NO. 2 THORACIC MECHANISM OF A GRASSHOPPER SNODGRASS Q.'J 



Scolopendra and Scutigera, but it is clear that the muscle attachments 

 have been altered by a transposition which is of a parallel nature in the 

 two cases. 



In the Chilopoda the sternal plates are uniformly developed through- 

 out the length of the body. In Strigmnia, Lithobiiis, and Scolopendra 

 there is a series of alternating segmental sternites (figs. 8 B, 15, Stn) 

 and intersegmental int erst emit es {1st). The intersternites are lacking 

 in Scutigera; they are highly developed in the geophilid Strigmnia 

 bothriopus, where their lateral ends extend upward on the sides of the 

 body between the subcoxae (fig. 8 A), but they are small and incon- 

 spicuous in Lithobius (fig. 15). In both the geophilid and lithobiid 

 as we have seen, the longitudinal ventral muscle bands have their at- 

 tachments on the intersternites. 



The presence of alternating sternites and intersternites in the chilo- 

 pods might suggest that this condition was the primitive one in in- 

 sects, and that the intersternites (spinastemites) have remained free 

 in the thoracic region or have united with the preceding sterna, while 

 they have fused with the segmental sternites following in the abdomen. 

 In the odonate larva shown at B of figure 16 there is a long interseg- 

 mental sclerite {^Ist) between the posternum {S^) and the mesoster- 

 num suggestive of the intersternites of the Chilopoda, and the fold 

 (list) in the ventral side of the neck (Cv), which bears the cervical 

 sclerites laterally (cv), appears to be likewise an intersternite between 

 the labial segment and the prothorax. In the Acrididae the spinaster- 

 nite between the prothorax and mesothorax (fig. 21, 6*^) is a well- 

 formed plate attached to the prosternum (S) ; that between the meso- 

 thorax and metathorax is indistinguishably fused into the posterior 

 border of the mesosternum, though the spina persists (fig. 31, 2Spn). 

 In many insects the first spinasternite is a free sclerite, and in the Blat- 

 tidae both the first and the second are distinct plates (fig. 19 A, 

 iSs, 2Ss) . 



The definitive thoracic sterna of most insects are undoubtedly com- 

 posite structures. The first and second intersternites are usually con- 

 tained in the posterior parts of the prosternum and mesosternum, 

 respectively, or at least are closely associated with them, though the 

 first frequently retains its independence. The ventral arcs of the sub- 

 coxae contribute laterosternal elements in many insects. The evident 

 union of the ventral rim of the subcoxa with the sternum has been 

 noted in the Hemiptcra (Heymons, 1899, Snodgrass, 1927), but 

 Weber (1928, 1928a) has given ample reasons for believing that this 

 fusion of subcoxal elements with the primary sternum has taken place 

 in the majority of insects. The frequent ventral articulation of the 



