INSECT SPINASTERNAL MUSCULATURE — CIIADWICK 12/ 



first and second thoracic intersegments. It is seldom ligamentous, and 

 its lateral insertion is on the anterior margin of the immediately suc- 

 ceeding episternum. Unlike the first type, it rarely lacks the median 

 attachment, though in Cryptoccrcus (Blattariae) the more dorsal fibers 

 of isps-epS2 are found as eps.-eps-i (Chadwick, 1957), while in Cyclo- 

 chila (Homoptera) the muscle that apparently corresponds to isps- 

 epSo has no midventral insertion (Larsen, 1945c). 



While this second type of transverse muscle may be ultimately a 

 derivative of the first, which represents the usual transverse interseg- 

 mental muscles of arthropods, there is evidence that both types have 

 existed independently for a very long time in the hexapod line. Thus, 

 there are in Lepisma two such ligaments, n and n' of Barlet (1951), 

 in the first thoracic intersegment. Cryptoccrcus (Blattariae) has two 

 transverse muscles, the anterior partly ligamentous, in both the first 

 and second intersegments, while a similar arrangement is found in the 

 first intersegment of some Isoptera (Chadwick, 1957; idem, unpub- 

 lished), and in both intersegments of certain sialids (Czihak, 1953; 

 Chadwick, unpublished). Elsewhere, only one or the other of the two 

 types is seen ; and in some mantids the data suggest that the same 

 muscle which in immature forms is attached on Us may appear with 

 the episternal attachment in the adult. 



Several authors have listed as transverse muscles the bands fui-fui, 

 fuo-fu-y, fiis-fuz, whose proper affinity seems rather to be with the 

 spina f ureal muscles, as explained in the next section. Yet, a difficulty 

 in regard to this interpretation arises in the mallophagan genera Tri- 

 menopon and Myrsidea, where Mayer (1954) has described two 

 transverse muscles of the third thoracic intersegment. One of these, 

 /// trm I, is stretched between opposite sternal arms in the usual man- 

 ner and probably represents a former spinafurcal muscle, jsps-fus. 

 The other, /// trm 2, arises on ju^ and is inserted medially on the 

 membrane between metathorax and abdomen, i.e., at the presumed 

 locus of a third spina. Now, if the first band, juz-juz {111 trm /), of 

 these IMallophaga is to be taken as comparable with the similarly lo- 

 cated muscles and ligaments of other insects, then /// trm 2 must 

 represent a different muscle, and this could be only ^sps-sils. This 

 assumption, which seems the simplest that will account for the facts, 

 is however not wholly satisfactory, for it forces us to suppose too that 

 fu3 and jils were at one time contiguous, in order to facilitate the 

 transfer of the attachment of /// trm 2 from jiis to its present loca- 

 tion on /M3. Such a contention is at present without the support of any 

 other evidence, and indeed seems rather unlikely. Moreover, while 



