|308 HETEROCHRONY AND PAEDOMORPHOSIS
cific adaptation, it nonetheless recurs with very little difference in a group totally unrelated to the Cecidomyiidae, the beetle Micromalthus debilis (Scott, 1938, 1941). Micromalthus also has three paedogenetic forms (female-producing, male-producing, and male- and female- producing). (In an interesting variation that might intrigue Gilbert's Mikado, the male-producer gives birth to a single offspring, though several may begin to develop. After adhering to his mother's cuticle for four to five days, the larva inserts his head into her genital aperture and devours her.) The paedogenetic forms live in wet, rotting wood. If the wood dries out, they become sexual and seek a new resource.
Progenesis in Wingless, Parthenogenetic Aphids
Charles Bonnet, the first hero of this book, discovered parthenogenesis in aphids in 1746. Female monoecious aphids are either sexual (oviparae) or parthenogenetic (virginoparae). The parthenogenetic forms are either winged (alatae) or wingless (apterae). Wingless virginoparae have been identified as paedomorphs by several authors (Wigglesworth, 1966, p. 205; Johnson, 1959, p. 96; Mittler, 1973, p. 71; Kennedy and Stroyan, 1959). This assessment is based not only on the absence of wings, but on the coordinated retention of a large suite of juvenile characters.
The potential for rapid increase in population size among parthenogenetic forms is legendary. Again, this is not a function of individual fecundity (few mothers produce more than 100 offspring), but of extremely rapid maturation. Aphids have developed an extreme form of viviparity and ovulation is more a juvenile than an adult function (though aphids do not give birth before they become adult). Embryonic development actually begins in a mother's body before her own birth, and two subsequent generations may be telescoped within each "grandmother." Kennedy and Stroyan write: "Individual fecundity of the virginopara is modest . . . It is the telescoping of the generations which gives aphids their unequalled rates of multiplication and the oft-quoted astronomical totals of theoretical progeny from one female in one year—such as 524 billion for Aphis fabae Scopoli" (1959, p. 140).
Needless to say, parthenogenetic aphids have more rapid generations than sexual forms. Photoperiod seems to be the dominant influence in switching to sexual cycles. In most temperate aphids, the shortening of days induces the production of oviparae, which lay eggs that may overwinter until the spring flush of vegetation.