
562 THE AMERICAN NATURALIST. [VoL. XXXVI. 
there extends only a trichogen cell and a nerve. The larger 
number of the organs of special sense of arthropods are hairs 
of this type, more or less modified. Frequently, however, the 
modification has been carried so far that the organ of special 
sense is not at all hairlike. This is espe- 
cially true of many of the supposed organs of 
taste and of smell. I have made no attempt 
to study these. 
The investigation, the results of which are 
given in this paper, was confined to a study 
Bo hd: ie of those hollow hairs, or setze, which are dis- 
the silkworm (Bombyx tributed over the surface of the body and are 
an believed to be organs of touch. The primary 
object of this investigation was to determine as definitely as 
possible just what type of hair on the body surface is a sense 
hair; a secondary object was to determine in what ways this 
type is modified in various lepidopterous larvae. Some work 
was also done on several other orders of insects, with the result 
that a close correspondence was found in the structure of these 
hairs among even widely separated orders. 

METHODS. 
In order to definitely demonstrate which hairs were sensory 
it was necessary to have recourse to special histological methods, 
as the terminations of 
the nerves for touch in 
insects are exceedingly 
delicate, and the usual 
microscopical methods 
are entirely inadequate 
for the successful differ- 
entiation of the nervous 244599 
tissues. For general 
anatomy of the sense Fig. 2.— Sections through hairs from a tiger moth 
hairs the usual fixing is aid 
and staining fluids were used; but for staining the peripheral 
nerve fibers and cells the zuzra vítam methylen blue method 



