Page:Jardine Naturalist's library Entomology.djvu/120

114 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 circumstances render that necessary, and make it more flexible, the nervures being too rigid for that purpose. The longitudinal and transverse nervures, by intersecting 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 longitudinal portion of the wing that lies between the anterior margin and the postcostal nervure; intermediate 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