INTRODUCTION 9 



Hypermetamorphosis arises in a similar way among certain beetles for 

 the better adaptation of the very young and the older larval stages. 



The wings of insects are of necessity limited to the last stage because 

 provision for molting cannot be made in an organ delicate enough to serve 

 for flight. Wings are so complicated and so useless for this function until 

 they are large organs with hinge and muscles that they must have arisen 

 as an organ for another purpose. Aquatic insects often possess gills some- 

 what similar to wings in structure, and such organs are generally considered 

 as the precursor of the wings. An intermediate stage between the gill and 

 the wing is presented in the wing-like gill covers of certain May-fly nymphs. 

 The most important structures in the wing are the strengheniug ribs called 

 veins. The following elements of a venation can be distinguished: — 



1. Marginal veins, always present on both edges at the extreme base, 

 sometimes extending all around the wing. The anterior marginal connects 



Figure 3. Section of a wing showing the 

 folding of the cuticle just before the final 

 mo^t. A. section of vein. B. of membrane 

 C. tangentiar section showing the wrink- 

 ling Oi the membrane in both directions. 



with the tendon which pulls the wing from tae rest position, the posterior is 



continuous with the hind edge of the segment. 



2. The primary vein, extending across the wing in front of the middle and 

 at base bearing the lower articular condyle. This vein is also invariably 

 present in functional wings. 



3. The anterior and posterior veins, variable in number, usually one anterior 

 and two or three posterior. 



4. The independent veins and branches, occupying the spaces in the outer 

 portion of the wing between the veins previously mentioned. They proba- 

 bly all arose as independent veins tho many became attached as branches. 



5. Cross-veins, probably developing spontaneously over the wing surface 

 A theory of venation very largely accepted, derives the veins directly 



from the tracheae of the gill and bases a nomenclature upon a theoretical 

 primitive number and arrangement of these structures. As first presented 

 there were supposed to be eleven of these veins but all the even numbers were 



