152 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 81 



The various fibers of the head muscles are mostly arranged in 

 groups, and it is easiest to trace them from their points of insertion 

 on the back of the head. Inserted in the median notch of the vertex 

 there is a dorsalmost group of long fibers that diverge posteriorly to 

 the dorsal wall of the prothorax (B, C), the middle fibers of each 

 group going to the posterior margin of the segment. External to 

 these muscles, a group of short fibers, inserted serially on each side, 

 extends posteriorly and dorsally to the tergal plate of the prothorax. 

 Laterally there are inserted on the postoccipital ridge several fibers 

 that spread to their origins on the tergal plate, and a group of four 

 long fibers going dorsally and medially to the intersegmental fold 

 (Isg), with the two median fibers crossing the latter to the dorsum of 

 the mesothorax. Three lateral groups of fibers ( A, C) go ventrally and 

 posteriorly from their head insertions, one to the sternal interseg- 

 mental fold, another to the region just before the base of the protho- 

 racic leg, and the third to the median longitudinal fold between the 

 legs. Ventrally there are inserted on the ventral apodeme of the 

 hypostomal region (C, z'ap) the anterior ends of the ventral longitudi- 

 nal muscles of the prothorax (A, C), and a group of four long fibers 

 on each side that arise on the region above the spiracle. 



It is of particular interest to observe that, in the caterpillar, the 

 ventral longitudinal muscles of the prothorax are not inserted on the 

 tentorium (fig. 57 A, C) as they are in orthopteroid insects, and fur- 

 thermore, that all the principal longitudinal ventral muscles of the 

 thorax have their origin on the intersegmental folds, and not on in- 

 trasegmental apophyses. The primitive anterior insertion of these 

 muscles in the prothorax, therefore, should be on a ventral interseg- 

 mental fold between the prothorax and the last head segment. We 

 have already seen that there is evidence of the loss of the true labio- 

 prothoracic intersegmental fold, since the postoccipital ridge, which 

 bears the anterior attachments of the prothoracic muscles in all known 

 insects, appears to be the fold between the maxillary and the labial 

 segments. If so, the original attachments have been lost and the 

 muscles now extend through the length of two primary segments. 



Furthermore, the attachment of the ventral muscles of the cater- 

 pillar on the hypostomal regions of the head must signify a migration 

 of the muscles from their primitive sternal insertions, for the hypo- 

 stomal lobes clearly belong to the postgenae, and are, therefore, ventral 

 extensions of the tergal area of the head wall. In any case, an attach- 

 ment of the ventral muscles on the bridge of the tentorium certainly 

 represents a farther displacement of the muscle insertions by a final 



