SURROUNDINGS OF GROWING INSECTS 199 



blood-sucking females. Certain species of these anopheline 

 mosquitoes are notorious and of very great importance because 

 they serve alternately with human beings as hosts of micro- 

 scopic animal parasites which, in man, give rise to forms of 

 malaria and other dangerous diseases. The larva of Anopheles 

 (Fig. 103) is of the same general build as that of Culex, but the 

 spiracles (/), instead of being situated at the end of a long 

 siphon are found occupying a rather prominent area on the 

 dorsal aspect of the eighth abdominal segment. The Ano- 

 pheles larva is to be seen lying horizontally suspended, by pairs 

 of beautiful stellate hairs on the dorsal aspect of five of the 

 abdominal segments, from the surface film, which is broken 

 immediately over the spiracular region so as to bring the 

 cavity of the larval tracheal system into continuity with the 

 atmosphere (Fig. 104 c). While the larvae of culicine mosquitoes 

 many of which carry the germs of dangerous human maladies, 

 as the Anophelines do, in warm and tropical countries live in 

 ditches, in cisterns, or in the water accumulated in neglected 

 pots and cans, the anopheline larvae are found rather in clear 

 ponds, in swamps, and in slow-running streams in which there 

 is a fairly abundant growth of aquatic plants. The important 

 and fascinating subject of the relation of such blood-sucking- 

 insects to human diseases is beyond the scope of this book, 

 but it may be pointed out how closely knowledge of the insect 

 life-histories bears on practical measures of " protective medi- 

 cine." The incidence of certain diseases has been immensely 

 reduced in many tropical districts by altering the environ- 

 ment so as to render it unsuitable for the disease-bearing 

 insects to breed in ; l the spread of knowledge of these life- 

 histories has set men draining swamps, covering cisterns with 

 lids, and ditches and puddles with a film of oil impenetrable 

 by the gnat larvae, clearing away old cans and vessels, wherein 

 stagnant water may accumulate, from the neighbourhood of 

 settlements. The question of the surroundings of growing 

 insects has much to do with human life and industry ; it is 

 doubtful if without a drastic change in the surroundings of 

 the Canal Zone across the Isthmus of Panama, whereby the 

 number of mosquitoes was immensely reduced, that famous 

 waterway could ever have been completed. 



1 R. Ross : " Mosquito Brigades, and how to organize them ". London, 

 1902. 



