The thoracic skeleton and musculature of the adult bittacid mecopteron Bittacus strigosus Hagen is described. In its musculature, Bittacus shows only moderate differences from two panorpids (Neopanorpa, Panorpa) that have been studied by Maki ('38) and by Hasken ('39), respectively. Not only are these three genera much alike in their musculature generally, but in all of them, and in Boreus (Boreidae) too, the mesothorax is extremely similar to the metathorax. Functional emphasis (for flight) on either of the two pterothoracic segments has not appeared among neuropteroid insects at the metopteran evolutionary level.
Although the "snowfleas" of the genus Boreus possess striking alterations of pterothoracic structure in comparison with other mecopterons (Fuller, '54, '55), these are related to their unusual activities arid have not, to any great extent, affected the two pterothoracic segments differentially.
In terms of thoracic specialization, the overall mecopteran pattern represents a stage somewhat advanced beyond the primitive conditions exempIified by the Megaloptera and certain coleopterous larvae, but one that is in general less highly developed than is charatceristic of such neuropteroid orders BS the Siphonaptera, Diptera, Trichoptera, and Lepidoptera.