CHAPTER XXVIII 



ORDER DIPTERA* 



The unnged members of this order have only two imngs; these are 

 home by the mesothorax. The secotid pair of wings is represented by a 

 pair of knobbed, thread-like organs, the halteres, these are present in 

 nearly all flies, even when the mesothoracic wings are wanting. The 

 mouth-parts are formed for sucking. The metamorphosis is complete. 



To the order Diptera belong all insects that are properly termed 

 flies, and only these. The word fly forms a part of many compound 

 names of insects of other orders, as butterfly, Alay-fly, and chalcis-fly ; 

 but when used alone, it is correctly applied only to dipterous insects. 

 To some flies other common names have been applied, as mosquito, 

 gnat, and midge. 



The presence of a single pair of wings and of a pair of halteres is 

 sufficient to distinguish members of this order from those of all other 

 orders, except in the case of a few wingless forms. 



This is a large order both in number of species and of individuals. 

 Aldrich ('05) gives a list of about eight-thousand North American 

 species, distributed in more than a thousand genera. 



Different species differ greatly in habits. Some are very annoying 

 to man. Familiar examples are the mosquitoes, which attack his 

 person, the flesh-flies, which infest his food, the botflies and the gad- 

 flies that torment his cattle, and the gallgnats that destroy his crops. 

 Some species are extremely noxious, being disease carriers, as for 

 example the mosquitoes that transmit malaria and yellow fever. 

 Other species are beneficial. Those belonging to the Syrphidas and 

 the Tachinidae destroy many noxious insects; and ven- many species, 

 while in the larv^al state, feed upon decaying animal and vegetable 

 matter, thus acting as scavengers. 



There are certain structural features of flies that are used in the 

 classification of these insects and to which special terms have been 

 applied. The more important of these terms are defined below ; others 

 are defined later in the discussion of chastotaxy. 



The head and its appendages. — The head is ver\' mobile, being con- 

 nected to the thorax by a slender neck. It is variable in shape and in 

 its relative size. 



The cmnpound eyes are usually large, sometimes occup}-ing a large 

 part of the surface of the head. ^'VTien the eyes are contiguous on the 

 upper side of the head they are termed holoptic; when they are sep- 

 arated more or less broadly they are termed dichoptic. In some flies 

 each compound eye is divided into two parts, one of which is a day- 

 eye and the other a night-eye. See page 144. 



The ocelli are usually three in number. 



*Diptera: dis (5is), two; pteron (irrepdv), a wing. 



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