202 THE SENSES OF ANIMALS 



effected by the sense of smell with an acuity that is quite 

 unimaginable to us. We have already speculated on how 

 few molecules of an odorous substance are needed to stimu- 

 late a smell-receptor ; the assembling of moths must be an 

 extreme case, and one wonders whether a single molecule 

 is enough. On the other hand, an extremely small quantity 

 of matter contains a great many millions of molecules, so 

 that the number given off by the female moth may be very 

 large — did not someone say that we must all contain at 

 least one molecule that once formed part of the body of 

 Julius Caesar? 



The extreme sensitivity of assembling male moths to the 

 scent of the female may be because it is the only one that 

 they can perceive, so that no other scents can distract or 

 confuse them. It is also remarkable that in the species that 

 have this faculty the antennae of the males are very much 

 more elaborate in structure than those of the females, and 

 consist of feathery combs which greatly increase the surface 

 area on which the receptors are arranged. 



Many other insects besides moths have an acute sense of 

 smell ; usually it is important to them in finding their food. 

 Anyone who has travelled in the wilder parts of tropical 

 Africa, where there are no amenities of civilized sanitation, 

 will have noticed the extraordinary promptitude with which 

 the scarab beetles, which lay their eggs in a ball of dung that 

 they gather up, assemble the moment their favoured material 

 is provided. 



Among the butterflies there are many kinds that use the 

 sense of smell for another purpose — the males produce what 

 to us are sweet-scented perfumes in their courtship of the 

 female as aphrodisiacs or sexual stimulants to signal their 

 presence and stimulate her mating reactions. The perfumes 

 are produced by special glands and scales, and in some 

 species they are diffused into the air by special brushes that 

 act as powder puffs, but the exact kind of sensillum that 

 receives them in the female is not known. A close comparison 

 of the sense of smell in insects is not, however, possible with 

 ours because the particles that stimulate the sense of smell 

 in insects are not, as far as we know, dissolved in a film of 

 water covering the receptors, but act in the dry. The nature 



