io8 Anjtals Entomological Society of America [Vol. 11^ 



In the study of the tracheation in the Psyllidae the writer 

 has been confined to a single species, and we have here even a more 

 striking degree of normal coalescence of tracheae than in the Aphi- 

 didae. For instance, as is shown in figure 33, the seven tracheae, 

 are distinct to the body trachea in the nymphal wing pad, while 

 figure 34 records the normal tracheation of the freshly emerged 

 wing, the three most important tracheae, the radial, medial, and 

 cubital tracheae, are coalesced into a common stem at the base, 

 and for a considerable distance the medial and cubital tracheas^ 

 are stalked after the radial trachea has separated from them. 



We have, then, a very striking difference in the relation of the 

 tracheae of the nymphal wing pad and those of the freshly emerged 

 wing, in both of these families of insects. 



COSTA AND SUBCOSTA OF APHIDID^. 



The costal vein extends along the cephalic margin of the wing. 

 No trachea precedes this vein in the freshly emerged wing.* 

 Neither is a costal trachea present in any nymphal wing pad of 

 the species examined by the writer. Further, no evidence of a 

 vestigial costal spur from the body trachea is to be found in the 

 several species examined for this point. Perhaps the most 

 striking evidence of a costal vein aside from the stiffening of the 

 costal margin is the fact that in severing the wing from a freshly 

 killed aphid, the yellow body fluids frequently flow into this vein 

 and extend along to about the region of the stigma (fig. 4) . 



In like manner no trachea precedes the vein subcosta in any 

 species examined of the sixteen genera (see page 105) of aphids 

 from Lachnus to Chermes which I have studied for this point. 

 No item in this work has been more carefully investigated than 

 the possibility of a subcostal trachea. 



Indications of such a trachea have by no means been lacking. 

 For instance, the fold of the vein subcosta appears very early in 

 the freshly emerged wing (long before the other veins) and is 

 therefore frequently the only vein indicated at the time the tra- 

 cheae are visible. For many mounts of many species this form- 

 ing vein resembles a tracheal line closely enough to be mis- 

 taken for one unless exceedingly great care is taken to secure 



* Comstock and Needham. Wings of Insects. Page 8.5S. "* * * there 

 are often channels present which do not contain trache;E. This is oftenest true 

 of two large channels at the lateral margins of the wing. Of these the costal 

 remains abundantly lined with cells, which ultimately form the strong costal 

 vein. Its trachea is often atrophied, probably owing to the disadvantageous 

 position of its base in relation to air supply, as we have hitherto indicated." 



