The response of a homochrome grasshopper, Oedipoda miniata, to the dark-colour-inducing neurohormone (DCIN) of locusts.

The effect of the dark-colour-inducing neurohormone (DCIN=[His(7)]-corazonin) of locusts was investigated in nymphs of the grasshopper, Oedipoda miniata, which exhibit strong homochromy, but neither green-brown, nor phase colour polymorphism. Graded doses of synthetic DCIN were injected in 1 µl of olive oil into 0-24-h-old penultimate nymphs. DCIN induced dose-dependent darkening in the recipients, seen 4 days after injection (still in the penultimate nymphal instar), as well as in the subsequent last-instar nymphs. The dose-range obtained between discernible and maximum (almost black) darkening extended over three orders of magnitude, from 1 pmol to 1 nmol of DCIN. In spite of the darkening observed already in the penultimate nymphal instar, the exuviae in the moult from this to the last nymphal instar did not show marked dark patches. However, the exuviae of the next moult (from last-instar nymph to adult) showed such dark patches, which increased with the increase of the dose, revealing the presence of exocuticular melanin. We conclude that DCIN, or a very close peptide, is the endogenous hormone which plays the major or sole role in the control of homochromy of O. miniata and possibly also of other acridids which exhibit homochromy.