NO. 1 INSECT THORAX — SNODGRASS 37 



In the Pterygota, the eupleural arch of the subcoxa is commonly 

 differentiated into three regions, one forming a supracoxal plate, or 

 anaplciiritc (fig. i8 F, Apl), lying above the coxa, and the others a 

 precoxal plate (Acx) and a postcoxal plate (Pcx) lying before and 

 behind the coxa, respectively. The anapleurite is the primitive basis 

 of the episternum and epimeron, and is seldom undeveloped ; the 

 other two are variable, and one or both may be lacking. In the pro- 

 thorax of Plecoptera (figs. 13, 18 G), the anapleurite (Apl) of the 

 eupleuron, and the trochantin (Tn) are present and entirely separate 

 from each other, the latter carrying both the anterior and the dorsal 

 (posterior) articulations of the coxa (a, b). In other pterygote forms, 

 however, the dorsal part of the trochantin, with the dorsal articular 

 condyle of the coxa (b), is united with the anapleural region of the 

 eupleuron (fig. 18 H), The free anterior part of trochantin may 

 remain continuous with its dorsal part (H), but usually it separates 

 from the latter, which becomes an integral part of the episternum, 

 and constitutes a free sclerite lying before the coxa (I, J, K, Tn). 

 The anterior trochantinal piece, however, becomes rudimentary or is 

 lost entirely in most of the higher insects. The coxa then often ac- 

 quires a secondary ventral articulation with the furcisternum (L, c). 

 In a few insects the ventral rim of the subcoxa persists as a fold 

 around the mesal side of the coxal base (fig. 7), but generally it is 

 not distinguishable, and the precoxal and postcoxal parts of the sub- 

 coxa (fig. 18 H, Aex, Pcx) appear as ventral extensions from the 

 episternum and epimeron to the sternum, which, if chitinized, form 

 plates or i)ridges (J, K). The precoxal bridge is usually best de- 

 veloped (I), the posterior one being frequently lacking. Both niay 

 become confluent with the pleural plates above and with the sternal 

 plates below, uniting all these parts into a continuous chitinization 

 surrounding the coxal cavity (K), but the postcoxal bridge may be 

 absent, leaving the coxal cavity "open " behind (L). The numerous 

 other variations of the pterygote pleuron familiar to students of the 

 thorax need not be detailed here, for it will be clear that all are but 

 modifications of the basic structure given above. 



THE SPIRACLES 



It seems most reasonable to suppose that the spiracles of primitive 

 insects were situated in the pleural regions of the segments, between 

 the edges of the terga and the bases of the limbs. From this neutral 

 position, then, the tracheal branches from each spiracle went to the 

 dorsal and ventral parts of the segment, and the dorsal and ventral 



