40 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 7/ 



of construction cannot often be observed. Schwabe (1906) is the 

 only writer who has said definitely that the sense rods lie within the 

 distal part of the sense cell process, though Erhardt (1916) gives 

 figures of the chordotonal organs from the wing base of Eristalis 

 Horeus in which the scolopalae are shown distinctly inside the long 

 distal processes of the sense cells. 



Eggers (1923) would explain the scolopalse of the chordotonal, 

 organs and of the organ of Johnston as originating from a fibrous dif- 

 ferentiation of the terminal part of the sense cell process, a condition 

 which he finds in the sense cells of the primitive organ of Johnston 

 in the antenna of a dragonfly larva. The distal parts of the fibers, 

 Eggers suggests, come together to form the terminal filament of the 

 scolopala, while their proximal parts fuse to form the ribbed walls 

 of the scolopala itself. This explanation, however, leaves us to assume 

 that the fibers are chitinizations of the lateral walls of the sense 

 cell, and, therefore, meets with the same objection above noted, viz., 

 that it violates the rule of similar internal chitinizations being formed 

 otherwise than as a surface deposit, or as a prolongation of the sur- 

 face deposit within the body of a cell. 



THE AXIAL FIBER 



An axial fiber (fig. 15 A, C, F, H, and fig. 26, AxF) has been ob- 

 served in all the dififerent groups of insect sense organs, though it 

 has not been shown to exist in every organ. When present, it tra- 

 verses the distal process of the sense cell from the body of the cell 

 to the sense rod (fig. 26). Its distal end terminates in the apical body 

 (fig. 15 H, AB), when the latter is present, or continues to the end 

 of the rod (C) when an apical body is lacking. Proximally, the fiber 

 is usually lost in the body of the sense cell, but Hess (1917) says, in 

 the chordotonal organs of Cerambycid larvae, it can be traced through 

 the sense cell into the nerve, and Schwabe (1906) claims that it 

 separates within the sense cell into fine fibrils which unite again into 

 a single fiber entering the nerve. 



The nature of the axial fiber has not been determined. Some investi- 

 gators believe that it is the end of the true nerve fiber of the sensillum ; 

 others claim that it is of a chitinous texture, though Sihler observes 

 that it is not cast ofif with the molted rod during ecdysis. 



IV. THE HAIR ORGANS 



All sense organs in which the cuticular part has the structure of 

 a hair, whether setiform, bristle-like, club-shaped, scale-shaped, cone- 



