l6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I3I 



short lils-fui; and a longer and more slender 2ils-fug was found in 

 all but Pycnoscelus. The presence of jils-jiis was definitely ascer- 

 tained only in the genera Periplaneta (4 species), Eurycotis, Blatta, 

 and Cryptocercus. Failure to find a structure of this delicate nature 

 is, of course, no proof of its absence ; but the observations cited never- 

 theless do indicate quite well the tendency for these ligaments to 

 weaken in the more posterior segments. The genera where their 

 presence in the metathorax is most doubtful {Leucophaea, Nauphoeta, 

 etc.) are notably those judged to have a more specialized muscular 

 pattern on the basis of other criteria ; and several of these are large 

 insects, where such a structure, if present, should be relatively easy 

 to find. 



The origins of the postcoxal ligaments of cockroaches, though 

 clearly intersegmental, are at sites anterior and dorsal to the small 

 sclerites indentified as the true Us by their reception in Cryptocercus 

 of the transverse ligaments and of the usual dorsoventral muscles. 

 In some other insects, the two sites are closer together or even indis- 

 tinguishable, and I can offer no explanation for their separation in 

 blattids. 



d. Spinasternal muscles of the abdominal Us. — Only two such mus- 

 cles have been found in cockroaches, namely 2sps-siiA and "^sps"- 

 SjiA' The usual abdominal attachments for both are near the antero- 

 lateral angle of the second sternum, somewhat anterior and ventral 

 to the suspension of the ventral diaphragm. As explained above, this 

 region of the definitive sternite is believed equivalent morphologi- 

 cally to the thoracic Us. 



Identification of this attachment site with the Us renders dubious 

 the homology, indicated as possible in table i, of "3sps"-SiiA with the 

 spinaf ureal muscles isps-fu2, ^sps-jus; for it is very unlikely that 

 the Us have contributed to the f ureal structures of cockroaches. (See 

 this section, c, above.) 



The muscle from the second spina 2sps-SnA clearly has no serial 

 homolog in blattids. It is ordinarily inserted on Sua somewhat mesad 

 and ventrad of "3sps"-SuA, and is thus two full segments in length. 

 The variant attached on suja has been discussed in section i,e. 



3. THE FURCAE 



The consensus of morphologists has been that the furcae (/») of 

 higher insects have been produced, in phylogeny, by the approxima- 

 tion in the ventral midline of paired segmental sternal apophyses 

 (Weber, 1928; Snodgrass, 1929). The resulting Y-shaped structure 



