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 distention 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 he 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 
