Pioneering experiments of hormonal action and control in insects were done on the bloodsucking bug Rhodnius prolixus, which undergoes incomplete metamorphosis. Newly hatched Rhodnius lack certain adult features and molt five times before developing into a mature adult. A blood meal triggers each episode of molting and growth. Rhodnius is an amazingly hardy experimental animal—it survives for quite a long time after its head is cut off. If decapitated within an hour after a blood meal, Rhodnius can survive for up to a year, but it never molts. If decapitated a week after its blood meal, however, it does molt into an adult.

This observation suggested that molting was triggered by some substance slowly diffusing from the head. To test this hypothesis, two decapitated bugs–one soon after a blood meal and one a week after a blood meal–were connected together with a glass tube that allowed body fluids to cross between the two bugs. Both molted into adults proving that a diffusible substance from the bug’s head triggered molting.

Since those pioneering experiments on Rhodnius, the chemical nature of the diffusible substances (hormones) controlling molting in arthropods have been identified and their functions described.