Notched Sternite: a Mutant of Blattella germanica, with Possible Implications for the Homology and Evolution of Ventral Abdominal Structures1

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Abstract
Evidence from various sources indicates that blattids are dose to the stem which gave rise to the Pterygota. In addition, they still retain morphologic characteristics present in the more generalized of the living Apterygota. The notched-sternite mutation of Blattella germanica (L.) results in the appearance of additional body structures resembling those which presumably characterized ancestral insects, and which are found today among the Apterygota. These include 2 pairs of valvulae (gonapophyses) in females, styli on the eighth abdominal segment, apparent rudiments of styli on some of the pregenital segments, and a partial or complete division of the abdominal sterna into 3 plates. Mutant females are also characterized by mid-ventral membranous outgrowths in the pregenital segments. The only structures present in living insects to which these outgrowths seem at all comparable are the eversible sacs of apterygotes. A detailed study of the development and morphology of the mutant structures leads to a number of suggestions concerning the homology and evolution of some of the ventral abdominal structures. These include the proposals that structures of the genital chamber which lie anteriorly to the genital papilla belong to the seventh segment, that the first valvifer arms are homologous with the laterosternal shelf, and the first and second valvulae are homologous with eversible sacs in the sense that they appear to have developed from common antecedent structures. It is suggested also that the coxopodites or coxosternites of blattids may be composite structures, in that they include the fused elements of the telopodite as well as the coxopodite of a primitive appendage.

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