THE OLFACTORY SENSE 125 



developed antemiiie, have only moderate-sized compound eyes, 

 and ocelli are usually wanting. Many dipterous larvae are devoid 

 of eyes of any kind, but their olfactory sense is highly developed, 

 as for example in Callip/wra. Among many of the higher 

 Hymenoptera and Diptera, however, both senses are acutely 

 developed in the adults. 



For many years it has been believed that the true seat of the 

 olfactory organs is the antennae. It will serve no useful purpose 

 to discuss the extensive literature on this subject, and it will be 

 sufficient to stress certain features lending sui3port to the belief 

 mentioned. Elongated, movable organs, projecting outwards 

 from the body like antennae, are especially favourable as situations 

 for the location of olfactory sensilla^. As Forel has pointed out, 

 the position of such sensillae, externally on the antennae, gives to 

 the insect better olfactory powers at a distance than if the 

 odoriferous particles had to be drawn into an olfactory chamber, 

 as occurs in terrestrial vertebrates. It is a matter of common 

 observation that the antennae of many insects betray, by their 

 agitation and differential movements, an apparent acute sensitivity 

 to odours in their vicinity. Furthermore, elimination of their 

 functions by coating the antennae with impervious material or 

 by the amputation of those appendages has shown that certain 

 insects so treated no longer respond to odours, or do so much 

 more tardily. Mclndoo rejects the view that the antennae in 

 general are the seat of the olfactory sensillae. In a long series 

 of papers he has investigated the structure of supposed olfactory 

 sensillae and described the results of a number of experiments. 

 He criticises experiments involving amputation of the antennae 

 in that it induces abnormal reactions, yet, nevertheless, this 

 operation was carried out in some of his own experiments ! There 

 is no doubt that amputation should be avoided wherever possible, 

 and more natural reactions are likely to result when these 

 appendages are put out of action by coating over only. Mclndoo 

 maintains that the olfactory sensillae occur scattered over various 

 regions of the body, particularly on the bases of the legs and 

 wings. When these sensillae are varnished over a much slower 

 response results than when the antennae are amputated. Without 



