152 



THE SENSE ORGANS 



[CH. 



in chitin preparations, being purely a nervous formation. It 

 consists essentially of a ring of large bipolar nerve-cells (J), whose 

 fibres run back to form two bundles passing into the two main 

 divisions of the antennary nerve. It is difficult to understand 

 how such an organ could respond to anything except the vibration 

 or movement of the antennae as a whole, or, more particularly, 

 to the movements of the distalia inserted into the pedicel just 

 above it. 



r 



Fig. 65. Pedicel of antenna of larva of 

 Aeschna (x 70). D 1 first distale; 

 J Johnston's organ; na the two 

 main branches of the antennary 

 nerve; P pedicel; S scape; th sen- 

 sillae. Methylen blue preparation, 

 after Zawarsin. 



Fig. 66. Sensilla (tactile hair) from 

 antenna of larva of A eschna ( x 140). 

 can canal ; cut cuticle ; d circular disc 

 of hard chitin; h hair; nc nerve- 

 cell ; nuc its nucleus ; nv nerve-ending. 

 Methylen blue preparation, after 

 Zawarsin. 



Each segment of the antenna carries a few sensillae or sensory 

 hairs (fig. 66). These are organs of touch. They are most 

 numerous, though shortest, on the scape and pedicel. Each hair 

 is freely movable, set in a rounded depression (d), from which a 

 canal (can) runs inwards through the cuticle. This canal carries 

 the fine nerve-ending (nv) of a large bipolar sensory nerve-cell (nc). 



Zawarsin also found tympanulae on the scape and pedicel. 

 These are tiny circular grooves, with a raised wall covered over 

 by a chitinous membrane. A canal lying in the cuticle, in close 



