92 ON THE ANATOMY OF THE FLY. 



Dr. Braxton Hicks describes them as closed by a thin membrane, 

 which he calls the outer layer of the antenna, evidently my protoderm. 

 It appears to me however, from the most careful investigation, that the 

 protoderm is reflected into them, but forms a thin perforated collar 

 around the opening. The early period at which they appear in the pupa, 

 seems to indicate that they are mere sacculi of the surface ; the larger or 

 compound sacculi are moreover, in all probability similar organs, and un- 

 doubtedly open upon the surface. 



The larger spots (Plate VIII., figs. 11 and 12) are compound, 

 consisting of several sacculi, with a common opening ; the 

 opening in these is often as large as l-1500th of an inch. The 

 several chambers are arranged in a radiating manner. 



The fourth and fifth joints are mere rings, on the outer side of the third 

 joint, near its proximal extremity ; and the sixth forms a flattened plumose 

 tapering bristle, which is solid through the greater part of its length in 

 the mature insect. 



The cavity of the first and second joints is chiefly occupied by cells, 

 probably of endodermic origin ; they are traversed by the antennal nerve. 



The third joint is filled with pulp, which consists of cells 

 and large nuclei, which are especially abundant, together 

 with the terminal branches of the antennal nerve. This pulp 

 is surrounded by a layer of very delicate brown pigment cells, 

 which are immediately beneath the integument. 



The antennal nerve splits up into numerous fibrilla3 on 

 entering this pulp, some of which terminate in ordinary nerve 

 cells ; but by far the greater number are covered with minute 

 corpuscles, clustered round their terminations, like bunches of 

 grapes (Plate VIII., fig. 13). A large tracheal vessel ac- 

 companies the nerve and ramifies in the pulp. 



I believe the sacculi are minute expansions of surface, open 

 freely to the atmosphere, and that the entire third joint is an 

 olfactory organ. This seems probable, as the extensive ter- 



