In all of these systems entail a general mechanism. A signal molecule is released from a cell, and the action of the signal molecule follows interaction of the molecule with a specific receptor protein. Peptide hormones are usually very short chains of amino acids. Others are commonly a dozen or so amino acids in many insect and vertebrate peptide hormones. Insulin is a protein hormone composed of over 100 amino acids linked cysteine residues in two chains.
Prostaglandins are oxygenated metabolites of certain polyunsaturated fatty acids.
Peptides, prostaglandins and a few other classes of signal molecules share the physiological feature that their actions are mediated by way of specific receptor cites located on cell surfaces. For example, DH is released from neurosecretory cells into hemolymph, and it acts only on Malpighian tubules and on hindgut. This hormone does not act on brains or on reproductive tissues or on flight muscles. What these questions amount is wondering how a hormone signal is transduced into cellular events. The whole business is known as signal transduction mechanisms.