Vol. 1.] Woodu'ortli. — Wl)ig Veins of Insects. 59 



The production of the cuticle on the vein areas presents no 

 very peculiar features. It proceeds at the same time and in 

 the same manner as the cuticle production on the membrane, 

 from which it differs simply in being thicker. There must be 

 differences in the chemical composition of the cuticle in different 

 regions, as is shown by the development of color after emergence, 

 but this is not evident to the eye at first. The cells are not 

 usually so completely exhausted by cuticle production as are 

 those of the membrane. They thus often form a considerable 

 layer around the inside of the expanded vein. 



After the final molt all the folds of the cuticle straighten 

 out, the membranes of the two sides flatten against each 

 other, and after drying are usually hardly distinguishable as 

 two layers. The veins, at first flat, soon become tubular and 

 harden in that shape. 



Resume. — According to this account the development of the 

 vein begins with cells not at all differentiated from those of 

 the body wall, but constituting a sac, opening into the body 

 cavity, which may or may not be associated with trachea?, 

 connective tissue, etc. The first trace of a differentiation is 

 the formation of the vein cavities, caused chiefly by a slight 

 shortening of the cells of these regions. The differences 

 between the development of veins in higher and lower insects 

 are not important, as in all cases scarcely any progress is made 

 in- development until the penultimate stage is reached, in which 

 the two types closely resemble each other. Even in this stage 

 the veins are scarcely different from the membrane, except in 

 the matters already mentioned, and this condition persists until 

 near the last molt, when the grooving occurs. The veins fold 

 only transversely, while the membrane wrinkles in a very 

 complicated manner in both directions. An explanation of 

 the folding is attempted by assuming the secretion of a layer 

 that swells, and molds the outer cuticle-producing surface into 

 an appropriate shape, and thus provides for the ultimate 

 expansion of the wing into a flat organ. The cuticle produc- 

 tion and the expansion of the wing then proceed in the man- 

 ner that is normal for expanding regions in any part of the 

 bodv. 



