ON THE STRUCTURE OF THE ANTENNA. 589 



the larger superficial cells, but many apparently end in the small 

 round cells beneath them. 



The large Sacculi have an average diameter of 50 /z (PL XLI., 

 Fig. 2, B). Each sacculus consists of a very thin cuticular 

 layer, usually inflected so as to form a group of sacs open- 

 ing into a single central cavity, more rarely as a simple 

 flask-like sac. This membranous cuticle supports a vast 

 number of very fine, straight, broad-based setae, which con- 

 verge towards, and sometimes project from, the orifice of the 

 sacculus. The orifice of the sacculus is generally irregular in 

 form, bounded by several short curves ; but it is occasionally 

 circular, when the sacculus has no secondary sacculi in its 

 walls. The nervous structures beneath the sacculi are iden- 

 tical with those under the rest of the integument, but the 

 large superficial ganglion cells are more numerous in relation 

 with the sacculi. I have occasionally seen flame-shaped cones 

 in the sacculi, and suspect their origin is similar to that of the 

 cones on the surface of the antennas. Those in the sacculi are 

 smaller, proportionately to the finer setae which they contain. 



c. On the Functions of the Antennae in Insects generally. 



Oken regarded the antennae as organs of hearing, and 

 Lefebvre [262], in 1838, was apparently the first who attempted 

 to controvert this opinion and to attribute an olfactory function 

 to them. It is true that Reaumur suggested the possibility 

 that they subserve the olfactory sense, but he brought forward 

 no evidence in favour of this view ; and the majority of authors 

 who touched upon the subject before 1847 followed Oken. 



Erichson [263], in the latter year, investigated the minute 

 structure of the antennae, and concluded from his anatomical 

 investigations that they are olfactory organs. Erichson's 

 knowledge of their minute structure was necessarily very im- 

 perfect, and, although his conclusion was apparently correct, 

 he had no facts to justify it. 



Leydig [122], in 1855, reinvestigated the subject, and traced 

 the antennal nerve to the end organs discovered by Erichson, 

 although he gave a figure of the olfactory setae of Calliphora, 



