38 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. y] 



the apex of the rod only a fourth or a third of the length of the cap 

 cell. Usually, in the tympanal organs of the Orthoptera, according 

 to Schwabe, there is no terminal filament. 



In the sensilla of the sensory pegs, cones, plates, and some of the 

 smaller thin-walled hairs, in each of which there is a group of sense 

 cells (figs. 18, 24, SCls) with their terminal processes bound together 

 in a bundle or fasciculus (Fas), the minute sense rods (SRs) are also 

 removed a considerable distance from the cuticula. They are attached 

 to the latter by a long terminal strand (TS) composed apparently 

 of the individual terminal filaments (fig. 14 B, t) of the sense 

 rods (SR). 



From a review of the descriptions of the sense rods in the various 

 insect sense organs given by recent investigators, we may conclude 

 that all the rod-like structures are homologous organs. Most in- 

 vestigators believe that the rods are dififerentiations of the ends of 

 the sense cell processes, and this idea appears to be substantiated 

 by all the known facts bearing on their nature. The appearance and 

 staining properties of the rods suggest that they are weakly chitinous. 

 Vogel says that the scolopalse of the chordotonal organs in the wing 

 bases of Lepidoptera have the same optical qualities as other thin 

 chitin, and that the inner walls and the ribs stain in eosin and hema- 

 toxylin, just as does the inner part of the chitin of hairs and of 

 the body wall. The recent observation by Sihler (1924), however, 

 that the sense rods in immature Orthopteran insects are shed during 

 a molt is a most important addition to our knowledge of these here- 

 tofore puzzling structures, and, if correct, fixes their status by show- 

 ing conclusively that they are not only of a cuticular nature, but that 

 they belong to the cuticula of the body wall. Sihler bases his claim 

 of the molting of the sense rods on observations on the sense rods of 

 the tactile hairs of the cerci of an Acridian, Gomphocems rufns. He 

 says that when the exuviae are separated from the cuticula during a 

 molt, there is usually to be seen attached to the base of each hair 

 of the exuviae a tubular appendage in which are distinguishable 

 both the head and the ribs of the sense rod as observed in specimens 

 prior to molting (fig. 15, A, B). 



If, then, we are to regard the sense rod, or scolopala, as a chiti- 

 nous product of the end of the sense cell, we must next consider how 

 it may be formed. Sihler and most other recent writers regard the 

 rod as a cuticular sheath (fig. 16 A, SR) covering the end of the dis- 

 tal process {d) of the sense cell, and attached by its apex either 

 directly, or by means of a terminal stalk or filament, to the cuticula 

 of the body wall. However, from a study of other internal chitinous 



