THE TWO-WINGED FLIES 127 



the adult state, these flies bear a general resemblance to 

 the familiar house fly, although commonly they are some- 

 what smaller and are easily distinguished by experts. The 

 Cabbage Maggot, which is also known as the Radish Maggot, 

 and the Turnip Maggot, is a good example of these insects. 

 The adult flies appear in the cabbage fields when the plants 

 are set out, and deposit their 

 small white eggs about the 

 base of the stalks. A few 

 days later these eggs hatch 

 into tiny, whitish, footless 

 maggots that attack with 

 their rasping mouth parts the 



outer tissues of the roots. "'^^^^^'~'~' CABBAGE 



They continue to feed and "^^P MAGGOT 



grow for three or four weeks, 

 commonly causing the death 

 of the plant on account of 

 the destruction of the sec- 

 ondary roots or of the girdling of the primary root. By 

 the end of this period they become full-grown as larvae 

 and change to pupae. 



This process of changing to the pupa state in a large 

 proportion of the two-winged flies differs from that of most 

 other insects. The insects are unable to spin cocoons, but 

 they get the protection of an outward covering in this way : 

 The skin, which in most insects is cast off when the larva 

 changes to a pupa, in these maggots gradually hardens and 

 becomes brownish in color. It does not split open any- 

 where, so that it forms a covering to the insect, inside which 

 the latter becomes a pupa after this outer skin, which is 

 now called f he puparium, has hardened. It remains within 

 this puparium about a fortnight when it again changes into 



