114? INTRODUCTION TO 



threads, but immediately reunite and recover their 

 former figure. The substance of the nervures being 

 thus spread over a greater surface, necessarily loses 

 its usual depth of colour, and the transparency which 

 distinguishes these spots is the result. The trachea 

 however, is never interrupted. These interruptions 

 are always accompanied by a slight fold of the 

 membrane of the wing, and when the direction of 

 this fold changes they change along with it. It is 

 thence inferred that their principal object is to admit 

 of a slight detention of the wing, when circum- 

 stances render that necessary, and make it more 

 flexible, the nervures being too rigid for that purpose. 



The longitudinal and transverse nervures, by in- 

 tersecting and anastomosing with each other, enclose 

 small spaces of the surface of the wing, which are 

 called areolets or cells. These are pretty constant in 

 their forms and position in the several orders and 

 families, and therefore will be described hereafter 

 as aiding in the discrimination and determination of 

 groups. Kirby regards the wings of all insects as 

 divisible into three longitudinal areas, which he 

 names and defines as follows : costal area, the lon- 

 gitudinal portion of the wing that lies between the 

 anterior margin and the postcostal nervure ; interme- 

 diate area, the longitudinal portion lying between the 

 postcostal and anal nervures : anal area, the portion 

 between the anal nervure and the posterior margin. 



The names given to those parts of a wing which 

 Determine its general form, require to be accurately 

 defined. The part by which the wing articulates 



