INSECT SPINASTERNAL MUSCULATURE — CHADWICK 1 29 



correct, /// trm z of these species, which is fuz-fih, is comparable with 

 the inter f ureal muscle in other insects and hence with the presumed 

 ancestral muscle 3sps-fth, as well as with serial homologues in the 

 more anterior segments. An alternative hypothesis would be to take 

 /// inn 2 at face value, as being ssps-fu^. The muscle /// trm i would 

 then have to be interpreted as equivalent to ssps-sils, which would 

 pose the same requirement for a transient fusion of j//.y and fus to 

 which we have objected above, and would also leave the status of 

 / trm (fu^-fih) and // trm i (fuz-fus) in doubt. We have rejected 

 the alternative for this reason. 



4. Only rarely is the spina connected by a muscle to the Us of the 

 following intersegment, except for a band from the third spina or its 

 equivalent to the Us between the first and second abdominal interseg- 

 ments, i.e., to His. Such a muscle, ssps-Iils, is known to be present 

 in the Thysanura and 7 pterygote orders ; but since the vestiges of the 

 third spina are not always recognized as such, and because of some 

 confusion as to the identity of the anterior abdominal sterna, what 

 may be the same muscle may have been described in different terms 

 in some other groups and hence overlooked in this discussion. 



Lepisma has equivalent muscles in all three thoracic segments (Bar- 

 let, 1953, Nos. 34, 42, 51) ; larval dytiscids, in two (Speyer, 1922, 

 11 4d, IIl4d; Chadwick, unpublished). In Dermaptera, the muscle 

 isps-2ils (Maki, 1938; No. 28 in Anisolabis and Labia; Chadwick, 

 unpublished, in Forficula) is obviously the functional substitute for 

 isps-fu2, which is lacking here just as isps-2ils is in most pterygote 

 insects. Larval Corydalus (Megaloptera) possesses a muscle, distinct 

 from its 2sps-fth, that runs from 2sps to the intersegmental ligament, 

 %3, which is in this form the equivalent of ssps-stis, as noted in sec- 

 tion I above. This muscle, 2sps-lig:,, evidently stands in this situation 

 for 2sps-sils, and appears to have furnished part of 2sps-Iils in the 

 adult (see section 6). Kelsey's (1957) description of Corydalus does 

 not recognize the existence of larval lig-i and 2sps-ligz, nor of adult 

 2sps-Hls. 



It is probable that the muscles numbered 2C) in Lcpidocampa and 

 24 in Ncanura by Maki (1938) are serial homologues of the type 

 under discussion, and represent a osps-iils that is not found in other 

 insects. 



Other examples of the occurrence of muscles of this nature are 

 less certain. According to Maloeuf (i935). ny^phal and adult dragon- 

 flies have a muscle (his No. 68) which may represent our 2sps-3ils, 

 and this may correspond to the muscles numbered 43 by Maki (1938) 



