Box 51 Courtship and mating in Mecoptera

Sexual behavior has been well studied in hanging-flies (Bittacidae) of the North American Hylobittacus (Bittacus) apicalis and Bittacus species and the Australian Harpobittacus species, and in the Mexican Panorpa scorpionflies (Panorpidae). Adult males hunt for arthropod prey, such as caterpillars, bugs, flies, and katydids. These same food items may be presented to a female as a nuptial offering to be consumed during copulation. Females are attracted by a sex pheromone emitted from one or more eversible vesicles or pouches near the end of the male's abdomen as he hangs in the foliage using prehensile fore tarsi.

Courting and mating in Mecoptera are exemplified by the sexual interactions in Harpobittacus australis (Bittacidae). The female closely approaches the "calling" male; he then ends pheromone emission by retracting the abdominal vesicles. Usually the female probes the prey briefly, presumably testing its quality, while the male touches or rubs her abdomen and seeks her genitalia with his own. If the female rejects the nuptial gift, she refuses to copulate. However, if the prey is suitable, the genitalia of the pair couple and the male temporarily withdraws the prey with his hind legs. The female lowers herself until she hangs head downwards, suspended by her terminalia. The male then surrenders the nuptial offering (in the illustration, a caterpillar) to the female, which feeds as copulation proceeds. At this stage the male frequently supports the female by holding either her legs or the prey that she is feeding on. The derivation of the common name "hangingflies" is obvious!

Detailed field observations and manipulative experiments have demonstrated female choice of male partners in species of Bittacidae. Both sexes mate several times per day with different partners. Females discriminate against males that provide small or unsuitable prey either by rejection or by copulating only for a short time, which is insufficient to pass the complete ejaculate. Given an acceptable nuptial gift, the duration of copulation correlates with the size of the offering. Each copulation in field populations of Ha. australis lasts from 1 to a maximum of about 17 minutes for prey from 3 to 14 mm long. In the larger Hy. apicalis, copulations involving prey of the size of houseflies or larger (19-55 mm2; length x width) last from 20 to 31 minutes, resulting in maximal sperm transfer, increased oviposition, and the induction of a refractory period (female non-receptivity to other males) of several hours. Copulations that last less than 20 minutes reduce or eliminate male fertilization success. (Data after Thornhill 1976; Alcock 1979.)

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Body size (mm)

Fig. 5.2 Relationship between length of horn and body size (thorax width) of male scarabs of Onthophagus taurus. (After Moczek & Emlen 2000; with beetle heads drawn by S.L. Thrasher.)

seeking to oust the resident from the resource-rich nest site, provisioned with dung, his mate, and their young (Fig. 9.5). However, the system is more complicated, at least in the North American Onthophagus taurus. In this dung beetle, male horn size is dimorphic, with insects greater than a certain threshold size having large horns, and those below a certain size having only minimal horns (Fig. 5.2). However, nimble small-horned males attain some mating success through sneakily circumventing the large-horned but clumsy male defending the tunnel entrance, either by evasion or by digging a side tunnel to access the female.

Darwin could not understand why the size and location of horns varied, but now elegant comparative studies have shown that elaboration of large horns bears a developmental cost. Organs located close to a large horn are diminished in size: evidently resources are reallocated during development so that either eyes, antennae, or wings apparently "pay for" being close to a male's large horn. Regular-sized adjacent organs are developed in females of the same species with smaller horns and male conspecifics with weakly developed horns. Exceptionally, the species with the female having long horns on the head and thorax commensur-ately has reduced adjacent organs, and a sex reversal in defensive roles is assumed to have taken place. The different locations of the horns appear to be explained by selective sacrifice of adjacent organs according to species behavior. Thus, nocturnal species that require good eyes have their horns located elsewhere than the head; those requiring flight to locate dispersed dung have horns on the head where they interfere with eye or antennal size, but do not compromise the wings. Presumably, the upper limit to horn elaboration either is the burden of ever-increasing deleterious effects on adjacent vital functions, or an upper limit on the volume of new cuticle that can develop sub-epidermally in the pharate pupa within the final-instar larva, under juvenile hormonal control.

Size alone may be important in female choice: in some stick-insects (also called walking sticks) larger males often monopolize females. Males fight over their females by boxing at each other with their legs while grasping the female's abdomen with their claspers (as shown for Diapheromera veliei in the vignette for this chapter). Ornaments used in male-to-male combat include the extraordinary "antlers" of Phytalmia (Tephritidae) (Fig. 5.3) and the eyestalks of a few other flies (such as Diopsidae), which are used in competition for access to the oviposition site visited by females. Furthermore, in studied species of diopsid (stalk-eyed flies), female mate choice is based on eyestalk length up to a dimension of eye separation that can surpass the body length. Cases such as these provide evidence for two apparently alternative but likely non-exclusive explanations for male adornments: sexy sons or good genes. If the female choice commences arbitrarily for any particular adornment, their selection alone will drive the increased frequency and development of the elaboration in male offspring in ensuing generations (the sexy sons) despite countervailing selection against conventional unfitness. Alternatively, females may choose mates that can demonstrate their fitness by carrying around apparently deleterious elaborations thereby indicating a superior genetic background (good genes). Darwin's interpretation of the enigma of female choice certainly is substantiated, not least by studies of insects.

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