115 



A study of the causes of the changes which we are de- 

 scribing is beyond our present purpose, which is merely to 

 determine the homologies of the wing-veins. But we can 

 gain a hint of the probable reason for the development of 

 the pectinate type of veins without entering very deeply into 

 questions of the mechanics of flight. 



It is obvious that many styles of flight exist among in- 

 sects, and that for the different styles of flight different 

 kinds of wings are required. In Corydalis, which will be 

 studied a little later, it can be seen that the wing is stiffened, 

 along a line parallel with the costal margin of the wing, .by 

 the subcosta, the main stem of the radius, and veins R^ and 

 R^. Back of this line there is a broad, flexible area, which 

 bends up daring the downward stroke of the wing, forming 

 an inclined plane, the pressure of which against the air 

 forces the insect ahead. The flexibility of this area of the 

 wing is increased by those changes which result in the for- 

 mation of the pectinate type of branching. 



The wings of Sialis. — Among our more common neu- 

 ropterous insects those of the genus Sialis have retained the 

 most generalized condition of wing-venation. It is sug- 

 gested, therefore, that the student begin his practice in de- 

 termining the homologies of the wing-veins of Neuroptera 

 with a member of this genus. Make a drawing of the wings 

 and letter the principal veins, and the accessory veins. 



The wings of Chauliodes.— 



The wings of Corydalis. — In this genus the radial sec- 

 tor is an example of a high development of the pectinate 

 type of venation. 



The wings of Ant-lions. — Photographs of the wings of 

 representatives of several genera of the Myrmeleonidae will 

 be furnished the student for study. The photographs repre- 

 sent the wings grea^^ly enlarged and are, therefore, much 

 more available for study than the actual wings. Each 



