EREZYILMAZ*, D.F.*; RIDDIFORD, L.M.; TRUMAN, J.W.: Juvenile hormone and the origin of insect larvae.

Insects use two different life history strategies. In the ancestral hemimetabolous, or direct developing insects, embryonic development typically produces a miniature version of the adult. The nymph that emerges from the eggshell simply increases in size through the successive nymphal molts, acquiring wings and genitalia at the last instar. In holometabolous insects, by contrast, embryonic development produces a morphologically altered larval stage, and adult development is delayed until metamorphosis. Truman and Riddiford (Nature, 1999. Vol 401; p447) propose that the evolution of holometabolous larvae involved an advancement of juvenile hormone (JH) secretion during embryogenesis. We are testing this hypothesis by exposing hemimetabolous embryos (crickets and locusts) to JH. We find that precocious treatment causes premature differentiation of the embryo. In addition, JH suppresses the normal pattern of axial growth. In the developing leg, for instance, we find that the growth of distal segments is more greatly suppressed than that of proximal ones. This growth defect is due, at least in part, to a suppression of cell division, and we are currently analyzing the relative contributions of cell size and cell shape to this growth defect. The data provided so far describe a scenario in which earlier secretion of JH induces precocious differentiation of features required for life outside of the eggshell, while creating an impetus for morphological divergence that resulted in the holometabolous larval stage. These data also attest to the ability of JH to shift suites of characters with respect to one another and modularize insect life histories.