II FLIES WITH AQUATIC LARV^ 187 



climb. As soon as its wings are dry, it flies to the 

 trees or bushes overhanging the stream. 1 



The ascent of the Simulium fly through the waters 

 of a rushing stream is rendered safe by the hairy 

 surface of the body. The surface-film, by which the 

 air is imprisoned, clings to the hairs, and refuses to 

 enter the fine spaces between them. In the same 

 way a covering of velvety hairs prevents the Diving 

 Spider, as well as many diving Insects, from wetting 

 that part of their bodies which bears the spiracles. 



The flies of Simulium congregate, sometimes in 

 extraordinary numbers, on bushes and low trees near 

 the streams which harboured the larvae and pupae. 

 The males are distinguished from the females by 

 their large heads, a great part of which is covered 

 by the enormous compound eyes. In the male, but 

 not in the female, the upper part of each eye is built 

 up of larger facets than the lower half. No reason 

 is known for this curious feature, which is not un- 

 paralleled among Insects. The thorax is of large 

 size, the abdomen small. The legs are little used for 

 walking, but are continually exercised in what look 

 like exploring movements, the flies patting the 

 surface of a leaf with their feet. They are said to 



1 Verdat's original description is graphic and worth quota- 

 tion. When the fly reaches the surface of the water, he says, 

 " tous ses membres se deploient a la fois comme par explosion. 

 II se tient facilement sur 1'eau, ou il marche par le secours des 

 pelottes de 1'extremite des tarses, qui sont alors bien developpes, 

 et il gagne une tige voisine sans se mouiller. En tres peu de 

 temps ses ailes sont deployees et affermies. II quitte son ber- 

 ceau en prenant son essor, se repand partout dans la campagne 

 pour y chercher des alimens et sa femelle." 



