222 
MR WALTER RITCHIE ON THE STRUCTURE, BIONOMICS, AND 
or retarding development, I may say that while on a stem, chosen for experiments 
and observations, adults were issuing after pupation from the sunny side of the 
stem in the latter half of August, a corresponding brood in the north side of the 
stem was still in the pupal stage in October. 
Quality of Tree used for Brood Purposes by M. minor and M. piniperda. 
I have no doubt whatever, from a very large series of observations, that M. 
minor for brood purposes, seeks a better quality of food material than M. piniperda. 
In a thirty-year-old suppressed stem, where I found the two species at work, 
M. piniperda for brood purposes had chosen the lower parts of the stem of the 
tree, M. minor the upper, thinner-barked parts. In another pine stem similar to the 
last mentioned, which I felled for purposes of minute examination, M. piniperda 
brood galleries were plentiful below the bark of the lowermost 4 feet of the stem, 
without a trace of M. minor ; whereas, from this height upwards to the crown, 
M. minor brood galleries were very plentiful. In another stem of a similar age, 
minutely examined, there was no trace of M. piniperda brood galleries, M. minor 
galleries commenced at 1 foot above the surface of the ground and continued 
upwards to the crown. In still another stem of similar age examined, which was 
comparatively thin barked throughout its whole length, M. minor galleries were 
very plentiful from the surface of the ground to the crown of the tree. In a 
number of fifty-year-old-tree stems examined, the foliage of which was much reduced, 
partly by the beetles themselves, M. minor galleries were very plentiful from 3 feet 
above the surface of the ground to the crown. No galleries of any kind were found 
below that height. 
I conclude from these and other observations that M. minor prefers the thinner- 
barked portions of standing stems, i.e. the upper and middle portions, avoiding the 
thicker-barked portions nearer the surface of the ground. This does not exclude 
altogether the use of thicker-barked parts. This is also the experience of Continental 
foresters. 
M. piniperda, on the other hand, prefers these lower portions of standing stems 
where the bark is thicker. In older stems, with thick bark, M. piniperda may be 
found breeding almost throughout the whole length of the stem. 
To ascertain whether M. minor bred on stumps of felled trees, I examined in 
June 1916 a large number of stumps of trees which had been felled in the previous 
year. Some of these stumps were situated at intervals throughout a wood, where 
I found M. minor breeding freely on weakened stems which were still standing : 
others were in an area of clear felling adjacent to this wood. In none of these 
stumps could I find any trace of M. minor. On the other hand, I repeatedly 
found M. piniperda breeding on them. M. minor does not breed on stumps, 
whereas M. piniperda breeds quite freely on them. 
