8 THE BIOLOGY OF INSECTS 



receives an impression from the surroundings, resulting in 

 the transmission of a nerve-impulse to the central system, 

 its arrival at which may or may not result in a definite 

 sensation, but will surely result in the transmission from the 

 central system, to muscle-fibres or other tissue, of an impulse 

 which will lead to suitable movement or other response — 

 the whole operation aflFording an example of what physio- 

 logists call a '' reflex." In insects and other arthropods the 

 most characteristic feature of nervous action results from 

 the fact that, the living skin being everywhere covered by 

 the cuticle, impressions on the nervous system can be made 

 only through some modified part of this cuticle. Although 

 that envelope may be generally described as a protective 

 armour defending its wearer from outside influences, yet 

 it possesses hundreds or thousands of admirable modifica- 

 tions adapted for the reception of stimuli. An insect's 

 body or limb-segment often bears many " hairs " or 

 ** bristles " — stiflF projections, each jointed to the general 

 cuticular surface by a flexible basal region (Fig. i, A). Each 

 hair is the secretion of a special cell of the skin, and if 

 this cell in touch with the hair or bristle, be prolonged 

 into a nerve-fibre running towards a ganglion, the hair is 

 ** sensory " in function, the resulting sensation being com- 

 parable to a sense such as touch, which we ourselves know 

 by experience. Sensory hairs of this type are often especially 

 numerous on certain appendages of the head — notably on 

 the feelers, which are modified Hmbs belonging to one of 

 the brain-segments, and on the jointed leg- like " palps " 

 borne on some of those hinder head-limbs that sei-ve as 

 jaws. Even the highly specialised senses of hearing and 

 sight depend upon specialised cuticular areas. Thin, tense 

 patches of the cuticle in many insects are capable of being 

 thrown into vibration by impinging sound-waves, so that 

 adjacent nerve-endings can be aflfected ; while in almost 

 every insect the paired eyes, so prominent on the head, 

 show a reticulated surface of transparent cuticular facets, 

 each of which overlays a set of nerve- elements modi- 

 fied from the skin and in connection by means of the 



