Diversity of neurohormonal release sites in insects: coexistence of two different (alpha and beta) types of neurohemal structure in the perisympathetic organs.

A comparative ultrastructural study of insects was made whose primitive perisympathetic organs consisted either of a single median neurohemal formation per segment (Periplaneta americana) or of two transverse formations (Carausius morosus), or again, of three--one median and two transverse--as in Locusta migratoria. The results showed that the perisympathetic organs always comprised two types of structure (alpha and beta), even when a single formation exists. In Periplaneta and Carausius, both types were seen to coexist in the same neurohemal formation. In Locusta, however, they were separated, the alpha structure constituting the median organ and the beta structure, the two transverse organs. Each of the two structures have special features: the beta structure displays the usual characteristics of a loose neurohemal organ, i.e., it is penetrated by sinuses and has neurosecretory endings devoid of a glial coat; the alpha structure, on the contrary, forms dense compact organs whose endings are covered with a continuous glial layer. During the evolution, this last structure gradually separated from the other neurohemal formations. It was observed to correspond to a single type of neurosecretory cell, distinct from all other cell types by the particular mode of its release. These results for primitive perisympathetic organs confirm earlier findings for advanced organs. They indicate that both the alpha and beta structures are generally present in insects, thus showing their importance in the physiology of these animals.