Vol. 1.] Woodwortli. — M^iitg Veins of Insects. 7 



and Pseudoneuroptera, and are, therefore, supposed by him to 

 be secondary adaptations, as he seems to doubt that these 

 orders represent the primitive insects. 



In answer to these arguments it may be admitted that the 

 tracheal gills are of many sorts, and are not necessarily 

 homologous structures in any strict sense. Gegenbaur did 

 not suggest that the wings arose from organs extending from 

 the end of the abdomen, or from those growing on the base of 

 the legs, but rather that there are tracheal gills on the sides 

 of the abdominal segments, which by their position and struc- 

 ture suggest that other similar, perhaps homologous, structures 

 situated on the corresponding part of the thoracic segments 

 might have been modified into wings. The second argument 

 can be easily disposed of by citing the recent work of Com- 

 stock and Needham ('98-99), where it is shown that the wing 

 trachese all arise' from the side of a single trunk, or at most 

 from two. The weight of any argument based on the arrange- 

 ment of the tracheae is greatly diminished by the fact that, in 

 many wings at least, the tracheation is a comparatively late 

 and entirely secondary matter. The tinal argument advanced 

 by Packard does not seem to have much force. Granting that 

 none of the groups now existing represents the one in which 

 wings first arose, and that tracheal gills as they now occur are 

 always adaptive and secondary, or even that this was true in 

 the primitive insects, it is hard to see any bearing the facts 

 would have on the question at issue. 



It is not difficult to find weakness in the ideas advanced by 

 Packard. What, for instance, can be supposed to account for 

 the first development of broad lateral expansions on the sides 

 of the body in ancient Paleozoic times? Certainly these were 

 not developed as a means of protection from the kinds of 

 enemies that are supposed now to account for such structures. 

 Again, there is much more difficulty trying to imagine the 

 process of the conversion of a plate of this nature into a wing 

 than to imagine one produced from such a structure as a tracheal 

 gill; and further, there is no evidence that leaping insects 

 existed in those early times — certainly the remains of jumping 

 insects from the Paleozoic era are not abundant, and the 

 family groups possessing this power have never been recog- 

 nized as ancestral. 



Pancritius's suggestion is open to the same criticism as that 



