Neuropterid male terminalia revisited, ground-plan and the gonarcus as a key element to understand morphological diversity

The insect ground-plan, incorporating segmentation and appendicular serial homology, provides a basic framework to approach the structural complexity of male neuropterid terminalia. Otherwise, lacking appendages, the abdomen typically displays paired ninth gonostylus (gonopods) and paired tenth gonocoxite plus gonostylus (fused into a gonarcus), there may also be tenth tergites (ectoproct), and essentially no elements of the eleventh segment. Locating and establishing identity of the tenth gonocoxites (basal component) of the gonarcus is crucial, as they may have an associated mediuncus, and a closely articulated ninth gonostylus at each base of lateral apodeme. Often detached, posteroventrally close tenth gonostyli (distal component) may be simple or modified, even fused, becoming an acute process or a tubular structure (e.g., a penisfilum). A detached distal component of the gonarcus may develop its own apodemes, appearing as a secondary gonarcus or a deceivingly serially homologous structure. Considerable structural variation of these basic components of male terminalia across families challenges a correct identification. Broad opportunities in this line of research, individually or preferably through comparative approaches are foreseen.