The functional morphology of species-specific clasping structures on the front legs of male Archisepsis and Palaeosepsis flies (Diptera, Sepsidae)

Abstract

Several possible explanations for the elaborate species-specific morphology of male front leg clasping organs were tested by comparing six species of Archisepsis, Palaeosepsis and Micmsepsis flies. The only previously published hypothesis regarding these clasping organs was refuted by the finding that species-specific portions of the male femur and tibia consistently meshed tightly with prominent veins and folds in the female's wing, rather than meshing with each other. Female wing morphology in the region grasped by the male was relatively uniform and in general did not vary in ways that would prevent non-conspecific males from grasping them, arguing in all but one species against both simple lock-and-key and male-female conflict of interests hypotheses based on morphology. Interspecific differences in male front leg morphology generally represent alternative ways to accomplish the same basic mechanical function of holding tightly onto the relatively invariant female. Despite the fact that female resistance behaviour indicates that male-female conflict over male mounting is common, only one female wing structure in one species resembled an anti-clasper device, giving a second reason to doubt the morphological male-female conflict of interest hypothesis, at least for five of the six species. The positions of probable sensory structures on the wings of females were relatively similar in different species and did not correspond in any obvious way to species-specific features of male clasping structures. This, plus the intraspecific variation in both the positions of these sensilla and the exact site where the male grasped the female's wing, argued against simple 'sensory lockand-key' ideas about male front leg function. By a process of elimination, it appears that generalized female receptors are able to sense species-specific differences in male front legs. This idea was supported by increased female rejection behaviour in cross-specific pairs. (C= 2001 The Linnean Society of London ADDITIONAL KEY WORDS: genitalic evolutionsexual selectioncryptic female choicelock-and-keymale-female conflict.

Evolution of genitalia: theories, evidence, and new directions

“…The male's clamp fits very precisely with the female's wing (Eberhard 2001a), but experimental modification of the male's clamp did not impair his ability to hold onto the female with his front legs (on the basis of the durations of riding times; Eberhard 2002a), arguing against a SAC interpretation. Female receptors that sense stress in the cuticle occur in the area contacted by the male's front leg in this species (Eberhard 2001a; as well as in other sepsid species with species-specific male front legs- Ingram et al 2008), and could thus enable her to sense his grip, supporting a CFC interpretation. The female's wing base is quite sturdy, and there were no signs of damage (a possible prediction of SAC).…”

“…Female sense organs are also possible, though not necessarily predicted, on rigid structures that are contacted by species-specific male structures (e.g., the wing bases of sepsid flies). This prediction thus constitutes a strong test of CFC for some types of female structures, but female sense organs have almost never been studied (see, however, Battin (1993) and Robertson and Paterson (1982) on the thorax of damselflies; Córdoba-Aguilar (2005) on the oviduct of a damselfly; Eberhard (2001aEberhard ( , 2005 and Ingram et al (2008) on the wings of sepsid flies; M. Djernaes et al (unpublished) on genital sclerites in four species of cockroaches). It is also not clear whether females utilize generalized receptors with other functions that were already present in the area that is contacted by the male, or whether they tend to evolve special sensors whose placements or other characteristics coevolve with the form of the male.…”

“…Parenthetically, male damage to female wings by clasping them was claimed in another sepsid with similar male grasping devices (Mühlhäuser and Blanckenhorn 2002), but the wing damage that was observed was in other parts of the wing, and likely occurred when flies beat their wings against the walls of their small glass containers (see Baena and Eberhard 2007). In addition, in only one of the [10 sepsid species (in the genera Archisepsis, Microsepsis, Palaeosepsis, Sepsis, and Themira) is there any even potentially defensive modification of the female's wing in the area where the species-specific modifications of the male's front legs grasp her (Eberhard 2001a(Eberhard , 2005Ingram et al 2008).…”

“…Take, for instance, the sepsid flies, a relatively well-studied group in which SAC seems especially unlikely to explain the present-day morphology of the modified male front legs that clamp the female's wing. The area near the base of the female wing that is contacted by the elaborate, species-specific clamping devices on the males' front legs is relatively uniform in different species and genera, and seldom has potentially defensive traits, so female morphological coevolution with males has generally not occurred (Eberhard 2001a;Ingram et al 2008). In addition, experimental data support the female choice rather than the SAC hypothesis (Eberhard 2002a).…”

Hairy kisses: tactile cheliceral courtship affects female mating decisions in Leucauge mariana (Araneae, Tetragnathidae)

“…In addition, such modifications can test for the effects of stimulation per se, as opposed to other possible physically coercive effects that the male may have on the female (Arnqvist and Rowe 2005). In addition, simple morphological measurements such as the comparative abundance of female tactile sense organs in the area contacted by the male, compared with adjacent areas on the same female (Eberhard 2001) and with those of the male and of the females of related species (Córdoba-Aguilar 1999Ingram et al 2008), can reveal evolutionary adjustments of females to male stimulation. Documenting such male-female coevolution is particularly important for sexual conflict hypotheses, which predict frequent coevolutionary reduction in female sensitivity to male stimulatory structures that diverge rapidly (e.g., Córdoba-Aguilar 2002).…”