Hypotheses regarding the function of elaborate male genitalia were tested in a sample of insects and spiders by comparing their allometric values (slopes in log-log regressions on indicators of body size) with those of other body parts. Male genitalia consistently had lower slopes than other body parts. Perhaps as a consequence of this pattern, genitalic size also tended, though less consistently, to have lower coefficients of variation than did the size of other body parts. The morphological details of coupling between males and females in several species clearly indicated that selection favoring mechanical fit is not responsible for these trends. Sexual selection on male courtship structures that are brought into contact with females in precise ways may favor relatively low allometric values, in contrast to the high values seen in the other sexually selected characters (usually visual display devices) that have been studied previously, because a female's own size will influence her perception of the contact courtship devices of a male.

Abstract.-Hypotheses regarding the function of elaborate male genitalia were tested in a sample of insects and spiders by comparing their allometric values (slopes in log-log regressions on indicators of body size) with those of other body parts. Male genitalia consistently had lower slopes than other body parts. Perhaps as a consequence of this pattern, genitalic size also tended, though less consistently, to have lower coefficients of variation than did the size of other body parts. The morphological details of coupling between males and females in several species clearly indicated that selection favoring mechanical fit is not responsible for these trends. Sexual selection on male courtship structures that are brought into contact with females in precise ways may favor relatively low allometric values, in contrast to the high values seen in the other sexually selected characters (usually visual display devices) that have been studied previously, because a female's own size will influence her perception of the contact courtship devices of a male.Key lVords.-Allometry, genitalia, sexual selection.Received February 14, 1997. Accepted December 9, 1997 Animal genitalia show unusual patterns of evolution, and many hypotheses have been proposed to explain these patterns (summary Eberhard 1985; also Shapiro and Porter 1989; Alexander et al. 1997). Although male genitalia may often evolve under sexual selection by cryptic female choice (Eberhard 1985(Eberhard , 1996, they might also show positive allometric relationships to body size like many other sexually selected traits (e.g., Petrie 1988Petrie , 1992 Alatalo et al. 1988; Meller 1991;Green 1992;Burkhardt et al. 1994). This latter possibility has never been systematically studied.Genitalic allometries might be important for female choice in the following manner. Large male size may be associated with superior abilities to accumulate resources and to survive (Andersson 1995). According to a "good viability genes" model of male genitalic evolution, females might use male genitalia to evaluate overall male size, and choose sires with superior viability genes by favoring males with larger genitalia. This could result in selection that favors males with relatively large genitalia. Analysis from the female point of view yields a similar conclusion. A male's genitalia would be an especially useful cue to a female attempting to judge the male's overall size if the genitalia of larger males were disproportionately large. If, on the other hand, the slope of male genitalic size on body size is low, female attempts to judge male size would be expected to utilize other cues rather than the male's genitalia.Another hypothesis regarding genitalic evolution also predicts relatively high allometric values. Male genitalia may function as weapons in male-female conflicts over control of events associated with copulation (Lloyd 1979; Alexander et al. 1997). In as much as these conflicts are decided by force, then male genitalia should have relatively high allometric values, just as ...

If species-specific male genitalia are courtship devices under sexual selection by cryptic female choice, then species-specific aspects of the morphology and behaviour of male genitalia should often function to stimulate the female during copulation. The morphology and behaviour of the complex, species-specific male genitalia of the tsetse fly, Glossina pallidipes Austen, were determined from both direct observations and dissections of flash-frozen copulating pairs; we found that some male genitalic traits probably function to stimulate the female, while others function to restrain her. The male clamps the ventral surface of the female's abdomen tightly with his powerful cerci. Clamping does not always result in intromission. Clamping bends the female's body wall and her internal reproductive tract sharply, posteriorly and dorsally, and pinches them tightly. The male performed sustained, complex, stereotyped, rhythmic squeezing movements with his cerci that were not necessary to mechanically restrain the female and appeared instead to have a stimulatory function. Six different groups of modified setae on and near the male's genitalia rub directly against particular sites on the female during squeezing. The designs of these setae correlate with the force with which they press on the female and the probable sensitivity of the female surfaces that they contact. As expected under the hypothesis that these structures are under sexual selection by female choice, several traits suspected to have stimulatory functions have diverged in G. pallidipes and its close relative, G. longipalpis. Additional male non-genitalic behaviour during copulation, redescribed more precisely than in previous publications, is also likely to have a courtship function. The elaborate copulatory courtship behaviour and male genitalia may provide the stimuli that previous studies showed to induce female ovulation and resistance to remating.

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We observed courtship and aggressive behavior in males of 13 species in four families of earwigs in order to determine the functional significance of sexually dimorphic morphology of cerci and associated structures in males. Criteria for determining function included appropriateness of design and consistency of use. In 12 species, at least 17 secondary sexual modifications of male cerci and associated structures appeared to serve in aggressive interactions between males, as weapons and/or as display devices. In two species, modifications of male cerci served to clasp females just prior to and during copulation; in one case, they were used to force copulations on females; in the other, they apparently were used as a courtship device. Functions could not be assigned confidently to many minor features, such as positions of teeth on the cerci. OFFICIAL PUBLICATION DATE is handstamped in a limited number of initial copies and is recorded in the Institution's annual report, Smithsonian Year. SERIES COVER DESIGN: The coral Montastrea cavernosa (Linnaeus). Library of Congress Cataloging-in-Publication Data Briceno, R. Daniel. The functional morphology of male cerci and associated characters in 13 species of tropical earwigs (Dermaptera:Forficulidae.