45° THE INVERTEBRATA 



to the waves of light and sound, to changes of temperature, to chemi- 

 cal stimuli by contact or at a distance, e.g. as in the sensations of taste 

 and smell, and to tactile impressions. The sensory equipment is 

 complicated, and the solution of the functional problem which many 

 of its parts present is not made easier by the fact that though the 

 principle of the reaction may be the same as in ourselves, insects often 

 react to stimuli of an amplitude which is beyond our receptive 

 capacity. For instance, they do react to pitches of sound which 

 the human organ cannot detect, and though they do not appreciate 

 the full spectrum in colour vision, they can perceive ultra-violet rays. 



No matter what the sense organ may be, the fundamental element 

 is the sensilla. In the case of a simple sensory hair {trichoid sensilla) 

 the following elements are present: a trichogenous cell which gives 

 rise to the seta ; a hair membrane cell which produces the fine mem- 

 brane at which the seta is articulated to the body wall, and a bipolar 

 nerve cell which lies within the trichogenous cell (Fig. 315). Such 

 sensillae are generally tactile, though in certain cases olfactory, gusta- 

 tory and heat-perceiving functions have been shown to rest in them. 

 Olfactory sensillae commonly occur on the antennae. These are 

 generally placoid (with plate-like cuticle covering the sense cell) or 

 coeloconic (where the covering plate is thin and sunk in a depression 

 below the surface) (Fig. 315). But though the antennae are usually 

 olfactory in function, this sense is also located elsewhere, since re- 

 moval of the antennae does not entirely inhibit olfactory sensation. 



The power of insects to diffuse scents from special glands is well 

 known. These serve for defence, or to attract the sexes to each other, 

 and their prevalence, and wide distribution throughout the class, 

 postulate the existence of an olfactory sense. In moths the faculty 

 possessed by males of discovering the exact position of unpaired 

 females is of so astonishing a character that many observers have dis- 

 believed the olfactory explanation, and resorted to theories of etheric 

 wave-transmission. The production of a volatile chemical is clear, 

 however, in those cases where male moths have assembled at an 

 empty box in which a female had been recently housed. It is com- 

 paratively simple to demonstrate the existence of a taste sense in 

 insects. Preferences for sugar to other substances in solution can 

 readily be shown in a feeding butterfly. To find however, for example 

 as in Pyrameis, the red-admiral butterfly, that the taste organs lie in 

 the feet, is perhaps sufficient reason for using the term chemo-tactile ^ 

 for a sense which has no exact parallel in our own experience. Taste 

 organs occur also in the mouth, and on the palps of the mouth-parts. 



Many insects, such as grasshoppers and cicadas, are provided with 

 sound receptors known as tympanal organs, with which are incor- 

 porated chordotonal sensillae. Each of the latter consists of a sense 



