6 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 
convincing even the veriest layman that much damage is being done, 
and that more is to be feared. For the proper understanding of this 
subject, some considerable knowledge of the habits and life-history 
of our forest insects is desirable, but perhaps I may, by taking a 
special case, be able to convince even the non-professional man 
that insects do play a very important and undesirable part in forest 
economy. 
Suppose we go into a middle-aged or old Scots pine wood some- 
time during winter, and proceed to examine the thick bark at the 
base of the trees, more especially of those that are semi-isolated or 
situated near the edge of the wood, we shall find that it is perfor- 
ated in exactly such a way as would result from the discharge of 
a gun loaded with No. 5 shot at a distance of about twenty yards. 
On following these perforations into the bark, we would find that 
they extended for an inch or so, but did not penetrate so far as 
the wood, and that each, or most of them, contained a small dark 
brown beetle in a semi-comatose condition, which we would have 
no difficulty in recognising as the well-known pine beetle (Hylur- 
gus piniperda). I fear that at this stage our non-professional 
friend would be apt to remark that if this small and apparently 
lifeless creature cannot show more destructive work than that of 
boring into the dead tissues of trees, it can hardly be said to have 
earned the amount of reprobation to which it has been subjected. 
So much I am prepared to admit, merely remarking that if not 
distinctly destructive, neither is it positively beneficial to tree 
growth in its winter quarters. 
Towards the end of March—a little later or earlier according to 
the character of the weather—the pine beetles withdraw from their 
winter quarters, and for the next two months are to be found 
under the bark of a certain class of tree. The conditions which 
the insects demand are very stringent, that is to say, so long as 
the conditions can be obtained, no trees will be attacked which do 
not fulfil these conditions. ‘The trees most in request are pines 
which are sufficiently old to be provided with thick bark, and 
which have been dead for a month or more. The thick bark is 
necessary, because the transition stage from larva to pupa being 
passed not between the wood and the bark, but actually in the 
bark, there would not be space for the accomplishment’ of this 
process were the bark not of a certain thickness, Trees in robust 
health are, under ordinary circumstances, free from attack, prob- 
ably because the insects are inconvenienced by the presence of 
