ABSTRACT
Invertebrate animals rely heavily upon neuroendocrine control systems as opposed to classical endocrine systems. Within the vertebrate phyla, there are vast arrays of organs which have an established endocrine function. Insects are similar to crustaceans in the wide variety of physiological functions that are controlled by endocrine organs and the predominance of neuroendocrine control systems. The chemical nature of the hormone is important because ultimately it decides how the hormone exerts its biological effect. Given the chemical nature of these hormones, they would be unable to cross the plasma membrane to influence cellular processes and therefore require a receptor located in the cell membrane. The specificity of hormone action, that is, the fact that only particular cells will be affected by a particular hormone, is determined by the presence or absence of receptors for that hormone in a given cell.
