Chap. XIII.] THE COCKROACH. 271 



tion of muscular movements is perhaps mainly the function of 

 the so-called brain ; but it would seem to be, in part at least, 

 performed by the sub-cesophageal mass. 



Special Senses in Insects. The chitinous exoskeleton of in- 

 sects rests upon a hypodermic or chitinogenous layer. Its free sur- 

 face is, in the cockroach, divided into polygonal areas, and from 

 the midst of some of these there rise large sense-hairs, developed 

 from long flask-shaped hypodermic cells. These are tactile in 

 character. The nerve filament that passes to such a seta ex- 

 pands at the base of the hair into a nucleated ganglion, a fila- 

 ment from which pierces the hypodermic cell, and is continued 

 to the tip of the hair. 



The paraglossae and inner ends of the maxillae may be in the 

 cockroach, as they are in the wasp, supplied with end-organs of 

 taste. No one who has seen a cockroach spitting, as he sucks 

 from his antenna some distasteful substance, can doubt his being 

 powerfully affected by sapid substances. 



The sense of smell is located in the antennae, in which there 

 are developed a great number of minute sacculi, filled with 

 serous fluid, the orifices of which are covered over with a deli- 

 cate membrane. The nerve-fibres which proceed to these sacculi 

 end in rod-like bodies. If the antennae be extirpated or coated 

 over with wax, the insect exhibits no repugnance to such sub- 

 stances as turpentine and carbolic acid, and does not eagerly 

 rush to such food substances as bread sopped in beer. 



No auditory organs have been described for the cockroach. 

 But other insects possess such organs. In the grasshopper, for 

 example, a thin chitinous membrane is stretched on a chitinous 

 bow over a cavity in the tibia of the fore-leg. The cavity 

 covered over by this membrane is in communication with the 

 exterior, and within the cavity is a ganglionic mass at the end of 

 a special nerve-fibre. In the antenna of the bee, at the bottom 

 of certain open cavities, there are cone-like structures, which are 

 regarded by some anatomists as auditory in function. 



The many-faceted eye of the cockroach is constructed on the 

 same principle as that of the crayfish. The organ is large, and 



