THE INSECTS. 351 



The order DIPTERA (two wings), metamorphosis complete, 

 have only one pair of membranous wings fixed to the 

 centre of the sides of the thorax, but the}' also have two 

 slender organs behind the wing, which, in entomological 

 language, are called halteres, or balances. The mouth is 

 provided with a sucker composed of six scaly lancet-like 

 pieces. Body in three principal divisions, head, thorax, 

 and abdomen. Legs, six. The eyes in the males are 

 generally very large, uniting in the crown of the head, 

 and in some occupy the entire head. This order com- 

 prises all the true flies gnats, gad-flies, wasp-flies, ant- 

 lions, &c. 



Westwood says : " The immense profusion with which 

 they are dispersed over the whole globe causes them to 

 fulfil two very important functions in the economy of 

 nature. First, they serve for food to a vast number of 

 higher animals swallows, fly-catchers, and many other 

 birds ; whilst, in the second place, they cease not in 

 effecting the disappearance of all substances in a state of 

 decomposition, both animal and vegetable." They are 

 universal scavengers, and so great is their activity and the 

 rapid succession of their generation, that Linnaeus declared 

 that these flies can consume a horse as fast as a lion. 

 That pest to the fly-fisher, the grouse- shooter, and all who 

 come in contact with it the midge Chironomus is of this 

 order ; and well may Westwood say : " Accordingly many 

 species cease not to make man their prey." 



The GOLDEN DUN MIDGE of Ronalds, who says it is a 

 good fly on hot days, is Chironomus plumosus, one of the 

 Tipulidce. 



The Cow- DUNG FLY (Scartophaga stercorarid) is a well- 

 known fly, and when blown on to the water has a very 

 peculiar way of buzzing along the surface. The body is 

 thickly covered with a yellow tomentum ; head luteous ; 

 the eyes and proboscis are black ; antennae black ; wings 

 are greyish, with a slight tawny tinge along the borders 

 of the veins ; abdomen and legs tawny. These flies are 

 plentiful in March and April, and at times the trout will 

 take them freely. 



