SENSATION AND REACTION 73 



therefore directly pressed or bent when the hair touches an 

 external object. As far as physiological mechanism can 

 guide us, we may conclude that the insect's reception of 

 and reaction to tactile stimulation are far more delicate than 

 our own. But we have no evidence that an insect's " sense 

 of touch," experienced in whatever form of consciousness 

 the creature may possess, is more vivid than ours, or indeed 

 that it is like ours at all. 



Besides these " sensory " hairs and spines whose function 

 is undoubtedly tactile — that is to say, the nerve-ending can 

 be stimulated only if the hair actually touches some external 

 object — the feelers and parts of the jaws of many insects 

 carry blunt peg-like outgrowths of the cuticle in which the 

 chitin is markedly thinner than in typical tactile hairs. 

 Into the cavity of those also pass delicate processes of nerve- 

 cells in the skin. Most students of the sense-organs of 

 insects — for example, A. Forel (1908) — have regarded these 

 as concerned with smell or taste, considering that the cuticle 

 covering the nerve- endings is sufficiently thin and delicate 

 to permit the permeation of vapour and thus allow the 

 nerve-endings to be stimulated chemically. Such peg-like 

 organs are abundant on the feelers, maxillae and labium of 

 many insects, and experiments made by Forel and others 

 show conclusively that insects can be affected from a 

 distance by pungent substances, the sensibihty diminishing 

 or disappearing if the feelers be cut off. N. E. Mclndoo 

 (1916), on the other hand, has pointed out the improba- 

 bility of delicate chemical stimulation, such as that which 

 results in sensations like smell or taste, acting through 

 cuticle even though it be thin and delicate ; therefore he 

 regards these peg-hke organs as tactile Hke the slender and 

 more rigid hairs or spines. 



The feelers and palps of insects, however, are often 

 provided richly with sensory nerve-endings of apparently 

 another type, which have been described by K. M. Smith 

 (19 1 9) as well as by several previous writers. The positions 

 of these are evident through the presence of pits or pores on 

 the surface of the cuticle. Study of sections of the organs 



