THE EXTERNAL ANATOMY OF INSECTS 33 



In addition to the trichogen there may be a gland-cell opening into 

 the seta, thus forming a glandular hair, or a nerve may extend to the 

 seta, forming a sense-hair; each of these types is discussed later. 



The most common type of seta is bristle-like in form; familiar 

 examples of this type are the hairs of many larvae. But numerous 

 modifications of this form exist. Frequently the setae are stout and 

 firm, such are the spine-like setce; others are Jurnished with lateral 

 prolongations, these are the plumose hairs; and still others are flat, 

 wide, and comparatively short, examples of this form are the scales 

 of the Lepidoptera and of many other insects. 



The taxonomic value of setce. — In many cases the form of the setae 

 and in others their "arrangement on the cuticula afford useful charac- 

 teristics for the classification of insects. Thus the scale-like form of 

 the setae on the wing-veins of mosquitoes serves to distinguish these 

 insects from closely allied midges; and the clothing of scales is one 

 of the most striking of the characteristics of the Lepidoptera, 



The arrangement of the setae upon the cuticula, in some cases at 

 least, is a very definite one. Thus Dyar ('94) was able to work out a 

 classification of lepidopterous larvae by a study of the setse v\ith 

 which the body is clothed. 



A classification of setce. — If only their function be considered the 

 hairs or setae of insects can be grouped in the three following classes : 



(i) The clothing hairs. — Under this head are grouped those hairs 

 and scales whose primary function appears to be merely the protection 

 of the body or of its appendages. So far as is known, such hairs con- 

 tain only a prolongation of the trichogen cell that produced them. It 

 should be stated, however, that this group is merely a provisional one; 

 for as yet comparatively little is known regarding the relation of these 

 hairs to the activities of the insects possessing them. 



In some cases the clothing hairs have a secondary function. Thus 

 the highly specialized overlapping scales of the wings of Lepidoptera, 

 which are modified setae, may serve to strengthen the wings; and the 

 markings of insects are due almost entirely to hairs and scales. The 

 fringes on the wings of many insects doubtless aid in flight, and the 

 fringes on the legs of certain aquatic insects also aid in locomotion. 



(2) The glandular hairs. — Under this head are grouped those hairs 

 that serve as the outlets of gland cells. They are discussed in the next 

 chapter, under the head of hypodermal glands. 



(3) The sense-hairs — In many case a seta, more or less modified 

 in form, constitutes a part of a sense-organ, either of touch, taste, or 

 smell; examples of these are discussed in the next chapter. 



