The alimentary tract of arostrate, dwarfish males of the aphid genus Stomaphis (Insecta, Hemiptera)

Abstract

Dwarfish males are a relatively common phenomenon in animals, occurring in various taxa, most often when females conduct a sedentary life mode. Such males, beside smaller size, exhibit a series of morphological and anatomical alterations, mostly structural reductions. Here we present the case of the alimentary tract of dwarfish males of the aphid genus Stomaphis where, despite it being structurally normal, it does not serve its original function due to a lack of mouthparts. Cross-sections through mounted specimens revealed also that nervous and reproductive systems are fully developed. The question arises as to whether such a system may be considered vestigial, or an exaptation with secondary function shifted to play new primary function. Because the aphid genus Stomaphis is known from the fossil record to have existed for at least ca. 14 My and all of its species have dwarfish, arostral males, this indicates that it may be too short a period for reduction of the whole system. It also raises questions about the mechanisms of loss of the primary functionality of the whole physiological unit, and the influence of such modification on the evolution of its phylogenetic successors. Secondary functions of the tract are speculated to be responsible for its endurance.