142 THE SENSE ORGANS AND REFLEX BEHAVIOUR 



The simplest type of chordotonal organ consists of a single sensilla 

 which exhibits the following structure (Fig. 60, B). An ovoid or 

 fusiform sensory cell is provided with a distal prolongation 

 terminating in a sense-rod or scolopale. The scolopale is 

 strengthened by a variable number of internal ribs and is usually 

 capped by a deeply staining apical body. A delicate axial fibre 

 extends from the sense-cell and traverses the axis of the scolopale 

 to terminate in the apical body. The prolongation of the sensory 

 cell, together with the scolopale, are enclosed within an envelope- 

 cell. The latter, in its turn, is closely associated with an elongated 

 cap-cell, which serves to connect the sensilla with the integument. 

 The scolopale itself is often connected with the integument by 

 means of a terminal fibre which traverses the envelope-cell. At 

 the point where the distal extremity of the sensilla is fixed to the 

 cuticula, there may be a pit or a thickened disc, but there is no 

 special receptive structure. Proximally, the sensilla is attached 

 to the integument by a special ligament which is inserted into the 

 base of the sense-cell. It will be observed, therefore, that a 

 typical chordotonal sensilla is suspended between two points of 

 the integument. In some cases, it may be added, the scolopale 

 appears to be hollow, and its cavity is connected basally with a 

 vacuole filled with fluid. The scolopale is immersed in the semi- 

 fluid protoplasm of the envelope-cell, within which it is free to 

 vibrate. 



Chordotonal organs may be grouped into three classes arranged 

 in order of increasing complexity of structure : (1) simple liga- 

 mentous organs ; (2) Johnston's organs ; and (3) tympanal organs. 

 These three types are only briefly dealt with in the following 

 sections, since their structural features are well described in 

 modern text-books. A very full and admirably illustrated treatise 

 on the subject will be found in the recent work by Eggers (1928). 



Simple Ligamentous Chordotonal Organs. Organs of this type, 

 comprising small groups of one or more sensillse, occur in a great 

 many different groups of insects and their larv?e. Among larval 

 insects they are commonly segmentally arranged with an organ on 

 either side of each of the first seven or eight abdominal segments 

 (Fig. 60, B). Structures of this type are well known from Graber's 



