Insect Progenesis

Prothetely and Metathetely

Entomologists generally make a distinction between two types of juvenilization in adult insects. They speak of prothetely when the adult develops too "early" (perhaps at a precocious molt), leaving some characters in a larval state. When development proceeds at its normal pace, but certain characters retain their juvenile form in the imago, metathetely has occurred (Singh-Pruthi, 1924; Wigglesworth, 1954; Novak, 1966; Matsuda, in press). This classification duplicates the distinction between progenesis (prothetely) and neoteny (metathetely).

Before the elucidation of endocrine control for metamorphosis, entomologists often denied the validity of such a separation, arguing that prothetely and metathetely merely represent the end points of a continuum (which, of course, they do) and that the distinction had no significance either in mechanism or adaptive meaning (just as students of other animal groups often denied a meaningful distinction between progenesis and neoteny).

The demonstration of potentially different mechanisms for the two modes of juvenilization has verified the importance of separating prothetely and metathetely. Southwood (1961) has pointed out that brachyptery (short-wingedness) is often a juvenile trait, especially when it represents part of a polymorphism or spectrum of geographic variability within a species. It may arise in two distinct ways: by excessive influence of the juvenile hormone, leading to juvenile characters in the adult (metathetely); or by depression of juvenile hormone levels, leading to adult characters in the larva (prothetely), perhaps by suppression of a molt (prothoracic glands are often lost in the adult; if a pronounced drop in juvenile hormone induces the adult state at an early molt, later molts are usually suppressed). Southwood discusses

the problem of distinguishing between the former, production of an adult with larval characters, which could perhaps be called neoteny, and the latter, a larva with adult characters, which is a condition homologous with paedogenesis [= progenesis] . . . Such a differentiation may appear artificial, but it does have the important distinction, in the present theory, that the former results from a lengthening of the influence of the juvenile hormone and the latter from a reduction in this period. (1961, pp. 63–64)

If progenesis and neoteny usually have such different efficient causes, then my hypothesis of their separate status is supported.

Southwood (1961) uses instar number to test his hypothesis of different causes for brachyptery. For geographic variation within species, he finds a correlation between metathetelous (neotenic) bra-