The Internal Anatomy of Spiders 



n. 



Fig. 146. SECTION 



OF A SETA 

 n, nerve (after Hilton) 



Fig. 147. GLANDULAR 



HAIR OF A CATERPILLAR 



(after Hilton) 



termed setae; the two are morphologically the same, and they 

 grade by insensible degrees into each other. 



As regards their internal structure, the hairs of spiders, like 

 those of insects and other arthropods, differ markedly from 

 the hairs of mammals, being hollow. 



Each hair of a spider arises from a more or less cup-like 

 cavity in the cuticula of the body-wall, which may be made more 

 pronounced by a ring-like elevation of the cuticula surrounding 



it (Fig. 144); but the latter feature is not an 



essential one. The sheath of the hair is con- 

 tinuous with the cuticula 



of the body-wall, but at 



its base it is infolded and 



more or less flexible, thus 



forming the joint. The 



cup-like cavity in the 



cuticula from which the 



hair arises is situated at 



the end of a pore in the 



cuticula, the trichopore. 

 The trichopore and 

 the base of the hair is filled with a prolongation of the hypo- 

 dermal cell that formed the hair, the trichogen (Fig. 144, tr). 



The above may be regarded as the essential features of an 

 arthropod hair. Certain writers refer to protecting hairs, which 

 are supposed to present only these features, and to serve merely 

 to protect the body. My own observations, based, it is true, 

 chiefly on the hairs of insects, lead me to doubt the existence 

 of hairs that are merely protective in function. 



In most cases, if not in all, each hair is supplied with a minute 

 nerve, connecting the hair with a subhypodermal nerve plexus 

 (Fig. 146); this feature has been carefully worked out with insects, 

 and it is not probable that it is radically different in spiders. 

 Hairs furnished with a nerve are regarded as organs of special 

 sense; in most cases they are organs of touch, the tactile hairs; 

 these are discussed later. 



A very important modification of the ordinary type of a 

 hair is that known as the glandular hair. These are hairs that 

 serve as outlets of hypodermal glands (Fig. 147); several kinds 

 of these, that are found in insects, have been described. The 



140 



