1919] Walker: Structure of Orthopteroid Insects 277 



sternum (Fig. 10), between the first pair and sometimes a 

 little in front of them. The outer pair on segment 9 form the 

 dorsal valvulas, the inner pair the inner valvulae, while the 

 pair from segment 8 become the ventral valvulae. Where 

 styli occur, they may appear at an earlier stage than the papillae, 

 which are destined to form the valvulae, and invariably occupy 

 the same position as the outer papillae of the ninth sternum, 

 so that as these elongate into the valvulae they carry the styli 

 with them as small apical appendages. The styli, in all Orthop- 

 teroid insects, ultimately disappear or become indistinguishable 

 from the apices of the dorsal valvulae, but in many Odonata, 

 including all the Zygoptera and some of the Anisoptera 

 (Aeshnin« and Petalurinae of the Aeshnid^), well-developed 

 styli persist in the adult females as functional organs. 



In early larval life, when the first rudiments of the ovipositor 

 appear, the abdominal segments are of relatively uniform size, 

 decreasing in length but little towards the caudal extremity. 

 In forms with a well-developed ovipositor (Orthoptera, Gryllo- 

 blattoidea, Mantoidea), however, as development proceeds, 

 the tenth segment becomes relatively smaller and the ninth 

 deeper, so that, when maturity is reached, the sternal region of 

 segment nine is shifted from a horizontal to a nearly vertical 

 position, thus bringing the bases of the dorsal and inner valvulae 

 into the same transverse plane with those of the ventral valves. 

 An enlargement of the bases of the valvulae usually accompanies 

 their increase in length and this brings about a crowding out, 

 as it were, of such portions of the ninth sternum as are not 

 occupied by the ovipositor itself. Thus, in the adult, the 

 lateral areas are represented merely by the valvifers, while the 

 median parts are the intervalvular membrane and the superior 

 and inferior interval vulae. 



In contrast to the reduction of the ninth sternum, the eighth 

 sternum is usually prolonged caudad as the subgenital plate 

 covering the vulva and bases of the ventral valvulae. In some 

 groups (Blattoidea, Mantoidea and Isoptera) it is, however, 

 much reduced and hidden by the greatly developed seventh 

 sternum, which replaces it as a subgenital plate. In such 

 cases the eighth sternum becomes more or less completely 

 dechitinized. 



The important question as to the homologies of the valvulag 

 will be deferred until after the consideration of both the male 

 and female structures in the various groups. 



