THE PINE INVESTIGATION. 373 



droctonus frontalis) were killed when different stages occurred 

 in the bark some distance above the base of the tree; but 

 Tomicus pini, Tomicus calligraphus, Tomicus cacographus and 

 Tomicus caelatus, which often occurred in the same bark with 

 the destructive species and pass the winter in the adult stage in 

 the bark or in the wood and bark of small twigs, were not af- 

 fected in the least so far as I could observe ; neither were species 

 of the genus Pityophthorus^ Pityogenes, Polygraphus and 

 Crypturgus, the latter being among the smallest of all the bark 

 beetles. All of these pass the winter in the adult, and some 

 of them in all stages except the eggs, in the same manner as 

 did Dendroctonus frontalis. Polygraphus rufipennis may be 

 found in all stages in spruce bark during the winter, which was 

 not apparently affected in the least by the cold, even on our 

 higher mountains where it was probably much colder than at 

 a place where the temperature was recorded. 



Insects, and especially bark beetles, are capable of with- 

 standing a very low degree of temperature within their natural 

 hibernating places, yet sudden changes from low to high tem- 

 perature, or the reverse, may, and doubtless does kill enormous 

 numbers of insects of various kinds. 1 In the case of southern 

 species extending northward during a series of mild winters, I 

 should not be surprised if a period of excessive cold tempera- 

 ture would exterminate them north of their natural limit. 

 Possibly some species of wide range of destruction over differ- 

 ent zones of temperature may be less capable of resisting ex- 

 treme cold or sudden changes than those which are confined to 

 a restricted area where extremes of temperature occur, but it 

 does not seem probable that this would be the case. 



It is not possible, from our present knowledge of the de- 

 structive pine bark beetle, to say whether or not it has extend- 

 ed its range of destruction from the south or from the north ; 

 but the fact that it has been taken in the northern, eastern, 

 western, and southern portion of the United States, would in- 

 dicate that it is a species which is capable of existing under 



1. observations on the effects of cold (12 degrees below zero, 1897) on three species of 

 bark beetles, leads me to conclude that D. frontalis may have been exterminated by 

 the same cold of Dec. 1892, and Nov. 1893. 



