NO. 2 THORACIC MECHANISM OF A GRASSHOPPER SNODGRASS 23 



fibers of the longitudinal muscles are on the intersegmental folds or 

 on intersegmental sclerotizations (figs. 2, 3). The dorsal muscles 

 throughout the length of the body, and the ventral muscles of the 

 abdomen are thus attached, except where the anterior ends of the fibers 

 may have migrated to the segmental regions of the definitive terga 

 and sterna. In the thorax of adult pterygote insects, how^ever, most 

 of the sternal muscles are stretched between paired apodemal processes 

 of the segmental sternites (fig. 4, SA^, SA2), except that the anterior- 

 most fibers are inserted anteriorly on the head, while the posteriormost 

 fibers extend into the abdomen. Only a few slender median muscles 

 retain a connection with the intersternites (iSs, 2Ss). The paired 



list 



Fig. 16. — Sternal structure of ephemerid and odonate nymphs. 



A, ventral surface of niesothorax, metathorax, and first abdominal segment 

 of an ephemerid nymph, showing ventral articulations of coxae (d) with sub- 

 coxal, laterosternal sclerites (Ls). 



B, ventral surface of neck and prothorax of an aeschnid nymph, showing direct 

 articulation of coxae (d) with laterosternal parts (Ls) of the definitive sternum. 



apophyses of the thoracic sterna are the so-called f ureal anus, which 

 in the higher orders are united upon a common median base and here 

 constitute a true furca. 



The anterior ends of the ventral muscle fibers, as we have noted, are 

 attached on the back of the head. In orthopteroid insects the attach- 

 ment is with the posterior arms of the tentorium (fig. 4, PT), but this 

 condition is clearly a secondary one since the posterior tentorial arms 

 are tergal apodemes. In many adult insects, and in most holometabo- 

 lous larvae, the anterior ventral muscles are inserted on the posterior 

 part of the head wall. Morison (1927) enumerates three pairs of 

 prothoracic sternal muscles in the honeybee, all of which are attached 

 anteriorly on the lateral occipital regions of the head. In the caterpil- 

 lars the corresponding muscles are inserted on apodemes of the ventral 

 margin of the foramen magnum. In all such cases the insertion points 

 of the ventral head muscles must have acquired their present positions 



