24 THE TRACHEATION OF WINGS 



tance and then changes its course suddenly and enters and follows another 

 parallel vein; and there is no regularity in these aberations. In some 

 forms there is a great reduction of the principal tracheae and correlated with 

 this reduction a remarkable increase of fine tracheal twigs; so that the 

 result is an increased tracheation of the wing. Examples of these devia- 

 tions from the typical tracheation are figured in the chapter devoted to the 

 wings of the Ephemerida. 



Limitations to the value of tracheae in determining the homologies of 

 veins. — Much stress has been laid in the preceding pages upon the value 

 of the tracheae of the wings as an aid to determining the homologies of the 

 wing-veins. In fact the most conclusive proof of the uniformity of the 

 fundamental type of the wing-venation of all orders of insects is drawn from 

 studies of the tracheation of the wings of representatives of those orders in 

 which the tracheation is well preserved. Fortunately in the case of those 

 orders where the tracheation is reduced the venation of the wings of adults 

 so closely resembles in its more general features that of the orders in which 

 the tracheae are well preserved that there is no difficulty in recognizing 

 the identity of the principal veins. 



There are cases, however, in which the evidence presented by the 

 tracheation of the wings is misleading and can not be accepted. This 

 does not imply that we are to accept the testimony of the tracheae if it 

 suits our purpose and to reject it if it does not; but rather that we are to 

 consider other evidence as well as that presented by the tracheation. Two 

 illustrations will serve to make this point clear. 



In the suborder Anisoptera of the order Odonata studies of the trachea- 

 tion of the wings have shown that in the adult wing of Gomphus, for exam- 

 ple, the radial sector occupies a position between veins M2 and M:j. Not 

 only does the basal connection of the radial sector trachea show this but 

 the successive stages of the migration of the radial sector trachea from its 

 normal position to this unusual one can be seen in the wings of a series of 

 nymphs of different ages. Figures illustrating this are given in the chapter 

 treating of the Odonata. 



If one studies the wings of a member of the suborder Zygoptera of this 

 order and compares the venation with that of Gomphus there will be no 

 difficulty in identifying the veins and in recognizing the fact that the radial 

 sector occupies the same position as in the Anisoptera. But when one 

 studies the tracheation of the wing of a nymph of one of the Zygoptera the 

 radial trachea is found to be unbranched and the trachea that precedes 

 what is doubtless the radial sector is a branch of media. It is obvious that 

 in this case the evidence presented by the tracheation is misleading and that 

 the evidence presented by the adult venation is more reliable. The 

 explanation of the discrepancy is that in the Zygo])tera the radial sector 



