156 SAWFLIES, ICHNEUMON-FLIES, WASPS, ETC. 



which time they eat voraciously. They go down into 

 the ground to change, and tlie new brood of Sawflies 

 in summer comes up, ready to lay eggs and start a 

 new attack in about three weeks. With this attack 

 we can, as with most others, do something by ensur- 

 ing vigorous plant growth ; but all methods that have 

 been found useful to check attack will be found to be 

 based on the special habits of the caterpillar. 



During its three weeks' life it changes its skin about 



Fig. 120.— Turnip Sawfly, magnified ; caterpillar, pupa, and 

 puj^a-case. 



'once a week, and to manage this it must hold fast, by 

 means of the pair of sucker-feet at the end of its tail, 

 to a leaf or stem, so as to have something to drag 

 against in the struggle to get free of its cast coat. If 

 it cannot do this it dies. Also it has only the power 

 of spinning a thread to let itself down, or come home 

 again by in passing alarms, during the first few days 

 of its life. After that, if it falls to the ground, it can 

 only return by crawling. 



The remedies which are chiefly used all turn directly 

 on these habits. The plan of driving sheep tln-ough 

 an infested field, or of brushing the plants with boughs, 

 either carried by hand or fixed on a scufticr ; or, 

 again, dragging a cart-rope over the plants ; — all turn 



