4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL, I3I 



derived from former muscles, ^sps-fus. A true third spina, which oc- 

 curs in some Apterygota (Maki, 1938), is known among pterygote 

 insects only in Gryllohlatta (Walker, 1938, 1943), but an arrange- 

 ment much like that of blattids has been reported for larval Dytiscus 

 (Speyer, 1922) and has also been seen in larval Cybister, in Zoo- 

 termopsis (fig. 6), and in larval and adult Corydaliis. Other vestiges 

 of the third spina and its musculature have been found but not recog- 

 nized as such by several students in a number of other insects. Alto- 

 gether, the facts constitute strong evidence that a third spina was 

 present in the ancestral Pterygota, and probably in early hexapods 

 generally. 



More thoroughly documented, since much of the testimony is still 

 available in a variety of living forms, is a general tendency toward 

 loss of the remaining spinae and their associated musculature during 

 the post-Carboniferous evolution of the pterygote thorax. Certain 

 cockroaches, however, have gone contrary to this trend, and have 

 experienced a prodigious extension of the first spina and, to a lesser 

 extent, the second, in the direction of the body axis. Compare, for 

 example, figure i with figure 2, or 17 with 18. This spinal elongation 

 is related, in part, to hypertrophy of the transverse spinal musculature 

 (isps-epSz, 2sps-epss) ; and it is probably no coincidence that sev- 

 eral species that manifest this development have also exceptionally 

 large transverse muscles of the first abdominal intersegment, sua- 

 SiiA (figs. I, 8, II, 12 : S4)- My judgment that these characteristics are 

 secondary rests partly upon the fact that they are peculiar to some 

 blattids, being unparalleled in others and absent, so far as I know, in 

 any other group of insects ; and partly upon the presence in the mus- 

 culature of these same cockroaches of other trends away from a primi- 

 tive condition, such as a tendency toward loss of certain spinacoxal 

 muscles. (See c, this section, below.) 



Table i provides a composite list of the spinasternal muscles of 

 blattids, as these are now known, and is so arranged as to indicate 

 the probable serial homologies. Most species possess a very large frac- 

 tion of the total complement. The relatively few exceptions are sum- 

 marized in footnotes to the table, and some of them are discussed 

 briefly in the text. Included in the spinasternal musculature are 

 (a) transverse spinal muscles; (b) spinal muscles of the preceding 

 or succeeding furca; (c) spinal muscles of the preceding or succeed- 

 ing coxa; (d) muscles stretched between successive spinae; and (e) 

 spinabdominal muscles. Each of these groups is discussed under the 

 corresponding subheading below. 



a. Transverse spinal muscles. — The transverse muscles of cock- 



