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ORDER VIIL DIPTERA. 



724. THE two-winged insects constitute one of the most 

 extensive Orders in the whole Class, not only in regard to the 

 number of distinct species, but also from the occasional excessive 

 multiplication of individuals of the same species. Many of them, 

 also, have been constant attendants upon Man in all ages. They 

 do not attract attention, however, from their size, for there are 

 few that exceed an inch in length ; nor is it on account of their 

 beauty, for the majority of them are of dull colours ; their forms, 

 too, are rarely elegant ; and of the habits and metamorphoses of 

 a large proportion of them, very little is known. They owe the 

 notice they have attracted, chiefly to the habits of certain species; 

 which affect Man and the Domestic Animals, in various ways, 

 both in their perfect and early states. However annoying these 

 may be, it must not be forgotten, that other Diptera are of 

 extreme service, in cleansing the surface of the earth of vege- 

 table and animal impurities ; and the carcass that is full of 

 maggots would be much more prejudicial in its decomposition, 

 than it is when principally eaten up by these voracious creatures. 

 The mouth in the insects of this order is formed for suction, as 

 already described ( 616); but there are considerable varieties 

 in the mode in which this is accomplished. Behind the wings 

 are found a pair of moveable slender bodies, termed halteres, or 

 balancers ; these are probably the representatives of the second 

 pair of wings. They are kept in continual motion, and are 

 usually present even when the true wings are not developed. 

 The Diptera all undergo a complete metamorphosis, as far as 

 respects the comparative structure of the Larva and the perfect 

 Insect; the former being generally cylindrical footless grubs, 

 with no representatives of legs, except in a few species. But in 

 many there is no proper transformation into the Pupa state ; the 

 skin of the larva not being thrown off, but hardening and con- 

 tracting, so as to form a kind of cocoon. Within this, the body of 



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