NO. 2 THORACIC MECHANISM OF A GRASSHOPPER SNODGRASS \J 



described the pleural sclerotizations of Eosentomon germanicmn as 

 consisting of a number of small sclerites, which, however, fall into 

 two concentric series. (See Snodgrass, 1927, fig. 8.) In the Collem- 

 bola, as shown by Ewing (1928), the subcoxal sclerotizations of the 

 mesothorax and metathorax (fig. 12 B, Sc.v^, Scxs) consist in each 

 segment of two slender, supra-coxal arches; the subcoxal sclerotiza- 

 tion of the prothorax (Scxi) is a single plate with an incomplete sub- 

 division. In the Thysanura the subcoxal pleurites likewise take the 

 form of two arches over the coxal base, or they become reduced to a 

 single sclerite. The coxal and subcoxal musculature of the Aptery- 

 gota has been but little studied. 



In the thoracic segments of the Pterygota the subcoxae evidently 

 become the sclerotized parts of the lateral segmental walls known as 

 the pleura (cf. figs, i and 13). The ventral rim of each subcoxa, lying 

 between the coxa and the sternum (fig. 13 A), may be reduced to a 

 membranous fold, though in rare cases it contains a large plate (fig. 17, 

 Lso, Lsa), and in others a rudimentary sclerite (fig. 16 A, Ls). In the 

 majority of insects, as has been shown by Weber (1928, 1928 a), the 

 ventral arc of the subcoxa has apparently fused with the primary 

 stemite to form a laterosternite of the definitive sternum (figs. 13 B, 

 iSD, Ls). 



The coxa of insects is universally hinged to the subcoxa by a dorsal 

 articulation (fig. i, c) ; it may also have either an anterior articulation 

 with the trochantinal piece of the subcoxa (fig. 13 B, ^), or a ventral 

 articulation (A, rf) with the ventral rim of the subcoxa or with the 

 subcoxal laterosternite. The trochantinal articulation of the coxa is 

 peculiar to certain insects and is, therefore, probably a secondary one. 

 The ventral articulation, however, so frequently recurs both in the 

 Chilopoda (fig. 15, d) and in the more generalized insects (fig. 16 A, 

 B, d) that there can be little doubt that the primitive axis of the 

 subcoxo-coxal hinge was vertical or approximately so. The writer, 

 therefore, would retract the opinion, expressed in a former study of 

 the thorax (1927, pp. 34-36), that the primitive axis of the coxal 

 movement was a horizontal one between anterior and posterior articu- 

 lations with the eutrochantinal arch of the subcoxa. The ventral ar- 

 ticulation of the coxa is highly variable in insects ; it is always absent 

 in the more generalized Pterygota that have a well-developed trochan- 

 tin. In the meml>ers of the higher orders lacking a trochantin it is 

 commonly present, but it is to be suspected in such cases that the 

 articulation is a secondary one developed between the coxa and the 

 sternum. 



