224 
MR WALTER RITCHIE ON THE STRUCTURE, BIONOMICS, AND 
maturity, -this feeding may take place in the place below the bark where the beetle 
was reared, i.e. in the neighbourhood of its birthplace, or in other species the 
imago makes an exit hole through the bark, and flies to a feeding place on another 
part of the tree. Myelophilui minor and M. piniperda are two excellent examples 
of this latter habit. 
It is clear, too, that the question of whether two broods — broods in the relation- 
ship of parents, children, grandchildren — are possible in a year is to a great 
extent dependent on two things, viz. whether a preliminary feeding is necessary 
before brooding, and if this feeding time be short or long. If a Scolytid species can, 
as soon as the imago stage is attained, issue and proceed without a preliminary 
feeding period to rear a brood, then in favourable environment this brood may 
issue and at once proceed to pair and lay eggs from which adults will be developed 
in the same year. That is, we can have two generations in the year, in the 
succession of parents, children, grandchildren. This seems possible with our elm- 
bark beetles and with Hylesinus crenatus of the ash. If the feeding, however, that 
follows the fresh appearance of the imago takes only a short time, a second 
generation in the year is also possible. But if the feeding preliminary to first pairing 
extends over a long time, then a second generation in the year will not be possible. 
This last seems to be the case in M. minor. Knocke, working in Central Europe, 
declared that this applied to M. minor, M. piniperda , and Hylesinus fraxini. It 
was part of the purpose of this research to verify Knocke’s statement for Britain, 
and later in this paper I give reasons in favour of the view that two generations in 
a year of M. ' minor or of M. piniperda are not found in Scotland. The proving of 
this is not so easy as it looks at first sight, because there is the possible compli- 
cation that there may be two broods in a year in the relationship of parents and 
children and a second brood of brothers and sisters, the parents of the first brood 
having fed and bred again. The facts can only be satisfactorily proved by a 
careful dissection and examination of the reproductive organs of young and old 
beetles. To Nusslin and Knocke belongs the credit of first directing attention 
to the need for anatomical information. For the proper understanding of the 
problem a knowledge of the structure of the reproductive organs of Scolytid beetles 
is necessary, and I take the reproductive organs of M. minor for illustration. 
The Male Reproductive Organs. 
Fig. 16 is a drawing of the male reproductive apparatus after dissection: 
testes, vasa deferentia, seminal vesicles, accessory glands, common vas deferens or 
duct, and penis. 
The testes are transparent glandular bodies, one lying on either side of the 
abdomen, ventrally. In side view they appear flattened from above downwards 
and rounded at the edges ; by these the sperms are produced. From the under 
