42 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. "JJ 



and in the anterior region of the aHmentary canal. The movement of 

 the hair may be supposed to register a more gentle tactile stimulus 

 than would the effect of pressure on the general body surface, es- 

 pecially where the latter is covered by a coating of dense chitin. By 

 means of the sensitive hairs, moreover, the insect can become aware 

 of the approach or nearness of an external object before coming into 

 actual contact with it. 



The sensillum of a tactile hair contains usually but one sense cell 

 (fig. 13 B, SCI), though some are described and figured as having 

 two or several sense cells. The distal process of the sense cell is at- 

 tached to the hair membrane or to the base of the hair itself, or per- 

 haps within the cavity of the hair. The attachment may be direct (fig. 

 17 A), but it is usually by means of a terminal sense rod (B). Some 

 tactile hairs appear to be closed at the base by an imperforate mem- 

 brane (C). 



The supposedly chemoreceptive hairs are short and weakly chiti- 

 nized, having thin, transparent walls. Each is innervated through a 

 group of sense cells, the terminal strand from which penetrates the 

 cavity of the hair to its tip (fig. 17 D). The sensillum of a chemo- 

 receptive hair, therefore, is identical with that of a chemoreceptive peg 

 (fig. 18) except for the length and shape of the cuticular part. Chemo- 

 receptive hairs are found particularly on the antennae and the mouth 

 parts. 



SENSILLA CH.ETICA 



The sense organs classed under this head are separated from the 

 tactile hairs of the trichodea type only by the more spine-like or 

 bristle-like character of the external parts, but the distinction is arti- 

 ficial and unnecessary since the setal organs vary in shape from slender 

 hairs to thick clubs (fig. 17 G). The sensory spines and bristles are 

 usually thick-walled and densely chitinous ; some are said to be solid 

 structures (fig. 17 E). Probably most of them are organs of touch. 



The organs along the sides of the abdomen in the Nepidse, consist- 

 ing of small cavities with a fringe of movable, innervated spines within 

 the respiratory chamber's of the young, and of plates associated with 

 the abdominal spiracles covered with a mat of recumbent spines in 

 the adult, probably offer an example of a special use of a tactile spine. 

 These organs have been elaborately described by Baunacke (1912), 

 who believes that they are static in function. Their spines lie hori- 

 zontally in the plane separating the air in the air space beneath them 

 from surrounding water, and Baunacke, pointing out that a tilting 

 of the body in any direction would alter the plane of the spines, 



