SMELL AND TASTE 20I 



sense of smell in these aquatic animals until we have reviewed 

 the sense in those invertebrates that live in air. 



The insects are by far the most numerous of those inverte- 

 brates but members of some of the other groups are also 

 terrestrial, the most important being the spiders and their 

 relations, millipedes, and some of the molluscs such as snails 

 and slugs. Some of the Crustacea and many of the worms 

 live on land but they, like numerous other isolated examples, 

 are the exceptions from the general habits of their groups 

 which are aquatic. Practically all the land-living inverte- 

 brates give some response to air-borne vapours, and thus 

 show that they have a sense of smell ; in many of them, how- 

 ever, there appear to be no special sense organs for smell, 

 or none that have been identified as such. Earthworms, for 

 example, are attracted from a distance by the presence of 

 food but the receptors for smell appear to be scattered 

 through the skin. In snails and slugs the receptors are be- 

 lieved to be more concentrated in the skin of the tentacles. 

 In spiders, too, there are specialized sense organs in different 

 parts of the body, but little is known of the particular function 

 of the different kinds, although it is known from their be- 

 haviour that certain spiders have a sense of smell. 



Insects have a well-developed sense of smell, and in some 

 of them it is so acute that it far surpasses anything that comes 

 within the range of our own experience. Much work has 

 been done on this sense of insects yet little is known of how 

 it functions. As previously mentioned, insects have numerous 

 sense-organs scattered over their bodies but because we do 

 not understand the exact use of many of them they are 

 merely called sensilla ; some of these certainly serve the 

 sense of smell. It is known that the sense is chiefly concen- 

 trated in the antennae of insects, and in the feet of some 

 butterflies, but it is also present to a lesser extent in the 

 general body surface ; furthermore the antennae bear more 

 than one kind of sensillum and consequently it has not been 

 possible to determine which kind is that for smell. Of course 

 it is possible, though improbable, that more than one kind 

 is sensitive to smells. 



The "assembhng" of the males of some kinds of moth 

 round a female, which was mentioned in Part I, page 98, is 



