and numbers of intense interactions seemed to decrease in the early afternoon when more direct sun fell on the dung, but no careful counts were made. Aggressive behavior among flies on the dung was relatively uncommon, and never es calated beyond single strikes by males and females. All of the estimated 30-50 intense fights I saw occurred in the leaf litter near the dung, and involved only males. Most bat tles involved a male on an object such as a dead leaf, apparently attempting to drive away any other within a distance of 3-5 body lengths. In some cases the male attacked ag gressively while the other fly was still hovering just before landing. Over the course of an on same 5-10 min aggressive male often stayed the object, and thus defended a par over ticular territory. But longer time spans males tended to move, so defense of particu lar territories was ephemeral. Presumably this territorial behavior enabled the male to re spond more effectively and mount females that landed nearby, but in some cases a male flew and collided with or struck at and drove a female from his leaf (in other cases the fe male did not leave after being struck, and the male turned away). Male aggression included the following behavior patterns, in order of increasing ap parent intensity: 1.Wing flicks: Often before darting toward another fly that was a body length or more away, a male alternately flicked one or both wings anteriorly in the space of approximately 0.03 sec to a position close to perpendicular to his longitudinal axis (Fig. la). At the apex of each forward movement the wing was twisted on its longitudinal axis, so its lower sur face was directly anteriorly. Wing flicks were performed both when the other fly was fac ing the male, and when itwas facing away. The time between flicks of alternate wings was approximately 0.1 sec, and from one to nine flicks occurred before the male darted for ward. In two cases the male also flicked his abdomen rapidly ventrally. 2. Fly and collide: In seven cases a fly flicked his wings and then flew 2-4 body lengths in the space of 0.03 sec to collide with another male. In some cases the attacking male bounced backward from the collision and then walked away, while the other fly gave no obvious response; in others the flies then squared off in mutual stilting (below). 3. Unilateral forward wing display: After closely approaching another fly, the male usu ally directed his wings anteriorly and held them still while facing his opponent (Fig. lb, c). The male kept all his legs on the substrate during low intensity forward wing displays, but in higher intensity displays he tilted the anterior portion of his body dorsally and raised his front legs (Fig. lb). 320 JOURNAL OF THE KANSAS ENTOMOLOGICAL SOCIETY

'' c ^ 'li: ^U Fig. 1. Male aggressive behavior, a) A male flicks one wing forward toward another fly (out of view) and twists it so the dark tip is displayed (arrows and dotted lines mark positions of wings 0.07 sec before), b) A male (right) performs a forward wing display and then strikes toward a female, which raises her abdomen (arrows mark po sitions of the male head and front legs 0.03 sec later), c) A mutual stilt leads to an unusual simultaneous mutual ram, which bent both males' heads dorsally (solid lines in d). Both flies fell to the substrate 0.07 sec later (arrow and dotted lines in d), and the fly on the left then walked away.

4. Unilateral strike with front legs (and chase): A common sequence involved one male performing a forward wing display with raised front legs, then lunging forward and force fully slamming his front legs ventrally (Fig. lb). Some strikes fell short, but inmany cases his tibiae came down on the dorsal surface of the other fly; particularly forceful slams drove the opponent all the way to the substrate or knocked him partly off his feet. Often the fly which had been struck then moved away, and he was often struck repeatedly from behind as he retreated. Occasionally a male being struck from the rear did not flee, but in VOLUME 75, ISSUE 4 321

stead raised his abdomen (Fig. lb). In these cases the attacker generally turned away af ter one or a few strikes. Occasionally a strike ended with the attacking male jerking rear ward, pulling the opponent toward him (up to nearly an entire body length in the space of 0.03 sec). Presumably he pulled the opponent with his front legs as he jerked rearward af ter the strike. In two cases the other fly was rolled partially onto his back by such a pull. Most (but not all) forward strikes were preceded by a forward wing display. 5. Mutual stilt and wing display: Occasionally aggression was less one-sided, and the a two males approached each other closely face-to-face, each performing forward wing display with his front legs raised (Figs, lc, 2a, c). Over the space of 0.1-0.2 sec, both flies stilted higher and higher off the substrate. Each male's entire body was elevated from the substrate during stilting, not just the anterior portion as in unilateral forward wing displays. = Mutual stilted poses were held for variable lengths of time (0.07-0.6 sec; mean 0.18 ? = 0.15 sec, n 25), and then one fly or the other attempted to strike or ram the other (Figs. Id, 2). 6. Lean rearward away from strike: When a mutual stilt was followed by one male strik ing ventrally with his front legs, the other male often leaned rearward (Fig. 2a, d). Some were one case a leans substantial, and caused the strike (or in ramming attack) to miss. In the most dramatic cases the impetus of the missed slam caused the head of the attacking male to crash against the leaf on which they were fighting. In one fight in which the males were more some move aligned nearly side-to-side than face-to-face, of the most rapid were or some ments rearward sideways; rearward leans occurred just (0.03 sec) before the opponent lunged forward. In a an 7. Ram: several intense battles, male lunged forward, delivering apparently pow erful blow with his head that knocked the opponent a body length or more rearward (Figs. Id, 2b, e). Some movements were so rapid that the attacking fly rammed and then regained his original position in the space of one frame (0.03 sec) (Fig. 2e). Ramming blows struck the opponent's abdomen, thorax, or head. Most but not all ramming blows were struck just after the male had leaned rearward (Fig. 2b, e). An unusual mutual ram caused both flies' heads to bend dorsally (Fig. Id), and both individuals then fell to the substrate. It was not always possible to distinguish the male's genitalic surstyli from the rest of his were abdomen during aggressive encounters, but his surstyli briefly deflected ventrally, away from the ventral surface of his abdomen in a variety of contexts: just before flying to collide with another fly, just prior to and immediately after a strike (Fig. lc), just after a was male struck while he held his abdomen raised, and even without any associated ag I never saw a an gressive behavior. male touch opponent with his genitalia, however, so the functional significance of these genitalic movements is unclear. The sternal brushes were never seen directed ventrally (Fig. 3c) during aggressive behavior. courtship before and during copulation: Males darted onto females from up to four body lengths away, with either no preliminary signaling behavior or after one or more wing flicks. Despite the fact that most females were on the dung rather than in the nearby leaf litter, most of the mounting attempts I saw occurred in the leaf litter (89.5% of 38 at tempts), and all four copulating pairs were found on leaves and twigs rather than on the dung. None of the mounts whose initiation I saw led to copulation, and most mounts were very brief (<5 sec). Females generally responded to being mounted by shaking their bod ies rapidly from side to side as in other sepsids (e.g., Parker, 1972a; Ward, 1983; Allen and Simmons, 1996; Blankenhorn et al, 2000; M. L. Baena, in prep.), and by flexing their ab so domens ventrally the male could not make genitalic contact (Fig. 3c) Successfully mounted males clamped the base of each of the female's wings with the femur and tibia 322 JOURNAL OF THE KANSAS ENTOMOLOGICAL SOCIETY

Fig. 2. Two ramming attacks, each preceded by a possible feint. In one pair (a, b) both males first stilted (solid lines in a), and then the fly on the right rocked far to the rear as the other fly leaned forward (arrows and dotted lines in a mark positions 0.07 sec later). The fly on the left then raised his front legs to slam (solid lines in b, 0.07 sec after dotted lines in a), slammed them ventrally (not shown), and was immediately rammed forcefully by the fly on the right and driven about one body length backward onto his back (arrows and dotted lines in b mark positions 0.07 sec later). In the other pair (c-e), mutual stilting (solid lines in c) was followed (arrows in c mark positions 0.03 sec later) by the male on the left rocking backward and crouching as the other male leaned forward as if to slam (solid and dotted lines in d show positions, respectively 0.07 and 0.03 sec later). But the male on the left then apparently rammed forward from his crouch and knocked his opponent rearward about a body length (e) (solid lines in e show positions 0.03 sec after the dotted lines in d).

of each front leg (Fig. 3a, b). One mounted pair was taped for just over 45 sec in the field as the male courted and struggled to copulate; another was observed briefly under a dis secting microscope. The following courtship movements were distinguished: 1.Middle legs forward: As in other genera (Parker, 1972a, and Schulz, 1999 on Sepsis; Mangan 1976 on Themira; Eberhard, 2001a on Microsepsis and Eberhard, unpublished on Archisepsis) the mounted male's middle legs were directed forward. They were held in two VOLUME 75, ISSUE 4 323

tarsi ^^-front

c d i

Fig. 3. Interactions between males (stippled) and females, a) and b) Dorsal view of a mounted male as he courts a female with wing buzzing and sweeps of his middle legs, while clamping her wings with his front legs. The male's middle legs, which in the resting position (solid lines in a) are directed anteriorly and laterally, are moved medially (dotted lines and arrow in a), and then posteriorly to near the anterior margin of the female's wings (solid lines in b) before returning to their resting positions (dotted lines in b). The male began wing buzzing and jerked his abdomen dorsally 0.03 sec before he moved his legs medially, c) A mounted male extends his surstyli and sternal brushes ventrally as he struggles with a female, which has bent her abdomen ventrally to avoid gen italic contact, d) A mounted male crosses his anteriorly directed middle legs over the female's head.

positions. During periods in which sweeping occurred (below), they were spread laterally so that their distal portions were above and lateral to the female's eyes (Fig. 3a). When the male pressed his abdomen to that of the female and she shook violently, his middle tarsi were directed medially and crossed over or in front of the female's head (Fig. 3d). The middle legs were still while they were directed forward, rather than vibrating as in some species (Eberhard, unpubl.). 324 JOURNAL OF THE KANSAS ENTOMOLOGICAL SOCIETY

2. Sweep with middle legs: Periodically both of the male's middle legs were brought forward for one frame (0.03 sec) just after a burst of wing vibration began (Fig. 3a), and then moved rearward to possibly touch the anterior edge of the wings in the next frame (Fig. 3b) (viewing angles precluded certainty regarding contact). They were then returned to their spread position in the next frame. Each sweep of the middle legs thus lasted on the order of 0.1 sec. There was no indication that the male's legs slid along the female's wing, as occurs in some Microsepsis (Eberhard, 2001b), and Archisepsis (Eberhard, un publ.). 3. Abdomen movements: On several occasions the male bent his abdomen ventrally and pressed his genitalia against the female in an apparent attempt to copulate. During some (but not all) attempts the male crossed his hind tibiae under the female's abdomen, prob ably to lift it dorsally so his genitalia could contact hers. Both the male's genitalic surstyli and the brushes of long setae on his fourth sternal lobes were directly ventrally, away from the surface of the male's abdomen just prior to these attempts (Fig. 3c). These were the only moments before copulation when the flies' positions made it feasible for the male to contact the female with the brushes of setae on the sternal lobes. It was not possible to see whether the male's brushes or surstyli moved when they contacted the female. A second type of abdomen movement involved quick dorsal flicks that brought the male's longitu dinal axis to an angle of approximately 45-60? with that of the female (as also seen in Sep sis punctum?Schulz, 1999, and Archisepsis pleuralis [Coquilett]?Eberhard, unpubl.). In one series of 12 such movements, each lasted 0.03-0.07 sec and they were separated by 0.27-0.43 sec. 4. Wing vibration: The male vibrated his wings rapidly in both long and short bursts. Short bursts of wing vibration lasted a mean of 0.14 ? 0.02 sec, and the mean time be = tween bursts of wing vibration was 0.12 ? 0.03 sec (N 25 during the mount taped in the field). During the first 0.03 sec, the male's abdomen jerked rapidly dorsally and his wings were probably more or less perpendicular to his body. The male's movements appeared to be relatively forceful: sometimes they displaced the female slightly, and she repositioneed continued to her legs. During the following 0.03-0.10 sec, the male's wings vibrate, grad ually returning to their resting position; the amplitudes of vibration (as indicated by the degree of blurring in video images) gradually decreased. Wing vibration began either in or one the same frame of the video as sweeping frame earlier. I did not witness the beginning of a copulation. Copulations that had already begun when first seen lasted for 96 and 112 min in the field, and 66 and 55 min in captivity. Thus cop ulation was relatively long compared with the 15-20 min copulations in other species of sepsids (Parker, 1972a; Zerbe, 1993; Eberhard and Huber, 1998; Schulz, 1999). The male's front legs did not grasp the female's wings during copulation, but rested on her thorax, and his middle legs were on her wings except during wing wiping (below). Males performed the following courtship movements during copulation (captive females were very restless, so it was not possible to obtain quantitative descriptions of frequencies and sequences): 1. Wing wiping: The male extended both pairs of middle and hind legs rearward and rubbed his middle tarsi rapidly across his hind tibiae, then immediately flexed his middle legs ventrally and rubbed their tarsi on the undersurfaces of the bases of the female's wings. As inMicrosepsis armillata Melander and Spuler (Eberhard, 2001b), these movements ap to female's parently resulted in the transfer of a liquid from the hind tibia the base of the wing (Fig. 4a, b). In one pair in the field, wing wiping occurred sporadically during at least the first 30 min of copulation. 2. Tapping with sternal brushes: At rest, the brushes of long setae on the lobes of the VOLUME 75, ISSUE 4 325 male abdominal sternite 4 were directed posteriorly, but during copulation the male di rected the lobes and their setae laterally and slightly anteriorly, and then vibrated them rapidly medially so that they tapped on the dorsal and lateral surfaces of the female's ab domen (Eberhard, 2001c). Tapping with the brushes occurred in all eight copulations. In the two pairs observed most clearly, the male performed a long, sustained burst of tapping early in copulation (in one case lasting at least 12min), and then several (at least six) short bursts that lasted from approximately 5 to 100 sec and that were interspersed with pauses lasting from 1 to 14 min. During some bursts of tapping the male also vibrated his ab domen. 3. Abdomen rubbing: The male periodically directed both hind legs ventrally, and rubbed the tips of his tibia and the basal segment of his tarsus briskly back and forth across the pleural membranes of the female's abdomen. The two hind legs rubbed in alternation. 4. Body vibration: The male's abdomen vibrated rapidly dorso-ventrally during the first part of copulation. 5. Genital squeezing: The inner margins of the male's surtyli grasped the sides of the female's sternite 6, with their tips and the long setae on them projecting beyond the ster nite's anterior edge (Fig. 4c, d). In both pairs observed under the dissecting microscope, the surstyli actively opened and closed (Fig. 4c, d). In one case it appeared that the surstyli opened and closed very rapidly, in at least approximate coordination with the tapping movements of the brushes, but in another it was clear that they were quiet and squeezed the female tightly during bursts of tapping with the brushes. Opening and closing move ments did not appear to be as strongly rhythmic as inMicrosepsis (Eberhard, 2001a). In one pair the surstyli slid slightly rearward as they opened, and then anteriorly as they This not seen closed. movement, which has been in other sepsids whose genitalic move ments have been observed, did not appear to be produced by pushing and pulling move ments of the male's entire abdomen, and may have been produced instead by movements of the intromittent genitalia inside the female (see Eberhard and Huber, 1998, on proba ble internal movements in Archisepsis and another species of Palaeosepsis). During the latter portion of both these copulations the surstyli did not move, but squeezed the female so tightly that the lateral margins of her sixth sternite were bent dorsally and the sternite's outline was nearly cylindrical (Fig. 4d). As in some other sepsids (Eberhard and Huber, 1998; Eberhard, 2001a), the male's sperm pump was visible just below membrane basal to the hypandrium. It vibrated very rapidly during the first portion of one copulation, but was quiet during the last 20-30 min. When copulation ended, a small mass of clear white material adhered to the dorsal sur face of the tip of the female's abdomen. Similar masses produced during copulation inM. armillata and M. eberhardi Ozerov have been hypothesized to function to stimulate the female chemically (Eberhard, 2001b). allometry: The relative sizes of male and female structures in individuals of different are sizes shown in Fig. 5 and Table 1. Three general body measures, head width, wing length, and abdominal sternite 2 width, were not sexually dimorphic. When regressed on as an thorax length indicator of overall body size, all had very similar slopes (Fig. 5a-c) (combined slopes for males and females were, respectively, 0.81, 0.81, and 0.85). Tibia lengths for all legs were somewhat greater in males than in females, and had slightly steeper slopes. These difference were larger in the middle and hind tibia than the front tibia (Fig. 5e-g). Wing spots were proportionally larger inmales than in females (Fig. 5d), and had much steeper slopes (1.57 and 1.30 respectively). A statistical test (Eberhard et al, 2000) for dimorphism in wing spot length between larger and smaller males revealed no 326 JOURNAL OF THE KANSAS ENTOMOLOGICAL SOCIETY

Fig. 4. SEM images of fluid transferred by the male during copulation (a, b) and of genitalia during copulation (c, d). a) Basal portion of the hind tibia of a male frozen during copulation, showing the accumulation of fluid on the dense patch of setae (the osmeterium). b) base of wing of female frozen during copulation, showing the accumulation of liquid along the rear margin, c) Male surstyli grip the female's sixth abdominal sternite loosely, leaving itmore or less planar, d) Male surstyli squeeze the female's sixth abdominal sternite tightly, bending its lateral edges dorsally.

significant dimorphism. The slope of the male's genitalia (hypandrium length) was, in con trast, much less than that of non-genitalic structures (0.39). Both sternal brush length and sternite 4 width of males had intermediate low slopes (0.61 and 0.73 respectively). One male had an unusually small sternite 4, middle tibia, and genitalic hypandrium for VOLUME 75, ISSUE 4 327

0.6 Wing Head ##. * ? %

* o <> 0 g> g = Q$ male slope 0.82 male slope = 0.81 * female = 0.87 ^o ^* = J?? 04 slope 0 female slope 0.81 i "' . A '.B 0.30.1 0.2 J 0.1 0.20.3 Log thorax length (mm) Log thorax length (mm)

"?4' o.i | Sternite 2 ? Wing spot ? v ! * ? -0.5- o ^ ^ 0> $ o ?. * . * CN ? 0J' . c ?o g_# ? a3 ? male = - = . oo^? slope 1.57 male^ slope 0.80 . female = 1.30 | * -0.3 slope ^ n7 *o female slope =1.00 '00? _

0.30.1 0.2 0.1 0.20.3 thorax thorax Log length (mm) Log0.4U length (mm) _ Tibial o,l _Tibia 2 . *.. I ' . E I *?" i0'3' C#5?I f .- o*oo?? ? * "^ ? = ??male = 0.99 -O.i- . * * male slope 0.96 ? slope o = o female = ?? female slope 0.95 Jg> 0 , 0 slope 0.90 . I ? F 0.30.1 0.2 E 0.1 0.20.3 - ,norax Log thorax length (mm) Lo9 lenS,h (mr") Tibia3 -A . . f 0. Sternite4

? 0.3- ^*L = I ?? J*^" male slope 0.58 1 *o = o 02 - o 5 female slope 1.15 - * ?? o * ? = 2 o ?? 5 male slope 0.93 3 ? *0? ? = o ?0 S?o? o o) female slope 1.03 jQ 0.8 ?o GI ?.. ? I H 0.30.10.2 0.1 0.20.3 thorax Log length (mm) Log thorax length (mm) ~ I" Brush sternite 4 ^ ? Hypandrium

| E

2 -0.4. ? ^ a = = male slope 0.32 2> male slope 0.45 ^ O 03 -0.6l-.-._,_ I J ** 0.30.10.2 0.1'-^-?-._ 0.20.3 thorax Log thorax length (mm) Lo9 length (mm)

Fig. 5. Log-log graphs of different portions of the bodies of males (filled circles) and females (open circles) plotted against an indicator of overall body size (thorax length) (arrows in h, i, and j indicate the same aberrant male, which was not included in calculations of slopes given here). WH

^?^-_^^?_^i^^_?.?_^^-_Male-female

(1.005)(0.923)(1.646)O ? (0.985)(0.974) (0.853)(0.914)O 3 (1.027)g (0.933)(1.432) O (1.574) (1.111)

Male(N= = (NFemale34-36) 13-16)

InterceptSloperIntercept Sloperdifference

Table1.Allometricvaluesforleastmean squaresandrootmeanssquare(RMAparentheses)slopesin regressionsoflogsofdifferent structures onloglength of??

Length tibia 3 thorax in male and female P. dentatiformis. All 0.075values of? .008both slope and ? 0.933are 0.9870.8130.960-0.1180.810.023-0.119.068?.013?.010Widthhead??ns at P.034 < 0.001 except? 0.294 0.8190.96050.9540.872.010.043.019.097Lengthwing?ns for the -0.002value marked? .016ns"ns". "Male-female differ 1.028 ? .055 0.982 *** significant Lengthtibia20.048 ?.0080.988?.0330.983?.0110.0092 0.900?.0570.975*** Lengthtibia1 -0.223? .0080.962?.0360.977?-0.244 .012 0.947?.0620.972 *** g intercept 0.979

Lengthwingspot-0.460?.0221.566?.0930.9512?-0.485 .057 1.30?.300.826 ***^ Widthsternite2 -0.752?.0200.798?.083 0.855 -0.801?.208 1.000?.140 0.900 ns ? (0.847) (1.047)(0.953)

? > 2 (0.619)2? 0.8060.714-0.9320.733.123.0441.154-0.279.029?.2274Widthallsternite?***males (0.616)> (0.821) (0.533) (0.372) (1.072)

fi-0.237without0.9490.585.008***aberrant .034male? withoutaberrantmale-0.196?.0070.325.0310.874g Length hypandrium all males ?.015 0.392 ? .062 0.736 w -0.215 withoutaberrantmale-0.251+.0180.449?.0740.725O

Length brush sternite 4all -0.298 ? 0.36 0.614 ?.150 0.573 ? ence" gives significance inanalyses of covariance. males level VOLUME 75, ISSUE 4 329

his size (points marked with arrows in Fig. 5 h-j). When the data from this atypical male were removed from analyses, the slopes of hypandrium length, sternal brush length, and sternite 4 width inmales were all sharply reduced (Fig. 5h-j, while that of the middle tibia remained unchanged (0.99).

Discussion

Inmany respects the behavior of P. dentatiformis resembles that of other sepsids. Males and females aggregated near dung, and fed, fought and mated there; the male grasped the female's wing with his modified front legs prior to copulation; he squeezed her sternite 6 with his surstyli during copulation; and he transferred liquid from his osmeteria to the un derside of the female's wings. However, two aspects of their behavior were unusual. Dung was used exclusively as food, and never for oviposition. The matings thus did not follow oviposition, as is typical for sepsids (Schulz, 1999) (for other exceptions see Schulz, 1999 on and Themira, and Eberhard, 2000, on Microsepsis). Attacking males of P. den tatiformis nearly always faced directly toward their opponents, in contrast to species such as M. armillata in which laterally directed attacks are common (W. Eberhard, unpubl.). Strikingly rapid rearward movements occurred during P. dentatiformis battles in several a were a contexts: jerking opponents after strike; when males misaligned; and when male or leaned rearward just before while the other struck. Males of P. dentatiformis have several sexually dimorphic characters, including a large wing spot, a brush of long setae on their modified fourth abdominal sternites, complexly modified front femur and tibia, and a dense patch of short setae on the hind tibiae (osme terium), in addition to their species-specific genitalic surstyli which are also adorned with setae. us to long The observations reported here allow identify those traits used in male male aggression versus those involved in courtship. The wing spots appear to function as visual signals in male-male battles. The wing flicking prior to darting toward another fly, the dramatic twist of the wing that occurred at the apex of each flick and exposed the wing spot to the other fly, and the dramatic anterior flexion of the wings during apparent threats and battles all seem to be associated with displaying the wing spots. This interpretation of wing spot function accords with the association between large wing spots and wing threats in this species, in contrast to lack of wing flicks and wing flex threats as well as the very small wing spots or completely unmarked wings in several other sepsids that I have ob served in the field, including P. pusio (Schiner), Archisepsis diversiformis (Ozerov), A. armata pleuralis, A. discolor (Bigot), A. (Schiner), A. ecalcarata (Thompson), and Mi crosepsis armillata?Eberhard, unpubl.). Wing spot displays played little if any role in courtship. The front legs were used as weapons both to slam and to pull on opponents, but contact in both contexts was probably usually with the tarsi or the distal portion of the tibia rather than the modified basal portion of the tibia and the femur. The length of the front tibia was only slightly sexually dimorphic (Fig. 5e). Thus at least most of the sexually dimorphic as pects of the male's front legs were brought into play only while the male's front legs clamped the female's wings prior to copulation, a context in which their species-specific morphology probably functions to stimulate the female (Eberhard, 2001c, in press a). The brushes on the sternal lobes and the genitalic surstyli appear to be used to stimu late females by touch, rather than visually prior to mounting as in the probably indepen dently evolved sternal brushes of Themira (Mangan, 1976). The brushes played no appar ent role in male-male setae on interactions. The long the surstyli were never seen in use obvious (they lay spread against the ventral surface of the female's abdomen during 330 JOURNAL OF THE KANSAS ENTOMOLOGICAL SOCIETY

copulation?Fig. 4c, d), but might be employed in the early stages of copulation or just before it begins, stages that I failed to observe. Several of the aggressive behavior patterns seen in P. dentatiformis also occur in other families of flies. Stilting on all six legs occurs in neriids (Mangan, 1979; Eberhard, 1998; Preston-Mafham, 2001), and on the middle and hind legs inmicropezids (Wheeler, 1924; Eberhard, in prep, on Tanaeioptera; P. Ortiz, in prep, on Ptilosphen), platystomatids (McAlpine, 1975, 1979), diopsids (Berkhardt and de laMotte, 1983), a richardiid (Pie, 1998), and tephritids (Wilkinson and Dodson, 1997). Slamming raised front legs ventrally onto the opponent occurs in micropezids (Wheeler, 1924; Eberhard, in prep.; P. Ortiz, in prep.), drosophilids (Eberhard, in press b on Chymomyza) and another sepsid (Eberhard, unpublished on M. armillatd). Twisting the wing to direct the flat, marked ventral surface toward an opponent occurs in drosophilids (Spieth, 1981), a richardiid (Pie, 1998), and many tephritids (Aluja and Norrbom, 2000). The uniformity among flies in these sorts of display traits was noted by Pie (1998). Possibly the similarities occur because flies are gen erally poorly equipped to damage opponents. They lack strong biting mouthparts, and are lightly built for speed and agility. Thus slamming and quick ramming movements with the body or legs, and stilting postures that probably arose as threats of slams, may be among the few aggressive options available to them. This limitation contrasts with beetles, for ex ample (Eberhard, 1980), or earwigs (Moore and Wilson, 1993; Bricefio and Eberhard, 1995), which can bite, or squeeze powerfully with horns, legs or cerci. The patterns of sexual dimorphism and allometry in structures of P. dentatiformis with different functions during sexual interactions give only partial confirmation of predictions when they are compared with those of traits with other functions, such as head width, wing length, and sternite 2 width. As expected, three structures that may function to accentuate visual threats (wing spots and the middle and hind tibiae?which allowed males to elevate during stilting) were proportionally larger in males than in females. The middle tibiae, which function in both pre-copulatory and copulatory courtship in addition to aggressive stilting, were no more sexually dimorphic than the hind legs (Fig. 5f, g); values of F in = = analyses of covariance were 144 for middle legs (d.f. 41), and 169 for hind legs (d.f. 47). Wing spots had especially steep allometric slopes (stronger in males than females), while middle and hind tibiae had only moderately steep slopes compared with non-di as morphic traits such as head width. A further structure thatmight function a visual threat, as well as weapons in male-male battles, the front tibia, was only barely dimorphic; the length of the front tibia did not have a steeper slope than those of the middle and hind tib iae, which were never used as weapons. Male front legs thus appear not to be specialized for male-male battles. The species-specific sculpturing on their ventral surfaces probably functions to clasp and stimulate the female's wing (Eberhard, 2001c). Possibly this func tion has precluded their further specialization as weapons. Just as in other and spiders (Eberhard et al, 1998) and a scorpion (Peretti et al, 2001), the male genitalia had an especially low allometric value (for an exception to this rule in a vertebrate, see Miller and Burton, 2001). This low slope was accentuated in the use ster analyses excluding the aberrant male. Interestingly, in view of the of the male's nal brushes in a "genitalic" role as contact courtship devices during copulation, both the sternal brushes and the sternite width of males also had relatively low slopes. Presumably these low slopes represent a convergence on the growth pattern for primary genitalia. This functional interpretation is tentative, however, because the simultaneous reduction in both one the primary genitalia and the sternite and sternal brushes in aberrant male intimates that these structures may share a developmental control that determines relative size. VOLUME 75, ISSUE 4 331

Acknowledgments

I thank Andrej Ozerov for kindly identifying the flies, and Maribelle Vargas for out standing help producing SEM images. I received financial support from the Smithsonian Tropical Research Institute.

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