Some locust species have a Jekyll-and-Hyde streak. Normally, they are a cryptic shade of green — shown in the photograph of Schistocerca gregaria, top — and lead solitary, unobtrusive lives. But at high population densities, their colour darkens and they become the gregarious, mobile animals that lay waste to crops. Now Amer Tawfik et al. have identified the hormone that triggers one aspect of this transmogrification (Proc. Natl Acad. Sci. USA 96, 7083–7087; 1999).

Tawfik et al. used an albino mutant of the plague locust Locusta migratoria (middle), which did not darken when crowded. They found that implanting either the brain or the corpus cardiacum — an organ close to the brain — from normal locusts into the albino caused the production of melanin in the insects cuticle, and the normal darkening response. The hormone is probably made in the brain, then transferred to the corpus cardiacum for storage and release.

Next, Tawfik et al. used the albinos to identify the messenger itself. They injected albino locust nymphs with purified extracts from normal locusts: any that darkened had received the active ingredient. It turns out that this is a short peptide, 11 amino acids long, called [His7] corazonin. The bottom photograph shows an albino nymph of L. migratoria after injection with corazonin (the effect on the green S. gregaria is similar). This hormone had already been found in another locust species, but its function was not known.

The hormone, christened ‘dark-colour-inducing neurohormone’, produces its effects even in locusts that are not crowded — the group is going on to investigate its effects on behaviour. Intriguingly, its amino-acid sequence is similar to that of the melanophore-stimulating hormone (MSH) found in mammals, and injecting MSH into albino locusts also causes their colour to darken.