stough: the hackberry psylla. 141 



trachea was torn more or less when the fore wing pad and the 

 hind wing pad, which are grown together at this point, were 

 separated, such evidence in itself is not conclusive. However, 

 from my studies I have every reason to believe that normally 

 only one anal vein is represented. Likewise, the basal connec- 

 tions of the costa of the posterior margin and the trachea of 

 the anal fold to the main trunk from the thorax could not be 

 accurately determined. 



For the further study of the development of the venation, a 

 number of Pachy psylla celtidis-mammse which had emerged 

 during the fall of 1908 were thrown into weak formalin im- 

 mediately after emergence, before the chitinized veins had 

 been laid down (26-i; 32-9, 10 and 11). In one specimen 

 (26-4 and 32-9) the wing was still delicate and transparent, 

 with its edges crumpled and folded, and the veins still prac- 

 tically in their tracheal condition, while in other specimens the 

 wing membranes had hardened, the pattern gained its pig- 

 ment, though still light, and the veins fully formed but with 

 the tracheae in some cases visible within them (32-10 and 11). 

 In the youngest specimen mentioned above the three parallel 

 tracheae of the subcosta plus radius, media and cubitus were 

 strictly separate and corresponded in position to those of the 

 nymph of P. c.-gemma. The distal ends of the veins, becoming 

 lost in the folds of the margin of the wing, could not be traced. 

 The position and direction of these tracheae here were more 

 closely like the veins found in the adult than were those of the 

 nymphal wing pads. 



These studies aided me much in studying the development 

 of the tracheae into veins, and the coalescence of two or more 

 of the former into one vein, as seen in the case of the sub- 

 costa plus radius, media and cubitus. In some specimens 

 these could be seen to be still single and parallel until they 

 reached the internal trunk (26-A and 32-9). The radius and 

 subcosta fused (32-10) was on the cephalic margin of the com- 

 pound vein in process of formation, the cubitus (fig. 11) being 

 on the caudal margin. 



The vein in the anal fold could still be plainly seen (26-4), 

 but its basal connection was not made out. Since there is no 

 thickening of the wing here, as soon as the air leaves this 

 trachea the only suggestion of a vein that is left is the trans- 

 parent "fold." 



