Growth and Retrogression in Larvae of Trogoderma glabrum (Coleoptera: Dermestidae). 3. Ecdysis and Form Determination1

Abstract

The number of larval stadia occuring in the life cycle of Trogoderma glabrum (Herbst) is an indeterminate characteristic. Under optimal, uninterrupted rearing conditions, there may be as few as 5 (males) or 6 (females) larval stadia, but there is no apparent species-typical maximum number of ecdyses possible. Under retrogressive conditions, ecdyses continue to occur and are accompanied by a reduction in larval size and weight. Under conditions that permit growth but prohibit pupation, a stable maximum larval size is attained (Dauer-larvae), and ecdyses continue to occur, but without significant changes in larval weight or dimensions. Different but relatively constant frequencies of ecdyses were observed under each of the 3 culturing conditions tested (progressive growth, retrogressive growth, and Dauer-larval conditions). The rates of occurrence of the molting cycle appeared to be determined by the general metabolic rates of the larvae, rather than by nutritional state or by increment of weight change (either positive or negative). Determination of post-ecdysial form is controlled, at least in part, by the titer of juvenile hormone (JH). JH at a concentration of 70 ppm in the dietary medium effectively inhibited pupation of regrown, previously retrogressed, larvae under conditions otherwise known to stimulate pupation. Exogenous JH also stimulated larval growth. Wing anlagen and other imaginal discs could not be demonstrated in larvae prior to their having been committed to pupation, supporting the interpretation that the duration of larval life is indeterminate and that an indefinite series of larval ecdyses is not complicated or limited by a concurrent differentiation of adult structures.