54 THE PHYSIOLOGY OF INSECT SENSES 



cross in many directions (Fig. 49) (Graber, 1882 a, 1882 b; Radl, 

 1905; Hess, 1917). 



Typically the sensilla are stretched between two undifferentiated 

 points of the body wall by a ligament at the proximal (i.e., axonal) end 

 and by the cap cell or accessory cells at the distal (dendritic) end. 

 Consequently, the organs have the appearance of thin strands 

 (Fig. 46) or sheets (Fig. 47). 



Cue 



Fig. 45. Dorsal view of the base of the left haltere of Tipula paludosa 

 showing the location and orientation of the group of campaniform 

 and chordotonal sensilla. Re, Cue, aac, radial cubital, and ante-alar 

 chordotonal organs respectively; Ri, Rg, R3, Scg, radial and subcostal 

 groups of campaniform sensilla with arrows indicating long axes of 

 sensilla. (Redrawn from Pringle, 1957 after Zacwilichowski.) 



The structure of chordotonal sensilla is very complex and has yet 

 given no clue as to their exact mode of action. Each sensillum consists 

 of one bipolar neuron and a minimum of two companion cells which 

 are probably homologous with the trichogen and tormogen of other 

 sensilla (Schwabe, 1906; Schon, 1911 ; Vogel, 1912; Snodgrass, 1926; 

 Eggers, 1928). The three cells are arranged in overlapping linear 

 fashion. One, the enveloping cell, surrounds the proximal portion of 

 the dendrite ; the other, the cap cell, surrounds the remaining length 



