1890.] ENTOMOLOGICAL NEWS. 157 



ELEMENTARY ENTOMOLOGY. 



Sixth Paper. THE WINGS. 



The wings of an insect are membranous expansions of the body 

 wall. They first appear in the pupal or later nymphal stages as 

 flat, sac-like projections near the lateral borders of the terga of 

 the meso- and metathorax. As the wings develop, the greater 

 part of the dorsal and ventral walls of this flat sac become closely 

 united. Along certain lines, both the dorsal and ventral walls 

 become thickened. These lines are so situated that when the 

 union of the two walls takes place, those on the dorsal wall coin- 

 cide with those on the ventral wall; they thus form the frame-work 

 of the wings, and are called veins, nerves, or nervures. Many 

 veins enclose a tube formed by the union of two grooves, placed 

 one on the inner or opposing side of each of the two thickened lines. 

 Through these tubes blood circulates; often a trachea exists in 

 the centre of this tube. The veins or nerves of an insect's wing- 

 do not correspond to the similarly named parts of higher animals. 



That part of the wing by which it is attached to the thorax is 

 the base. The shape of the wing is roughly triangular; its three 

 edges, borders, or margins, receive distinctive names; the front 

 edge is also called the costal margin, or simply the costa; the outer 

 edge is also termed apical, the remaining border is known as the 

 inner, hind, or anal margin. The apex of the wing is the angle 

 where the front and outer margins meet. The inner and outer 

 margins meet at the hind or anal angle. 



All the veins of a wing taken collectively constitute the venation, 

 neuration, or reticulation. The venation is regular, and although 

 differing very much in various groups of insects, is based on a 

 general plan. Five (or more) large veins run from the base 

 towards the apical border, diverging from each other. These 

 veins branch ; at right angles to these large veins and their 

 branches are other short veins. The branches of the large veins 

 are sometimes known as sectors. Each litttle area or space of the 

 wing, bounded by veins, is called a cell, or cellule. No general 

 nomenclature has yet been established for the venation of all the 

 groups of insects; for each group a special nomenclature is in use. 



The front wings may be specialixed so as to form protective 

 covers for the hind wings. In beetles and earwigs tin- front wings 



