Vol. 1.] Woodivorth . — Wing Veins of Insects. 41 



makes a complete circuit of the wing and has two basal 

 attachments, or finally because it often differs from the other 

 veins in having no trachea" in the early stages of its develop- 

 ment. 



8th. This last argument has been carried so far as the main- 

 taining of an entire difference of nature between all veins 

 developed about trachea^ and those without trachea". 



9th. An entire difference of nature has been maintained for 

 veins occupying elevations or depressions of the wing membrane. 



10th. A similar distinction has been made between veins 

 according to the proportion of substance contributed l)y the 

 upper and lower layers of the wing membrane. 



11th. Again, a row of hairs or spines, a fold of the wing 

 membrane, or even a disturbance of the pattern, have been 

 held to be potential veins, though showing no other evidence of 

 vein structure. 



12th. Finally, the true vein has been supposed to lie deeper 

 than the skin and to have no necessary connection with any 

 of the surface indications. 



A typical vein is a tubular structure, the wall of which 

 differs from the wing membrane by its firmer texture, by more 

 or less evident peculiarities of pigmentation, both in its own 

 substance and in the adjacent parts of the membrane, by 

 peculiarities in the surface ornamentation, especially in the 

 shape and arrangement of the surface hairs, and by the pres- 

 ence in its lumen of body fluid, a tracheal trunk, a more or 

 less evident quantity of connective tissue, and occasionally a 

 nerve. Ontogenetically the vein is a region of arrested devel- 

 opment — a region in which cells are proliferated, but not 

 specialized. The cells of the vein merely remain more like 

 those of the body wall than do the membrane cells, and for 

 this' reason retain the power of chitin production unimpaired. 

 Not only is the chitin production similar to that of the general 

 body wall, but the pigmentation is also quite comparable. In 

 position the veins probably in every case represent wrinkles 

 in the embryonic wing membrane, but it is often difficult to 

 make this out. The lumen is simph' a continuation of the 

 body cavity into the wingpad, brought about at first mainly 

 by the smaller size of the vein cells, as compared with the 

 membrane cells. The position of the vein at the angles corre- 

 sponding with the folds of the membrane and the crowding, 

 and consequent bowing out, aid in keeping the cavity open. 



