Reproduction

Insects and like arthropods are normally bisexual and require sexual communion or mating (Blum and Blum 1979, Thornhill and Alcock 1983), with subsequent gamete fusion, for reproduction (Davey 1965, Englemann 1970). Only in a few cases has parthenogenesis—and in still fewer cases, hermaphroditism—evolved. The production of normal young by unfertilized females is part of the regular reproductive process in many Homoptera, alternating with the sexual process. Unfertilized eggs may be the means of sex determination in others, such as the honeybee, which produces drones by this method. In the cottony cushion scale (Icerya purchasi), both male and female gonads develop in the female, and self-fertilization takes place.

The gonads and their immediate ducts are almost always paired. The generative organ may be single or multiple in myriapods, derived from mesodermal embryonic tissue. The gonoducts join paired or single ectodermal invaginations that lead to the outside via the gonopore. This may be located either terminally as in most insects or near the base of the abdomen in arachnids.

Male insects and myriapods usually have a complex set of genitalia surrounding the gonopore, an extension of which terminates in an intromittent organ or penis (often called the aedeagus). These genitalia, especially the claspers of one sort or another, are important in locking the pair securely and precisely together while the penis is inserted, forming a physical connection that is normally species specific (Eberhard 1985). They may also play a part in physical or chemical stimulation necessary for successful copulation (their inner surfaces often bear sensillar patches) (Alexander 1964). The gonopore is un-elaborated in spiders, the function of the genitalia being assumed by the pedipalps.

The external female genitalia are relatively simple compared to the male's, but some special structures (ovipositors) may be present for egg placement.

0 0

Post a comment