Hormonal and Nervous Control of Tanning in the Fly

In the newly emerged fly, darkening and hardening of the cuticle (tanning) starts after 15 to 20 minutes but can be delayed under certain conditions. Ligaturing off the head abolishes tanning. Tanning is initiated by a signal from neurosecretory cells in the brain, transmitted to the thorax via both the central and stomatogastric nervous system. Blood from a 15-minute-old fly induces tanning when injected into the body of a head-ligatured fly. The active factor in blood is probably neither a substrate nor an enzyme necessary for tanning. It could be a new hormone, because it is neither ecdyson, nor the juvenile hormone, nor a brain hormone. This factor is entirely unspecific, being present in newly molted cockroaches ( Periplaneta ) and newly emerged adults of the beetle Tenebrio . A mechanism that inhibits tanning also plays a role in this reaction.