CHAPTER XXXI 

 THE DIPTERA 



The Diptera or Flies are small insects, the largest species known being 

 slightly more than two inches long, but the majority are much smaller, 

 and many are almost microscopic. The flies as a group are distinguished 

 from other insects by the presence of only one — the front — pair of wings, 

 attached to the mesothorax. Sometimes these are absent, the insect 

 being entirely wingless, but there are only a few such cases. The hind 

 wings have been transformed into a pair of curious structures known as 

 halteres. They are small and each resembles a sort of knob joined to the 

 body by a stalk, usually slender, and variable in length. They are 

 believed to have special functions but what these are is far from settled. 



The wings are usually transparent though sometimes smoky or other- 

 wise colored, and in some instances scales are present either along the 

 veins or elsewhere, and in one family they entirely cover both the body 

 and wings. The veins are usually quite numerous but often show a 

 tendency to unite toward the outer margin of the wing, forming closed 

 cells in this way as well as by the more usual method with cross veins. 

 In some families, the veins are very few and sometimes several appear 

 only as faint traces. The hinder margin of the wing not far from its 

 attachment to the body frequently has a notch called the axillary incision 

 or sinus, and the membrane from here to the base may form one lobe, 

 or by other incisions consist of two or even three lobes. The one nearest 

 the base in some instances appears to become enlarged and lie over the 

 base of the halter, often partly or entirely concealing this structure from 

 above. 



The head of the fly is connected with the thorax by a small neck which 

 permits considerable rotation. Much of its surface is occupied by the 

 very large compound eyes which frequently meet above, particularly in 

 the males. Between the two eyes, or behind their point of meeting, 

 are usually three ocelli on the top of the head. 



The antennae vary greatly. They may consist of as many as 16 

 segments or as few as three, in the latter case a bristle, frequently feathered, 

 being often present, joined to the outer segment. 



In one section, a crescent-shaped cleft occurs above the attachment 

 of the antennae to the head, curving downward on each side. This slit 

 is called the lunula, and at the time when the fly escapes from its pupa 

 case a large, bladder-shaped structure is pushed out through this from 

 the inside of the head, and pressing against the end of the case, forces it 

 off, enabling the fly to escape. Later, this structure which is call-ed the 

 ptilinum is drawn back into the head. 



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