NO. 8 MORPHOLOGY OF INSECT SENSE ORGANS SNODGRASS 55 



organs would appear to be poorly adapted for the reception of olfac- 

 tory stimuli, and the responsibility might be shifted to the numerous 

 small, thin-walled hairs which cover the fllagella. The plates them- 

 selves are about 1.5 microns in thickness, but the surface over the 

 inner groove is not more than half a micron thick. A liquid might, 

 therefore, exude through the groove, spread over the surface of the 

 plate, and absorb odor substances, as some writers have suggested, 

 but, as mentioned before in the case of other supposed olfactory or- 

 gans, no one has observed the presence of any such liquid. 



VII. THE CHORDOTONAL ORGANS 



The sensory organs of insects known as the chordotonal organs 

 consist of bundles of simple sensilla, each of which comprises a 

 cap cell, an enveloping cell, and a single sense cell. The distal end 

 of the organ is attached to the cuticula of the body wall, but there 

 is no specially dififerentiated external receptive part, though the 

 point of attachment may be marked by a pit, a thickened disc, or 

 a nodule of chitin. The base of the organ is usually connected with 

 the hypodermis, often by a special ligament. A typical chordotonal 

 organ, therefore, is suspended between two points of the body wall. 

 The organs are frequently associated with enlarged parts of trachse, 

 or with tracheal sacs, and in some cases with membranous tympana 

 of the body wall. 



In form, a chordotonal organ is usually elongate or fusiform (fig. 

 25) ; its elements may, however, constitute an oval mass (fig. 27 B, 

 SB), they may spread out in the form of a fan (fig. 28, SgO), or 

 they may be arranged serially (fig. 28, TmO). The cap cell is gen- 

 erally elongate (fig. 26, CCl), sometimes attenuate and tapering to 

 the point of attachment (fig. 25), but it may be short and thick (fig. 

 29 B, CCl). The enveloping cell apparently does not reach to the 

 cuticula through the cap cell (fig. 26, ECl), but its distal end is 

 buried in the base of the latter. The sense cell is of the usual oval 

 or fusiform shape (SCI) ; its long distal process (d), inclosed within 

 the enveloping cell, has a well-developed sense rod, or scolopala (SR) 

 at its end. The chordotonal ligament, when present (fig. 25, d), is 

 inserted on the bases of the sense cells, and attached distally to the 

 hypodermis at a point opposite the attachment of the cap cells. 



The structure of the chordotonal ligament is not well understood. 

 According to Graber (1882), the ligament of the chordotonal or- 

 gans of the larva of Corethra has the general appearance of a nerve, 

 appearing to be a thin-walled tube filled with a homogeneous granu- 

 lar mass, the membranous walls of which are continuous with the 



