NO. 8 MORPHOLOGY OF INSECT SENSE ORGANS SNODGRASS 35 



THE SENSE RODS, OR SCOLOPAL.E 



The scMise cell of the sense organs of adult insects, being a trans- 

 formed hypodermal cell, lies, during the formative stages, immedi- 

 ately beneath the cuticula. In the organs of vision the sense cells 

 usually become separated from the cuticula during development either 

 by invagination or by delamination ; in the other organs the sense 

 cells may retract inward, but they usually remain in connection with 

 the cuticula by chitinous strands of varying length. 



In the sensilla trichodea of the simpler types the distal process of 

 the sense cell may remain attached directly to the inner surface of 

 the cuticula of the hair membrane or the hair base (fig. 17 A, d). 

 In most of the other sense organs of the hair, plate, campaniform, 

 and chordotonal types, the end of the distal process of the sense cell 

 is more or less withdrawn from the inner surface of the cuticula, 

 but maintains its connection with the latter by means of a special 

 cuticular structure known as the sense rod, or scolopala (the " Stift " 

 or " Stiftkorperchen " of German writers). 



The sense rod usually has the form of a hollow cone, peg, or fusi- 

 form rod attached by its apex to the inner surface of the cuticular 

 part of the sense organ. The attachment to the latter may be either 

 direct, or by means of a long filament drawn out from the apex of 

 the rod. 



The form and complexity of the sense rods vary much in different 

 organs. In its simplest form, the rod is a mere cap, cone, or bulb 

 apparently investing the end of the sense cell process. According 

 to Hochreuther (1912) the sense rod in some of the tactile hairs of 

 Dytiscus is a simple arrowhead-shaped peg, attached by its apex to 

 the hair membrane or to the base of the hair. Sihler (1924) says 

 there are two forms of sense rods in the sense organs of the cerci 

 of Gryllus campestris; one form (fig. 15 A) has a pear-shaped head 

 and a long cylindrical shaft, the other (B) ends in a club-shaped 

 terminal enlargement. The walls of the rods of each form have ten 

 longitudinal thickenings or ribs {r, r), which, however, are not 

 continuous and their separated parts form two or three ribbed zones 

 in the walls of the rods, the positions varying in different organs 

 (A, B, r, r). The head of each rod contains a dark-staining apical 

 body {AB) from which there is continued a fine axial fiber (AxF) 

 into the body of the sense cell. In the campaniform organs of Dy- 

 tiscus marginalis, Hochreuther (191 2) describes a simple club-shaped 

 rod (fig. 15 C) with an apical point inserted into the dome-like ex- 

 ternal part of the organ (Do). There is here no apical body within 

 the rod and the axial fiber (AxF) is continuous to its tip. In the 



