Il8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I37 



1938) to have a third spina, and possibly also in Mallophaga (Mayer, 

 1954). The spinal apophysis intrudes between the nerve cords from a 

 sclerotized area of variable extent, the spinasternite, which in the 

 thorax may be distinct from or fused to a greater or less degree with 

 the preceding or succeeding segmental sclerite. Many Pterygota now 

 have two such spinae, others one, and some none ; in all these insects, 

 the abdominal counterparts of the thoracic spinasternites are said to 

 be included in the antecostae of the definitive abdominal sterna (Snod- 

 grass, 1929). The former cervical spina of most insects has evi- 

 dently been incorporated into the head, or even wholly lost; but its 

 fate apparently varies in different species and has not been thoroughly 

 studied. 



In this paper, all spinasternal muscles are regarded as intersegmen- 

 tal, since they have at least one of their attachments on what is con- 

 sidered the morphological equivalent of a primary intersegmental 

 groove. For similarly practical reasons, I shall refer here to all at- 

 tachments of muscles on the furcal arms as segmental, although I 

 have previously (Chadwick, 1957) argued that at least some of the 

 furcal attachment sites are morphologically intersegmental. 



SOURCES AND TREATMENT OF DATA 



The literature now provides adequate though sometimes imperfect 

 descriptions of thoracic structure for representatives of the Thy- 

 sanura and 24 pterygote orders. I have checked or supplemented 

 many of these observations by gross dissection of the same or closely 

 related species, and assume responsibility for any interpretations or 

 statements of fact in this paper that are not credited specifically to 

 others. I must also bear the burden of all mistakes. Apart from my 

 own failures and possible errors by the original authors, the necessity, 

 for comparison, of translating into a single idiom the varied designa- 

 tions used by different writers for the several muscles has opened 

 the way for misunderstandings on my part. Besides, there are in- 

 stances where different workers disagree as to the facts, and there are 

 others where the homologies of the parts that bear the muscles are un- 

 clear, or even in dispute. Fortunately, only a small fraction of the 

 data is subject to uncertainty from these causes and, though some 

 future corrections of detail are to be expected, they should have little 

 impact on the general import of the observations. 



There is still need for descriptive study in some areas. Among the 

 Apterygota, it is only for Lepisma (Barlet, 1951, 1953, 1954) that we 

 have sufficiently accurate detailed descriptions of both skeleton and 



