5.10.1. Parthenogenesis, pedogenesis (paedogenesis), and neoteny
Some or a few representatives of virtually every insect order have dispensed with mating, with females producing viable eggs even though unfertilized. In other groups, notably the Hymenoptera, mating occurs but the sperm need not be used in fertilizing all the eggs. Development from unfertilized eggs is called parthenogenesis, which in some species may be obligatory, but in many others is facultative. The female may produce parthenogenetically only female eggs (thelytokous parthenogenesis), only male eggs (arrhenotokous parthenogenesis), or eggs of both sexes (amphitokous or deuterotokous parthenogenesis). The largest insect group showing arrhenotoky is the Hymenoptera. Within the Hemiptera, aphids display thelytoky and most whiteflies are arrhenotokous. Certain Diptera and a few Coleoptera are thelytokous, and Thysanoptera display all three types of parthenogenesis. Facultative parthenogenesis, and variation in sex of egg produced, may be a response to fluctuations in environmental conditions, as occurs in aphids that vary the sex of their offspring and mix parthenogenetic and sexual cycles according to season.
Some insects abbreviate their life cycles by loss of the adult stage, or even both adult and pupal stages. In this precocious stage, reproduction is almost exclusively by parthenogenesis. Larval pedogenesis, the production of young by the larval insect, has arisen at least three times in the gall midges (Diptera: Cecidomyiidae) and once in the Coleoptera (Macromalthus debilis). In some gall midges, in an extreme case of hemocoelous viviparity, the precocially developed eggs hatch internally and the larvae may consume the body of the mother-larva before leaving to feed on the surrounding fungal medium. In the well-studied gall midge Heteropeza pygmaea, eggs develop into female larvae, which may metamorphose to female adults or produce more larvae pedogenetically. These larvae, in turn, may be males, females, or a mixture of both sexes. Female larvae may become adult females or repeat the larval pedogenetic cycle, whereas male larvae must develop to adulthood.
In pupal pedogenesis, which sporadically occurs in gall midges, embryos are formed in the hemocoel of a pedogenetic mother-pupa, termed a hemipupa as it differs morphologically from the “normal” pupa. This production of live young in pupal pedogenetic insects also destroys the mother-pupa from within, either by larval perforation of the cuticle or by the eating of the mother by the offspring. Pedogenesis appears to have evolved to allow maximum use of locally abundant but ephemeral larval habitats, such as a mushroom fruiting body. When a gravid female detects an oviposition site, eggs are deposited, and the larval population builds up rapidly through pedogenetic development. Adults are developed only in response to conditions adverse to larvae, such as food depletion and over-crowding. Adults may be female only, or males may occur in some species under specific conditions.
In true pedogenetic taxa there are no reproductive adaptations beyond precocious egg development. In contrast, in neoteny a non-terminal instar develops reproductive features of the adult, including the ability to locate a mate, copulate, and deposit eggs (or larvae) in a conventional manner. For example, the scale insects (Hemiptera: Coccoidea) appear to have neotenous females. Whereas a molt to the winged adult male follows the final immature instar, development of the reproductive female involves omission of one or more instars relative to the male. In appearance the female is a sedentary nymph-like or larviform instar, resembling a larger version of the previous (second or third) instar in all but the presence of a vulva and developing eggs. Neoteny also occurs in all members of the order Strepsiptera; in these insects female development ceases at the puparium stage. In some other insects (e.g. marine midges; Chironomidae), the adult appears larva-like, but this is evidently not due to neoteny because complete metamorphic development is retained, including a pupal instar. Their larviform appearance therefore results from suppression of adult features, rather than the pedogenetic acquisition of reproductive ability in the larval stage.