30 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 'J'] 



cap cell. Horizontal sections through some adult sense organs show 

 clearly that each enveloping cell completely encircles the one within 

 it (fig. 24 B, C) ; but we must believe that this condition is a secon- 

 dary one brought about by an overlapping of one cell about the other, 

 since all may retain their connections with the cuticula. Berlese ( 1909) 

 has shown that the cells of a sensillum are entirely separate at an 

 early stage of their development. 



CLASSIFICATION OF INSECT SENSE ORGANS 



Since so little is known definitely concerning the functions of the 

 various kinds of sense organs possessed by insects, except in the 

 case of the tactile hairs and the eyes, we cannot at present speak 

 of them as organs of smell, organs of taste, organs of hearing, etc. 

 We must therefore, classify them according to their structure. De- 

 parting from the typical hair type, the external parts of the sense 

 organs become peg-like or conical ; losing the hair form altogether, 

 they are reduced to papillse or low domes, or they are flattened out 

 to plates or membranes. In still others there is no external part 

 except a pit, a disc, or a nodule of chitin to which the internal parts 

 are attached. In the organs of vision the external parts are simple 

 transparent cornese or lens-like thickenings of the cuticula. 



Most of the sense organs of insects can be grouped, therefore, 

 according to the form of the external cuticular part, though with 

 some the internal structure must be taken into account. Hence, 

 following in part the well-known classification of Schenk (1903), 

 the various kinds of sense organs known in insects will be described 

 in this paper under the headings of hair organs, campaniform organs, 

 plate organs, chordotonal organs, the organ of Johnston, and the eyes. 

 The hair organs include those in which the cuticular part is typi- 

 cally setiform (sensilla trichodca), bristle-like {sensilla cluctica), 

 scale-like (sensilla squamiforma), or peg-like or cone-like (sensilla 

 basiconica) . The campaniform organs include the various sense pa- 

 pillse, domes and " pores." The plate organs (sensilla placodea) are 

 those in which the external part has the form of a thin membranous 

 or chitinous disc or plate. The chordotonal organs and organ of 

 Johnston are internal structures consisting of groups of simple sen- 

 silla attached to the cuticula of the body wall. The eyes are the vari- 

 ous light-perceiving organs. 



THE SENSE CELL 



The essential element in an insect sense organ is the sense cell 

 with its nerve connection. The truth of this is attested by the fact 



