EFFECT OF WEATHER. 15 



Again ; the caterpillar, or grub, feeds voraciously ; 

 and if there is nothing for it to feed on it soon dies. 

 This is another reason for thorough cleaning of arable 

 land. Many of the common weeds, such as Charlock, 

 Couch-grass, and also some of the marsh-weeds, keep 

 various of our pests alive and thriving, especially in 

 spring, till there is crop-food for them ; therefore, if 

 we bear in mind that if these voracious appetites are 

 not supplied the grubs die, we shall remember better 

 to have the jfields cleaned. 



The point of the caterpillar changing its coat from 

 time to time is sometimes most serviceably worked 

 forward, as in the case of the Turnip Sawfly, which 

 cannot quit its skin unless it is firmly fixed by the tail 

 of the old one ; if it cannot drag itself out, it perishes 

 in the stifling wrapping of its late coat. This know- 

 ledge is utilised by brushing ; or driving sheep through 

 the attacked fields ; and thus by simply loosening the 

 hold of a destructive pest we save much of the crop. 



Caterinllars can hear a great amount of cold without 

 any injury, so long as they remain where they have 

 placed themselves for the winter. Some will bear 

 being frozen hard without the least injury ; but if they 

 are disturbed from their cells under the surface of the 

 ground, and left exposed to freezing in wet loose soil, 

 they perish. 



Probably in this many circumstances act together, 

 ■ — the choking of the breathing-pores for one thing, 

 the drying up when exposed to wind for another, and 

 also the starvation of the creature if it wakes up from 

 its sleep in such circumstances. But however this 

 may be, if we know the habit of some caterpillars to 

 be helpless and torpid during winter, we can readily 

 act on it. 



But though some kinds of larvae will bear great cold, 

 yet the size, the colour, and the date of the appear- 

 ance, and also the presence or total absence of some 

 kinds of insects, have been found to depend greatly on 

 -difference in weather influence, and amongst other 



