94 ON THB ANATOMY OF THE PLT. 



of the proboscis. Its filaments chiefly end in nerve cells in 

 the extremity of the organ, where the sacculi are most 

 numerous ; a tracheal vessel accompanies the nerve in its 

 distribution. It may be both an olfactory and gustatory 

 organ, directing the insect to its food. However this may be, 

 it can scarcely fail to be connected with the sense of taste, 

 seeing that it is often enveloped in a drop of fluid regurgitated 

 from the sucking stomach (see Section IV., page 55). 



Section XVI. The Frontal Sac 

 PLATE I., AND PLATE VIII., FIG. 14. 



The evidence that induces me to include this organ under 

 those of special sense is, that its extensive surface, which is 

 thrown into numerous plications, is still further increased 

 by being covered with blunt papillee (Plate VIII., fig. 14). Its 

 internal surface is covered with yellow pigment, and with a thin 

 layer of nerve cells connected with filaments from the anten- 

 nal nerves. As has been already stated, its cavity is in free 

 communication with the atmosphere, and the air contained 

 must be frequently changed by the vibrations of the antennal 

 plate. 



With regard to the respective share which the antenna, 

 palpi, and frontal sac, probably have in the olfactory sense, we 

 have little to guide us ; but it appears to me likely, that the 

 olfactory organs of the antenna enable the insect to perceive 

 odors, which are quite inappreciable to the vertebrata. If, 

 as has been lately asserted, these are the organs which guide 



