Ecdysteroid levels associated with epidermal events during worker and soldier differentiation in Macrotermes michaelseni (Isoptera: Macrotermitinae)

Abstract

Radioimmunoassay of ecdysteroid levels coupled with morphological and histological studies have revealed a close association between hormonal titers and epidermal events during worker and soldier differentiation from lighter and heavier third instars of Macrotermes michaelseni, respectively. Two main peaks of ecdysteroid levels were found during the development of these larvae. The first low one occurred on Day 4 in both cases, although that of lighter larvae was short lived (only 1 day) while that of the heavier larvae continued for another day. Epidermal detachment (apolysis) was shown to start earlier in the heavier larvae (Day 4) than in the lighter ones (Day 6), indicating a differential influence of the first ecdysteroid peak on the epidermis (probably modulated by differing levels of juvenile hormones in the two types of larvae). It was also at this time that morphological differentiation of presoldier mandibles was observed. From this interval onward, hormonal levels remained higher and achieved a second peak in the heavier larvae earlier (Day 9–10) than in the lighter ones (Day 11–12). Epidermal events, i.e., complete apolysis, increased cellular growth, new cuticle deposition, and ecdysis, correlated well with the hormonal level increases, those of the lighter larvae always lagging behind by about 2 days. During presoldier-soldier transformation two distinct ecdysteroid peaks were demonstrated, one occurring on Day 2 (low) and the other on Day 9 (high). The third one was transient and not very distinct from the second peak. The significance of these peaks is discussed in the context of differentiation.