5.9. Ovoviviparity and viviparity
Most insects are oviparous, with the act of laying involved in initiation of egg development. However, some species are viviparous, with initiation of egg development taking place within the mother. The life cycle is shortened by retention of eggs and even of developing young within the mother. Four main types of viviparity are observed in different insect groups, with many of the specializations prevalent in various higher dipterans.
- Ovoviviparity, in which fertilized eggs containing yolk and enclosed in some form of eggshell are incubated inside the reproductive tract of the female. This occurs in some cockroaches (Blattidae), some aphids and scale insects (Hemiptera), a few beetles (Coleoptera) and thrips (Thysanoptera), and some flies (Muscidae, Calliphoridae, and Tachinidae). The fully developed eggs hatch immediately after being laid or just prior to ejection from the female’s reproductive tract.
- Pseudoplacental viviparity occurs when a yolk-deficient egg develops in the genital tract of the female. The mother provides a special placenta-like tissue, through which nutrients are transferred to developing embryos. There is no oral feeding and larvae are laid upon hatching. This form of viviparity occurs in many aphids (Hemiptera), some earwigs (Dermaptera), a few psocids (Psocoptera), and in polyctenid bugs (Hemiptera).
- Hemocoelous viviparity involves embryos developing free in the female’s hemolymph, with nutrients taken up by osmosis. This form of internal parasitism occurs only in Strepsiptera, in which the larvae exit through a brood canal (Box 13.6), and in some gall midges (Diptera: Cecidomyiidae), where the larvae may consume the mother (as in pedogenetic development, below).
- Adenotrophic viviparity occurs when a poorly developed larva hatches and feeds orally from accessory (“milk”) gland secretions within the “uterus” of the mother’s reproductive system. The full-grown larva is deposited and pupariates immediately. The dipteran “pupiparan” families, namely the Glossinidae (tsetse flies), Hippoboscidae (louse or wallaby flies, keds), and Nycteribidae and Streblidae (bat flies), demonstrate adenotrophic viviparity.