SENSATION AND REACTION 71 



impulse passes from cell to cell along the path of the 

 connecting fibres, a fibre-axis at its extremity coming into 

 touch by means of exceedingly minute branches with 

 relatively short finely branched outgrowths from other 

 cells ; thus a nerve impulse is passed on through a series 

 of " cell-relays." An insect's apparently purposeful move- 

 ments are therefore dependent on the co-ordinated trans- 

 mission of a large number of impulses through the various 

 parts of its nervous system, from receptive nerve-endings to 

 the ganglia and thence to the muscles. 



The nerve-endings of insects that can be affected by 

 outward stimulation are of many kinds, adapted for the 

 reception of varied types of influence. Attention has been 

 repeatedly directed to the horny cuticle which covers an 

 insect's body, and mention has been made of the hairs and 

 spines — often long and conspicuous- — which are specialised 

 portions of the cuticle, jointed on to the main surface by 

 basal flexible membrane (Fig. i). Many hairs are hollow, 

 and into their cavities pass fine thread-like processes of 

 nerve-cells lying in the skin, whence nerve-fibres lead to 

 groups of cells, situated in some ganglion. Such nerve- 

 endings as these are clearly adapted for receiving tactile 

 impressions. The slightest contact of any outside object 

 with the hollow hair must lead to stimulation of the 

 included thread-Hke process. Thus the skin-cell is affected, 

 and from the cell the impulse passes along the nerve-fibre 

 to a ganglion cell. Various objects that may be touched 

 will necessarily cause many nerve-endings to be affected, 

 and thus a number of impulses will simultaneously travel 

 to a gangHon, where, through the linkage of the receptive 

 cells, they will be co-ordinated and the impulses passed 

 on to motor cells, whence by efferent fibres they will 

 be reflected to the muscles whose contraction will result 

 in the reaction appropriate to the stimulation received. 

 The feelers carried on an insect's head, the palps con- 

 nected with the jaws (maxillae and labium) as well as 

 certain abdominal appendages such as the tail-feelers 

 (cerci) are often richly provided with tactile organs, so that 



