|306 HETEROCHRONY AND PAEDOMORPHOSIS
custs may be metathetelous in Southwood's sense (enhanced juvenile hormone for paedomorphosis with an extended period of development). But progenetic gall midges and aphids are not prothetelic; they are not precocious adults that happen to retain some larval characters. They are, at least for gall midges, true larvae with completed sexual maturity. Their precocious maturation is not a standard effect of juvenile hormone, for it involves none of the increased somatic differentiation that accompanies the precocious development of adults by lowered levels of juvenile hormone. As larvae, these gall midges maintain high titers of juvenile hormone. Their parthenogenesis is a true disruption of the normal correlation between somatic differentiation and reproductive maturation.
Paedogenesis (Parthenogenetic Progenesis)
The last of von Baer's contributions to the study of heterochrony (1866) may seem trifling compared with his earlier accomplishments: he gave the name Paedogenesis to a remarkable method of parthenogenetic reproduction in gall midges (Diptera, family Cecidomyiidae). These insects can reproduce by the normal route, with a full set of larval and pupal molts culminating in the production of winged, ovipositing images. But they can also propagate as larvae by viviparous parthenogenesis: larvae of the next generation grow within the body of their larval mother and devour her tissues (previously liquified for them by maternal history sis). When the larvae emerge, already prepared to rear the next generation, their own parent is often no more than a hollow shell. Greater love hath no woman.
Paedogenesis has arisen in at least four distinct groups of the Cecidomyiidae (Wyatt, 1967, p. 95). There can be no doubt that these larval paedomorphs are progenetic rather than neotenic, for their development is markedly accelerated relative to the sexual phase of their life cycle. The paedogenetic cycle of Miastor metraloas includes one larval instar with a single molt to the reproductive hemipupa. Up to 60 young may be reared by a single mother in 12 days. The sexual cycle, by contrast, includes two larval and one pupal molt before the emergence of a sexual imago (Wyatt, 1967). Mycophila speyeri undergoes only one molt and reproduces as a true larva; up to 38 offspring may emerge in a cycle of only five days (although 19 young in six days is the average). The sexual adults require two weeks to develop (Wyatt, 1964). In laboratory cultures of the same species, Ulrich et al. (1972) obtained up to 47 offspring from a single larval mother in four days.
These progenetic larvae maintain a phenomenal capacity for rapid