CHAP. XIV. FAMILIES OlP tttPTERA. 87 



CHAPTER XIV. 

 FAMILIES OP DIPTERA. (Two-winged Flies.) 



The insects belonging to this Order are divided into two 

 Classes, as follows: 



CLASS I. LONG-HORNED FLIES (Nemocera). In these insects 

 the antennae are usually long and composed of four or more 

 joints. None of these flies are beneficial, while several species 

 are occasionally very injurious. The pupa has the members 

 inclosed in separate sheaths. 



CLASS H.-^-SHORT-HORNED FLIES CBrachycera). In these 

 insects the antennas are short and have from two to three 

 joints, although in a few kinds the last joint appears to be 

 divided into two or more joints by impressed circles; but the 

 difference between these circles and the real divisions is easily 

 observed. These organs are sometimes bent down upon the 

 face, and the last joint is frequently furnished with a short 

 bristle (arista), which is sometimes pectinate, or plumose. 

 The pupa rarely has the different members inclosed in sepa- 

 rate sheaths, while in the greater numeer it is inclosed in the 

 hardened skin of the larva. Some of these insects are bene- 

 ficial, but the greater number are injurious. 



CLASS I. LONG-HORNED FLIES. (Nemocera.) 



MOSQUITOES (Culicidse). In these insects the mouth-parts 

 consist of six slender pieces or lancets. The larvae are aquatic 

 and feed upon decaying vegetable matter. It is only the 

 female mosquitoes that bite, and sucJfaf blood; the males being 

 perfectly harmless (Fig. 228). 



GALL-GNATS (Cecidomyidse). These insects are usually of 

 small size; the wings are furnished with three or four longi- 

 tudinal veins, and when at rest are generally folded flatly upon 

 the back. The greater number of these insects live in galls, 

 but a few kinds live in decayed vegetable matter, and one spe- 

 cies, which is closely related to the Wheat Midge, feeds upon 

 the gall-inhabiting species of the Grape Phylloxera. This 



