Comparative endocrinology helps to find the roots of homeostatic regulation in organisms. In this context, many years ago a series of experiments were done, which demonstrated the hormonal regula tion also on the invertebrate level. The mechanisms are partly similar, partly different, from those found in vertebrates. The new receptor era of mammalian endocrinology stimulated research on invertebrate hormone receptors, and sophisticated methods are applied also to determine hormones. The experiments demonstrated the existence and even similar function of these structures and signaling...