When a growing insect shrugs off its restrictive outer layer, ready to replace it with a looser fitting cuticle, the process is tightly choreographed by hormone peptides that trigger a set of pre-programmed manoeuvres to rid the insect of the unwanted layer. Dusan Žitňan and colleagues had already studied ecdysis in fruit flies and two moth species, and discovered that two of the key hormones involved in the three insects' life changes are released by a group of cells in their tracheas, called Inka cells. But would other insects use the same mechanism for triggering their own instar-to-instar transition?
Working with an international team of researchers, Žitňan began testing 26 species, ranging from primitive silverfish and dragonflies up to relatively advanced insects such as ants, for evidence of peptides that might trigger ecdysis (p. 1275). They tested Inka cells from each insect's tracheae, and found that the cells produced hormone peptides that were structurally similar to the moth and fruit fly ecdysis hormones. The team also injected silk moth larvae with tracheal extracts from several different insects to see whether the peptides could produce ecdysis; the silk moths began shedding their skins,proving that the extracts contained ecdysis-triggering hormones. But when the team looked at the number, size and distributions of Inka cells across the species, each insect had its own unique cell pattern and distribution, even though the hormones they produce are closely related.