Diversity of neurohormonal release sites in insects: Coexistence of two different (α and β) types of neurohemal structure in the perisympathetic organs

Abstract

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 (α and β), 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 α structure constituting the median organ and the β structure, the two transverse organs. Each of the two structures have special features: the β 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 α structure, on the contrary, forms dense compact organs whose endings are covered with a continous 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 α and β structures are generally present in insects, thus showing their importance in the physiology of these animals.