DIPTERA 



301 



and ten or twelve feet long, and in which the larvas were piled up from 

 four to six deep. The larvas crawl over each other so that the column 

 advances about an inch a minute. 



Some species of Sciara are sometimes injurious to house plants and to 

 plants in greenhouses for their larvas live in the soil and eat off the roots 

 of the plants. 



Family Cecidomyid^e 



The Gall-gnats 



The gall-gnats are minute flies which are extremely delicate in struc- 

 ture. The body and wings are clothed with long hairs, which are easily 

 rubbed off. The antennas are usually long and clothed with whorls of 

 hairs (Fig. 519); but they vary greatly in length and in the number of 

 their segments. The legs are slender and quite long, but the coxae are 

 not greatly elongated and the tibiae are without spurs. 



In most, the wing-veins are greatly reduced in number, 

 the anal veins being entirely wanting. 



To this family belong the smallest of the midge-like 

 flies. The larvae of many species cause the growth of 

 galls on plants. Other species arrest the growth of the 

 plants they infest, and cause very serious injury. 



The larvae are small maggots, with nine pairs of 

 spiracles. The head is small, poorly developed, and with- 

 out mandibles. Many species are brightly colored, be- 

 ing red, pink, yellow, or orange, and many species 

 possess in the last larval instar a peculiar chitinous organ 



on the ventral aspect of the prothorax; 



this organ is known as the breast-bone or 



sternal spatula, or anchor process (Fig. 



520). It varies in form in different species ; different views 



are held regarding 



its function, none of 



which seems well 



established. 

 The larval mouth-parts are fitted 

 only for taking liquid food; but the 

 nature of this food differs greatly in 

 different members of the family. 

 Some suck the juices from the bodies 

 of living aphids, coccids, mites, and 

 larvae and pupae of other flies; but 

 most live on the juices of plants. 



The different species vary as to the 

 method of undergoing their trans- 

 formation ; in some the pupa is naked ; 

 in others the pupa is enclosed in the 

 dried skin of the larva; and in still 

 others it is enclosed in a delicate co- 



519. — An- 

 tennae of gall-gnats; 

 m, male; /, female, 

 enlarged more than 

 that of the male. 



Fig. 520. — Head- 

 end of larva showing 

 the breast-bone. 



coon. 



Fig. 521. — The pine-cone willow-gall. 



One of the most conspicuous of the galls made by gall-gnats is the 

 pine-cone willow-gall (Fig. 521). This often occurs in abundance on the 



