44 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. "JJ 



distal processes from the latter form a cylindrical bundle (Fas) 

 traversing the vacuole (Vac) of the enveloping cell, and attached 

 by a terminal strand (TS) in the apex of the peg. In the Hymenop- 

 tera, at least, the terminal strand consists apparently of the terminal 

 fibers of the minute sense rods (SRs), which latter are here far 

 removed from the external surface. 



Fig. i8. — Diagrammatic structure of the sensillum of a thin-walled 

 peg (Pg) or hair (Hr) supposed to be receptive to chemical stimuli. 



The distal processes of the sense cells (SCls) form a compact 

 fasciculus (Fas) ending in a terminal strand (TS) of cuticular fibers 

 from the sense rods (SRs), attached within tip of external cuticular 

 part; enveloping cell (ECl) with vacuole (I'ac) containing fasciculus 

 and terminal strand. 



SENSILLA CCELOCONICA 



Sense organs of this type are simply sensory pegs sunken into 

 shallow cavities of the cuticula (fig. 19 A, B), and again, as with 

 the hairs and exposed pegs, some are thick-walled or solid and 

 innervated each through a single sense cell, while others are thin- 

 walled and innervated each through a group of sense cells. It be- 

 comes evident, that a truer classification of both hair and peg sense 

 organs, and one probably more coincident with their function, might 

 be based on the internal structure of the sensillum rather than on the 

 form of the external part, if the state of our knowledge would permit. 



Besides the simple pit organs, each with a single peg, there are 

 pits containing each a number of pegs. Organs of this kind occur 



