INSECT SPINASTERNAL MUSCULATURE — CHADWICK 1 23 



in their dorsolateral origins, but the corresponding bands from op- 

 posite sides are inserted separately on the ventral body wall at loci 

 that are mesad of the here widely divergent nerve cords (Sedgwick, 

 1888; Snodgrass, 1938). According to the scanty embryological evi- 

 dence on insects, the transverse muscles in the embryos of Dermaptera 

 and Blattariae (Heymons, 1895) arise as single bands that cross the 

 body at each intersegmental fold (cf. Roonwal, 1937, cited above). 

 During later embryonic development, the abdominal representatives 

 are lost, and the thoracic transverse muscles acquire the typical connec- 

 tions with the spinae. This sequence of events is not universal, since 

 Samtleben (1929) has described both abdominal and thoracic trans- 

 verse muscles in culicid larvae; while Ford (1923) and Maki (1938) 

 record the persistence of transverse abdominal muscles in the adults of 

 a number of pterygote insects. Such muscles are particularly well 

 developed in the first abdominal intersegment of certain cockroaches 

 (Chadwick, 1957). Czihak (1956) notes the presence of a transverse 

 cervical muscle in the adult of Mynneleon. From the descriptions of 

 Thysanura and Collembola given by Carpentier and Barlet in refer- 

 ences already cited, one judges that the transverse muscles of these 

 apterygotes have largely given way to ligamentous derivatives that 

 participate in the formation of the complex endosternum character- 

 istic of these insects. The endosternum here commonly has per inter- 

 segment one or more median ventral attachments, also of a ligamentous 

 consistency, that appear comparable with the spinal structures of 

 pterygote insects. The abdominal endosterna of Lepisma are some- 

 what simpler than their thoracic counterparts, but are otherwise very 

 similar (Barlet, 1954). 



Although a postmetathoracic spina is not found in most Pterygota, 

 the former general existence of such a structure is indicated not only 

 by its presence in certain apterygotes (Maki, 1938; Barlet, 1951, 1953, 

 1954), in the Grylloblattodea (Walker, 1938, 1943). and possibly in 

 Mallophaga (Mayer, 1954), but also by the muscular relationships in 

 other primitive winged insects, in which the postmetathoracic homo- 

 logues of the more anterior spinasternal muscles have a common mid- 

 ventral junction that lies above the nerve cords but is without connec- 

 tion with the integument (Badonnel, 1934; Chadwick, 1957; Kelsey, 

 1957). 



PRESENTATION OF DATA 



It has been impractical to prepare tables that would collate the desig- 

 nations of all the original authors with those adopted in this study. 

 Accordingly, I list in table i the principal spinasternal muscles together 



