How they become aware of their surroundings 



males, but both sexes have a tympanal organ for receiving the sound. 



A number of other groups of bugs are able to make sounds, and so 

 are many beetles. The mechanisms differ in detail, and use different 

 parts of the body, but nearly all of them involve rubbing one roughened 

 surface or edge against another. It is not always clear how the sound is 

 utilised by the insects, since they often have no obvious organ for hearing 

 it, other than the small hairs scattered over the body. It may be that 

 soimd production is a by-product of a movement made for another 

 purpose, and this may link up with the 'ticking' of the Death Watch 



Beetle — produced by tapping the head 

 against the wall — and the * click' of the 

 Click Beetle (Frontispiece) as it flips 

 over from lying on its back. 



Touch 

 The sensory hairs that we have men- 

 tioned in connection with hearing are 

 really delicate organs of touch, and 

 their response to sound waves is only 

 one of their uses. A hair is a modified 

 part of the body-wall, to which it is 

 joined at the base by a flexible mem- 

 brane. Our own hairs, being soft, do. 

 not feel a touch, but give a strong 

 nervous impulse if they are pulled; the 

 insect's hair, being stiff, rocks on its 

 joint if it is touched, or set in motion 

 by a sound wave. The joint is suppHed 

 with a nerve cell, and so every move- 

 ment of the hair is felt by the insect. 



Sensory hairs occur all over the 

 body, but they are most numerous 

 and most sensitive on the antennae 

 (Fig. 38), and on the tarsi, or feet, the 

 last few units of the segmented leg. 

 Insects may regularly be seen exploring 

 the object in front of them, either 

 with the antennae or with the fore-feet. 

 Watching an insect doing this is curi- 

 ously reminiscent of watching a blind 



67 



Fig. 42. Olfactory pits in 

 the antenna of a fly: A, 

 antenna held out during 

 flight, so that the pits are 

 exposed to the air-stream; 

 B, detail of an olfactory 

 pit, with nerve-cells pro- 

 jecting into its base as 

 sensory rods. From Wig- 

 glesworth, 1953 



