70 DISEASES OF CROPS. 



construction of the ovipositor,^ wMch. has procured for 

 them the name of " saw-flies." This organ (Fig. 29 C), 

 which is to be found on the posterior-ventral side of the 

 perfect insect, is composed of a pair of broad, serrated 

 plates, the analogues of the inferior bristles of the bee's 

 sting. By the agency of this curious organ, the female 

 cuts numerous minute slits in the leaves of the turnip 

 plants, in each of which she lays an egg. It is said that 

 each female lays from two to three hundred eggs during a 

 season. The eggs are hatched in from four to ten days. 

 The larvse (Kg. 29 B) (called " niggers," " blacks," " black 

 caterpillars,'' etc.) feed upon the leaves of turnips, which 

 they reduce to mere skeletons of fibres, and sometimes 

 cause the complete destruction of the crops over a con- 

 siderable extent of country. The yoting larvse are of a 

 green colour, but ultimately turn black. " The larva of 

 A. spinarum is large at the head tapering to the posterior, 

 and possesses a remarkable structure in its feet, some of 

 which are hooked, and others act as suckers." In about 

 three weeks after birth these caterpillars retire to the 

 ground, where they spin cocoons in which they turn to 

 the pupal stage. During the early summer the pupse are 

 transformed into perfect insects in about twenty-one days, 

 but if late in the season the larvse remain in the cocoons 

 throughout the winter, turning to pupae in the foUowing 

 spring. There are several broods in a season. The per- 

 fect insect (Pig. 29 A) is a pretty black and yellow fly. 

 with short antennse, and is common in the fields during 

 the stimmer. According to the late Mr. Curtis, these flies 

 come over from the North of Europe, but are probably bred 

 in small numbers annually in this country. 



1 An organ which aids in the laying o£ eggs. 



