Sexual Behavior and the Enlarged Hind Legs of Male Megalopus Armatus (Coleoptera, Chrysomelidae, Megalopodinae) Author(S): William G
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Sexual Behavior and the Enlarged Hind Legs of Male Megalopus armatus (Coleoptera, Chrysomelidae, Megalopodinae) Author(s): William G. Eberhard and Mary Carmen Marin Source: Journal of the Kansas Entomological Society, Vol. 69, No. 1 (Jan., 1996), pp. 1-8 Published by: Allen Press on behalf of Kansas (Central States) Entomological Society Stable URL: http://www.jstor.org/stable/25085643 . Accessed: 08/09/2011 20:07 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 69(1), 1996, pp. 1-8 Sexual Behavior and the Enlarged Hind Legs of Male Megalopus armatus (Cole?ptera, Chrysomelidae, Megalopodinae) William G. Eberhard12 and Mary Carmen Marin2 1 Smithsonian Tropical Research Institute 2 Escuela de Biolog?a, Universidad de Costa Rica, Ciudad Universitaria, Costa Rica abstract: Behavioral observations indicate the giant hind legs of male Magalopus ar matus beetles function as weapons in battles between males over sites where feeding and mating occur; they are not used to court females. Males court females actively during copulation. When male and female morphology show clear differences, it is reasonable to suspect that the sexually dimorphic male characters have evolved under sexual selection (Darwin, 1872). It is not so easy, however, to deduce whether a given male character has been under sexual selection by female choice, or by direct male combat. Given characters can, in fact, be under both types of selection. A first step in deciphering the function of a sexually dimorphic male structure is to observe the animal's behavior. If, for example, the structure is displayed prominently during male-female interactions in a way likely to increase the fem ale's perception of it, the male trait may play a role in sexual selection by female choice. If the structure is employed in aggressive interactions between males in a as to a such way apparently increase male's chances of defeating rival males, then a role in sexual selection by male combat is indicated. Males of the chrysomelid beetle Megalopus armatus Lacordaire have giant hind legs, with large spines on the femur and a curved tibia. This brief study indicates that these legs are used as weapons in fights between males over mating sites, and are probably not used in the context of female choice. Materials and Methods Megalopus armatus beetles were observed on 1 and 4 June, 1994 in the field on a Solanum sp. near americanum (Solanaceae) plant near the edge of the Rio Agres, about 2.5 km S of San Antonio de Escazu, San Jose Province, Costa Rica (el. about 1450 m). Four matings and four aggressive interactions were videotaped in the field using a Sony Hi8 camcorder with closeup lenses at 30 images/sec. An additional nine matings (one recorded with a National Omnipro camera, also with closeup lenses) and nine aggressive interactions (all recorded) between bee tles collected at about 1500 m on a second species of Solanum were observed in captivity. Feeding, mating (seven more times), and fighting were also observed in captivity under a dissecting microscope on 5 June, when an additional mating was taped. All drawings were traced from videotaped images. Voucher specimens are de posited in the Museum of Comparative Zoology, Harvard University, and the Museo de Entomolog?a of the Universidad de Costa Rica. Accepted for publication 25 August 1995. 2 JOURNAL OF THE KANSAS ENTOMOLOGICAL SOCIETY Results Feeding All of the approximately ten beetles seen on the Solanum sp. near americanum plant 1 June, and the nine beetles seen on 4 June were resting near the tips of shoots which had been broken or chewed off. In every case the beetle was facing the tip. When disturbed by a movement nearby, the beetle quickly swung sideways on the stem so as to place the stem between itself and the object which had moved. Each of the four females was accompanied by a male. The females were all near the tip of a broken shoot, at the basal edge of a portion which had apparently been chewed and where there were many small masses of plant tissue, while the male was just behind the female (Fig. 1 ). Observations in captivity confirmed that both male and female beetles produced this type of damage to plants by chewing on them. Under the microscope a feeding female repeatedly bit the shoot or (more often) a piece of plant tissue which had already been loosened. Just as the beetle seized the plant tissue, an area of the plant very near the beetle's mouthparts became appreciably wetter for a moment, and then immediately drier; it then remained this way for the approx imately 1-2 sec until the beetle released her hold. The beetle usually seized the plant again immediately in a slightly different place. In a pair observed for about an hour in the field, the male repeatedly fed briefly at the tip of the stem. The male moved forward and onto the side of the stem opposite the female, executed low level aggressive behavior such as one or more ventral leg swings (see below); the female (sometimes after a few swings of her own) backed 1-2 body lengths down the stem, and the male moved to the dorsal side of the tip and began to feed. Within 1-5 min the female moved forward again on the opposite side of the stem from the male, jerked her body and swung her hind legs, and the male backed down the stem, allowing her to resume feeding. Courtship and copulation Males and females copulated repeatedly. In two pairs which were observed for 1?1.5 hrs in the field, the male mated, moved back behind the female for up to 30 min, then moved forward and mated again. One pair mated three times in about 90 min. All observations in captivity and in the field were consistent with the following sequence of events. The male approached the female slowly from the rear, raised his anterior end slightly from the stalk, and lunged forward quickly to mount the female (Fig. 1). In at least some cases the male jerked his body several times and stridulated in the period just prior to mounting. With no preliminaries other than perhaps brief contact with his mouthparts against her elytra, the male flexed the tip of his abdomen ventrally and anteriorly until it encountered the female's abdomen. In two cases it was clear that the tip of the male's genitalia first contacted the female's pygidium, and then slid smoothly to its tip, and the male then inserted his geni talia. In no case was there any sign of resistance to intromission by the female. During the first portion of copulation the male repeatedly made sharp jerking movements of his body forward and dorsally (Fig. 2), but was otherwise quiet. Each jerk was accompanied by a small ventral movement of the male's antennae which never brought them into contact with the female (Fig. 2). In captivity body VOLUME 69, NUMBER 1 Fig. 1. A male Macropus armatus mounts a female with little or no preliminary courtship. After slowly moving about one half body length closer to the female from behind, he raised the anterior portion of his body (top), lunged forward (middle?0.07 sec after previous drawing), and mounted her (bottom?0.40 sec after previous drawing). Within 10 sec after the bottom drawing the male inserted his genitalia in the female. 4 JOURNAL OF THE KANSAS ENTOMOLOGICAL SOCIETY Body jerks intromission intromission begins ends J_L intromission begins intromission ends 2 min. Mouthparts tap | Taps with mouthparts i_4 sec. Fig. 2. A male Macropus armatus jerks his body forward and dorsally and lowers his antennae while copulating with the female (dotted lines indicate position 0.07 sec after solid lines). The pattern of body jerks and tapping with the mouthparts during two different copulations of the same pair are shown in the graphs in the middle (each vertical line is a body jerk). The bottom graph shows the pattern of taps with the maxillary palps during a period of mouthpart tapping. jerks were accompanied by stridulation (the noise of a nearby stream precluded observation of stridulation in the field). Toward the end of this first period of the move copulation male's mouthparts began to actively. During the second portion of copulation, the male leaned ventrally to bring his mouthparts into rhythmic contact with the female's elytra, and continued to jerk his body periodically (Fig. 2). Observation of mating pairs under a dissecting VOLUME 69, NUMBER 1 5 microscope showed that tapping with the mouthparts involved stereotyped, rhyth mic ventro-lateral extensions of the male's maxillae. With each extension the tips of the maxillary palps made a single, brief tapping contact with the female's elytra or the setae on their dorsal surfaces. Each cycle lasted about 0.5-1.0 sec (deter mined from video records of two pairs, one in the field, one in captivity). Most of the tapping movement was produced by movements of the mouthparts them selves, but sometimes the male's head also moved ventrally with each tap.