THE TRUE FLIES 



(Order Dipteral) 



All the true flies, that is, those insects which are called flies 

 and have but two wings, belong to the order Diptera. They are 

 the only insects which possess but two wings, with the exception 

 of the males of the scale insects, and a very few May flies (genera 

 Cloeon and 'Ccenis). Some insects in other orders have one 

 pair of wings so greatly aborted that they appear two-winged as 

 in the genus Psectra, one of the Lacewing flies. The wings are 

 membranous and usually transparent and bear no scales, except 

 in the mosquito family. The hind wings are represented only by 

 two knobbed projections called halteres, or poisers. The meta- 

 morphosis is very complete, the larvae being always footless and 

 usually apparently headless maggots and the pupae either some- 

 what resembling those of butterflies and moths, with compara- 

 tively free legs and wings, or they are enclosed in the larval skin. 

 Their mouth-parts are formed for sucking. The true flies com- 

 prise an enormous number of species. The most numerous of 

 all of the orders of insects are the Coleoptera, or beetles, the Hy- 

 menoptera, which we have just discussed, and the Diptera, and 

 for superiority in point of numbers the precedence must probably 

 be given to the Diptera. About forty thousand species are known 

 and it is estimated that the number yet to be described will bring this 

 number fully up to three hundred and fifty thousand, against three 

 hundred thousand which we have estimated for the Hymenoptera. 

 Not only have the true flies a superiority in point of numbers, but 

 entomologists are concluding that they probably stand at the head 

 of the insect system in point of evolution, that is to say, they 

 are the most highly specialized of insects. While they do not pos- 

 sess the apparent specialization in the way of intelligence and in 

 other respects seen with the bees, wasps and ants, the very com- 

 pleteness of their transformations and the highly specialized or- 

 ganization of the adults of several families support this view. 



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