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MR WALTER RITCHIE ON THE STRUCTURE, BIONOMICS, AND 
which I had planted young pines. At the same time I placed vertically in the cage 
fresh logs of Scots pine, some thick barked, other thin barked, in which the beetles 
could breed if they chose. A few days later I found these young beetles tunnelling 
in the shoots of the young pines. There they had gone to feed in order to ripen 
their reproductive organs. I examined the pine logs, caged with the young pines, at 
intervals of a few days, but still the beetles remained in the shoots. During the 
later months of the year — November and December 1915, — on examining some of the 
beetles in the shoots they appeared quite dormant, evidently feeding very little and 
using the shoots for hibernating purposes. This cage was kept outside, but all the 
beetles died in January 1916 because of unfavourable circumstances. In no case did 
they ever attempt to make a brood gallery on the pieces of stem supplied during the 
year 1915, a fact proved by external examination and dissection of the logs. 
In August 1916 I repeated this experiment, collecting the young beetles as before 
which emerged from their breeding stems, commencing August 25, 1916. Now I 
examined, after dissection, the reproductive organs of some of the females just as they 
emerged from their brood galleries. The reproductive organs were found to be quite 
immature (see fig. 18) and the beetles quite unready for egg-laying. The rest of 
these beetles I liberated on young pines protected against the entry of any other 
insects, and they proceeded to tunnel in the pine shoots for feeding and for the ripen- 
ing of their reproductive organs. On November 3, 1916, I killed some, and dissected 
out the reproductive organs, which were found to have undergone little further 
development and were still unripe. 
One must remember that with severer weather conditions in late October, 
November, and December the beetles feed very little, remaining dormant for most 
of the time. The daily temperature by this time is now much lowered, so that 
feeding practically ceases until the warmer weather in the following spring. It is 
impossible, then, that in the case of' AT. minor , whose first brood emerges in August, 
the reproductive organs of the females can be in a fit state for efficient copulation 
and egg-laying in the same year. They cannot be in such a state until the following 
spring. From the results of these experiments and dissections, I conclude that only 
one generation is possible in a year. 
In experiments similar to those already mentioned, which I carried out to obtain 
information as evidence on this particular point, in the case of M. piniperda I found 
that the young beetles did not breed in the same year as they emerged from their 
pupal chambers, but remained in the shoots throughout the winter months, breeding 
in the following spring. 
The Occurrence of Two Broods in a Year. 
While two generations in a year are impossible, the occurrence of two broods in 
a year is possible : that is to say, the adult beetles, after the laying of the eggs from 
