136 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I37 



developmental studies offer hope of direct information as to the true 

 relationships. Meanwhile, one had best recognize the existence of 

 both types of muscle, while deferring an opinion as to their homology. 



d. Spinas pinal muscles: 



13. isps-2sps 2sps-ssps ssps-Isps 



13. The spinaspinal muscles are limited in their known distribution 

 to Thysanura (Barlet, 1953, Nos. ^6, 44, and probably 34, in Lepisma) 

 and to a few orthopteroid orders. Larsen's (1945c) citation of Speyer 

 (1922) as an authority for their presence in larval Dyfiscns is appar- 

 ently an error. 



The bands isps-2sps are present in blattids (Chad wick, 1957, 

 No. 12), in nearly all the true Orthoptera and Mantodea that have 

 been studied, and in Stcnopsocus (Badonnel, 1934, No. LVM2). Maki 

 (1938) did not find them in Psocus. Chadwick (1957) failed to find 

 these muscles in nymphal Tenodera (Mantodea), but did find vestiges 

 of 2sps-jsps there ; this element is relatively well developed in cock- 

 roaches (No. 2j). Lepisma is the only other insect in which 2sps-^sps 

 is known. Certain usually weak muscular strands that connect "ssps" 

 of cockroaches with the ventral diaphragm may be serially homologous 

 with the preceding spinaspinal muscles and with Barlet's No. 54 in 

 Lepisma. 



Given the absence of a osps in most insects, one is not surprised 

 that the muscle osps-isps is ordinarily missing. However, what may 

 be this muscle has been recorded by Maki (1938) as No. 28 in Lepi- 

 docampa (Diplura), which seems to possess a cervical spina. 



Originally, the spinaspinal muscles are paired bilaterally, but the 

 two bands may coalesce during development to such an extent as to 

 become indistinguishable. The present distribution of these muscles, 

 as well as their location, suggests that their retention is a primitive 

 characteristic, although they are notably absent in several otherwise 

 unquestionably primitive forms. 



CONCLUSIONS 



Our principal conclusions have been anticipated to some extent in 

 the manner of presentation of the data and in the discussion of indi- 

 vidual muscle types. Thus, the mere fact that it is possible to construct 

 such a summary as table i seems to establish the point that all the 

 insects we have examined must be descendants of a common ancestral 

 stock, whose spinasternal musculature included most of those elements 

 that, as we have seen, are somewhat sporadically distributed among 



