138 SMITPISONIAN MISCELLANEOUS COLLECTIONS VOL. 1 37 



spina, may convert the still surviving muscle or ligament into the form 

 2ils-2ils, or occasionally even eps^-epSz. In a few instances, sclerotiza- 

 tion along the course of the ligament or former muscle has ended in 

 complete fusion and the formation of an endoskeletal bridge. 



2. The spina was regularly connected by a muscle with the immedi- 

 ately preceding f ureal region. In many orders, a corresponding muscle, 

 ligament, or fusion still persists, but in some this has disappeared with- 

 out a trace. Some form of union has been preserved more often in the 

 prothorax, a reflection no doubt of the greater degree of consolidation 

 of the winged segments and of the reduction that has generally taken 

 place in the anterior sternal region of the abdomen. 



3. An oblique muscle connected the primitive spina with the follow- 

 ing Us, but either a portion of this band soon became attached to the 

 corresponding furcal arm or else an independent muscle, 2sps-fu^, 

 already existed, for the furcal muscle is the more frequently repre- 

 sented in modern insects. The band ssps-^ils is still found in a few, 

 mainly larval forms ; and both it and the furcal muscle are present 

 together occasionally. By adding to itself the lateral longitudinal band, 

 pis-lils, the oblique muscle has given rise, in three orders, to 2sps- 

 lils; and, by an extension of this process, to 2sps-IIils in one. 



4. Another oblique muscle stretched between the spina and the 

 preceding Us; as in the previous example, a part of this muscle, or an 

 independent muscle, is now usually found attached to the correspond- 

 ing furca, as 2sps-fui. We may suppose also that addition of a pleuro- 

 intersegmental element at the anterior end of the muscle 2sps-iils 

 yielded the long pleuro-endosternal muscle, 2sps-ph, of Lepisma (Bar- 

 let, 1954), though details of the origin of such muscles are still in 

 doubt. A prothoracic homologue, isps-oils, has not been described, 

 while the also expected Isps-fu^ may sometimes be present though 

 unrecognized among the usual f urcabdominal muscles. 



5 and 6. The spinacoxal muscles, both remotors and promotors, are 

 still of frequent occurrence among insects. For the present, we have 

 lumped with them the corresponding cruciate intersegmental coxal 

 muscles, although these may originally have been independent entities. 

 The usual origin of the cruciate muscles is on the Us of the opposite 

 side. Those of the cervical intersegment originate on the icv, on the 

 "tentorium coUaire," or partly on the postocciput, this last in some 

 Mallophaga (Mayer, 1954). In a few acridids (Orthoptera), the in- 

 sertion has been transferred from the procoxa to the profurcal arm. 

 The cruciate remotors of the second and third coxae of larval Coryd- 

 aliis (Megaloptera) have shifted their origins to the respective furcal 

 arms. 



