SENS A T1ON IN INSECTS. 499 



ever, the case is very different. Kraepelin [273] says : ' The seat 

 of smell has for a long time excited the interest of authors. An 

 extensive literature has accumulated, and there is no subject, 

 perhaps, in zoological knowledge which has been treated with 

 more acumen, and in which so extensive a literature has pro- 

 duced so small a result. Even to-day, in spite of the many 

 investigations of the last decade, we are far from a complete 

 solution of the subject.' 



I think, however, Kraepelin's masterly work, and the further 

 researches which have followed it, more especially in relation 

 to the structure of the antennal ganglia, justify the view that 

 the antennae are the main agents of the olfactory sense. 



Tactile Sense. The antennae are usually regarded as tactile 

 organs, but this sense is also undoubtedly common to the 

 larger setae, and perhaps to the integument generally. Osten- 

 Sacken says of the Diptera : I assume, therefore, that the 

 macrochaetae (large bristles) are organs of orientation, con- 

 nected with the nervous system, being in their useful action 

 not unlike the whiskers of a cat.' 



I have already observed (p. 474) that the phenomena of reflex 

 action, in a decapitated fly, show conclusively that the general 

 integuments, horny as they are, possess a keen sensibility, and 

 the persistent manner in wtiich insects clean themselves, or in 

 which Ants and other social insects lick each other, indicate 

 that the integuments are sensitive. 



Sensations Peculiar to Insects. -It has been suggested by many 

 that insects possess senses of which we are totally ignorant ; 

 but it is by no means easy to understand what is meant by the 

 suggestion. Johannes Miiller wrote : ' The essential attribute 

 of a new sense is, not the perception of external objects or 

 influences which do not act on the senses of man, but that 

 external causes should excite a new and peculiar kind of 

 sensation different from all the sensations of our five senses. 

 The possibility of the possession of such a faculty, by some 

 animals, cannot be denied. No facts, however, are known 

 which establish the existence of such a new mode of sensa- 

 tion' [192]. 



