OP FRUIT TREES. 85 



one to four inches deep, and in ploughed land not 

 more than to the depth of seven or eight inches. 

 He has also ascertained that a part of the canker 

 moths rise in the autumn and deposit their eggs. 

 They are such as were an inch or two below the 

 surface ; those which lie deeper are not affected by 

 the transient changes of the atmosphere in Novem- 

 ber, and do not rise till the spring. The chrysalis 

 state comes in twenty-four hours after the larvae 

 has penetrated the earth, and it appears that the 

 insects are soon perfect, since a course of warm 

 weather has been found to raise some of them from 

 the earth in November. Those which rise in No- 

 vember are, not very numerous, compared with 

 those which rise in the spring, but being very pro- 

 lifick, are exceedingly injurious, if no means are 

 taken to prevent their ascending the trees ; as the 

 winter's frost does not kill the eggs. The warmth 

 of the season at the time of the descent into the 

 soil is favourable to the perfect development of the 

 insect in the chrysalis, particularly those which m* 

 nearest the surface, while those at the depth of 

 six or seven inches are longer in coming to maturi- 

 ty. The first are perfect in September, and re- 

 quire only to be excited to burst from their con- 

 finement ; but they cannot be excited until they 

 have passed through a degree of cold sufficient to 

 make them sensible of the mild temperature of the 

 atmosphere which occurs in November. The ex- 

 citability of such as lie deeper, and are not accessi- 

 ble by cold till a later period of the season, is not 

 so soon accumulated, nor are they sensible of slight 

 changes of temperature, which affect only the sur- 

 face ; they, therefore, do not leave the earth till 

 the spring, when the warmth of the air is longer 

 continued, and penetrates to the depth at which 

 they lie. To prevent the dreadful ravages of the 

 canker worm, the great object is to keep the female 

 from ascending the trees. For this purpose van- 



