32 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. ']'] 



case of the antennal sense organs of adult insects. Without conflict 

 with either side of this question, it might be supposed that the primi- 

 tive sensory apparatus of insects consisted of a specialized innervated 

 hypodermal cell, as in the earthworms ; but such a large number of 

 the sense organs of insects suggest by their structure an origin from 

 innervated hairs, that we must give serious consideration to the idea 

 that the hollow cuticular seta was developed first, perhaps as a pro- 

 tective structure, and later became a sense organ through having the 

 end of an innervated cell connected with its base. It, therefore, seems 

 reasonable to suppose that the tactile hairs were the first specific 

 sense organs to be acquired by insects, possibly excepting the eyes, 

 and that from them were developed organs for perceiving chemical 

 stimuli, sound stimuli, or whatever other stimuli are perceptible to 

 insects. On any other basis, it is difficult to account for the uniform- 

 ity of structure that runs through all types of insect sense organs 

 except the eyes. The extent to which the hair structure can be traced 

 in the various kinds of sense organs will be shown in the following 

 sections of this paper. 



The sense cell is, in most cases, easily distinguishable in the sen- 

 sory complex by its large regular elliptical nucleus, which contains 

 an abundance of chromatin, and by its oval or fusiform bipolar shape, 

 drawn out at one end into the distal process, and continued at the 

 other into a nerve. In many sense organs the sense cell is multiplied, 

 there being from two to many cells (figs. i8, 24, SCW), the group 

 sometimes being contained within the limits of the normal hypoder- 

 mis, sometimes bulging or protruding from it into the body cavity ; 

 and sometimes the sense cells of neighboring sensilla form a continu- 

 ous layer beneath the normal hypodermal cells (fig. 24). In all such 

 cases, however, the sense cells are limited basally by the basement 

 membrane of the body wall. Other sense organs consist of a bundle 

 of sensilla in which there is an equal number of cap cells, enveloping 

 cells, and sense cells. 



The work of vom Rath (1896), Vogel (1923), and others appears 

 to demonstrate that a nerve fiber extends into each sense cell from 

 the connected sensory nerve trunk. Vogel says that in the antennal 

 sensilla of wasps {Pollstes, Vespa) an extremely fine fiber, less than 

 half a micron in diameter, can be traced to the base of the sense cell 

 in specimens stained by the Golgi method, and can be followed in 

 the other direction into the nerve trunk of the organ, leaving thus no 

 doubt of its being a nerve fiber. From the distal end of the sense cell, 

 likewise a fiber somewhat thicker than the basal one can be traced out- 

 ward to the cuticular part of the organ. Figure 14 A, taken from 



