62 THE CAUSES WHICH PROPAGATE 



neuters or barren females follow a fertile matron to a con- 

 venient branch of a tree or nook suitable for placing- a new comb, 

 a proceeding probably due to the workings of rivalry, and which 

 many have thought guided by the hum of these insects. It may 

 be termed a celebration of a nuptial dance of barren females. 



But did not other attraction prevent display and music in 

 coercing insects into flocks to further reproduction and distribu- 

 tion, it is probable the smaller kinds would be extirpated. Insects 

 generally are aggregated by a common food, and in this case the 

 organs of scent and taste are the agents implicated ; others, as 

 bees and crickets, unite in nidification; and others are brought 

 together by secondary sexual characters. And, indeed, if we 

 examine the pageants in which love and rivalry promote the 

 reproduction and distribution of Insecta, and transmit these 

 forms of organic life in successive generations, w^e shall find the 

 sensorial organs implicated are those of hearing, sight, smell, 

 and touch, corresjDonding to the attractive qualities afforded by 

 sound, light, colour, shape, or odour, between which organs and 

 secondary sexual characters there is often correlative balance of 

 development. Thus the most musical groups, or those attracted 

 by odours, are usually dull in colour; and those bright in tone 

 have generally less efficient musical organs, or attractive secre- 

 tions are absent. But groups may be specialised by one or more 

 of these attributes. 



The employment of smell is markedly influential and widely 

 disseminated in insects ; and it may either be generally involved 

 in enticing the sexes to the emanation from flowery wastes, 

 fragrant leaves, fetid decay, or other pabulum ; or it may act 

 individually through a provision of specific or sexual secretions, 

 as those diffused by the '' fans " of Lepidoptera. There appears 

 also reason to suppose odorous glands exert their spell to conduct 

 invalid females to the momentary attractive upas plant of 

 oviposition, often alien from their accustomed food as regards 

 smell. 



The position of the organs of smell is yet in measure an 

 enigma. While some, with Cuvier, locate them at the mouth of 

 the spiracles, where, we shall shoAv, there exists in flies certain 

 structures that may be subservient, others find them in the head, 

 especially in the antennts. Huber, Kirby, and more lately Dr. 



