42 University of California Publications. [Entomology 



Nearly all the structures that appear as veins in the higher 

 groups — Diptera, Hymenoptera, and Lepidoptera — are veins 

 according to almost any criterion that may be used, and may 

 be taken as the standard for determining the soundness of the 

 criteria proposed. We will now consider in detail the signifi- 

 cance of the characters suggested for the recognition of veins. 



The presence of the lumen has been a very common test 

 ever since the name came into general use. This is partic- 

 ularly true with English entomologists, who follow Westwood 

 in the belief that flight is only one of the functions of the 

 wing, and that another of great importance is the aeration of 

 the blood, the veins being supposed to afford especial oppor- 



\^ 



FIG. 14. Diagramatio section of a wing of Cicada near the Ijase, 

 sliowing tliree principal veins in cross-section. 



A, a vein belonging entirely to the lower membrane. B, one largely 

 formed by upper membrane; and C, a vein about equally divided 

 between the two membranes. 



tunity for its accomplishment. If this is true, the lumen is of 

 prime importance. No evidence, however, has ever been 

 advanced to prove that the blood could receive any appre- 

 ciable amount of oxygen from the outside air through the 

 vein walls, even when the veins have large cavities. The 

 circulation of the blood through the veins is in most cases an 

 impossibility, because they are not continuous channels. 



The supposition that the vein function is the aeration of 

 the blood is not necessary in accounting for the shape of the 

 vein. There are two other explanations for the development 

 and maintenance of this shape. One of these is that the 

 presence of the blood in the wing prevents the drying of the 

 wing to such an extent as to make it brittle; a condition 

 which is quickly reached in dead insects. The other explana- 

 tion is of greater importance, namely, that a tubular form 

 combines the greatest strength with the least amount of 

 material. 



