46 Vniuersity of California PuUications. [Entomology 



before this time and is usually already occupied by a trachea. 

 In some insects the vein cavities are clearly distinguishable 

 for a considerable period before the tracheae enter them, while 

 in others the tracheae can be recognized before the vein cavities 

 appear. In these cases it is impossible to deny that the loca- 

 tion of the veins may have been really marked out, though 

 unrecognizable to the eye; but if so they were really estab- 

 lished before the entrance of the tracheae, as in other insects. 

 It is probable that the tracheae enter the cavity between the 

 lamina? and assume their fixed relation to the structure of the 

 walls, though they later occupy vein cavities. 



It is the contention of those who insist upon a close relation- 

 ship between the veins and trachea? that the presence of the 

 trachea? causes the change in the cells close to it which results 

 in the production of the vein. In the cases Avhere the tracheae 

 are not present, or only later enter these cavities, the supposi- 

 tion is that the veins, while originally requiring the presence 

 of a trachea, have reached a condition in which they no 

 longer thus depend on trachea?. . How such adjustment can be 

 brought about has never been explained, and until it is, the 

 facts must be considered as throwing a great deal of doubt 

 upon this explanation. 



There is reason to believe that the presence of a trachea pro- 

 duces a different environment for the neighboring cells; but 

 the fact is, that there is no appreciable difference in the struc- 

 ture or appearance between veins which enclose tracheae and 

 others in the same wing which are devoid of them, and 

 that there is likewise no difference between the regions of the 

 membrane which are traversed by tracheae and those which 

 are distant from them; all goes to show that the influence of the 

 tracheae extends far enough so that. all parts of the wing are 

 in practically the same condition in this respect. 



A trachea always takes a more or less sinuous course through 

 a vein, never conforms to the short, abrupt turns the veins 

 often make, and does not exhibit any evidence of closer rela- 

 tionship to the vein than that which exists between the tubu- 

 lar antenna or leg and its tracheal trunks. 



Evidence has already been cited discrediting the idea that 

 the wing performs the functions of a respiratory organ; more- 

 over, the course pointed out as probably that of the phylo- 

 genetic development of the wing — which gives the vein an 



