ON THE LIFE HISTORIES OF FOREST INSECTS 31 



Four weeks later an inspection may show that the scolytid grubs have 

 reached full size and have turned into pupae at the end of their galleries. 

 The parasitic larvae and the Niponius ones may also have pupated in the 

 galleries. 



Some six weeks or two months after our first visit to the tree, we may 

 find the plan of the galleries of the bark beetle complete, and at the end of 

 each larval gallery will be a pupa, white in colour and having some 

 resemblance to the beetle, or a beetle itself complete in every detail. This 

 will mean that the life-cycle of that particular insect has been watched 

 from start to finish. In other words we shall have proved that in all 

 probability that particular species passes through more than one life-cycle 

 or generation in the year; and its life history will not be completely 

 known until we have ascertained how many of these life-cycles it passes 

 through in this period. 



In most cases it will be useless to watch for further generations in 

 the tree in which the one cycle has been reared. The bist layer will now 

 be too dry, except perhaps at the thicker butt end, where some of the 

 insects of the new generation may tunnel in to oviposit. It is a safer plan 

 to fell two more trees close by as soon as you see your beetles nearly 

 mature and ready to leave the first trees. 



The first generation of beetles on leaving your first set of trees wnll 

 repair to the second set to oviposit, and your observations may now be 

 transferred there for the next period. Some of these bark beetles pass 

 through from three to five generations in the year, and this will entail 

 felling from three to five series of trees in order to definitely ascertain the 

 correct number. All that is necessary is to wait till the beetles of a 

 particular generation you are watching in the trees become nearly black in 

 their pupal chambers, and then fell your next series of trees. 



With the maturing of our first lot of beetles in the first series of trees, 

 our observations in this series will not have come to an end, however. 

 Maturing with them, we must remember, will be their insect foes the 

 predaceous pink larvae and the parasitic grubs. Careful observations will, 

 therefore, enable us not only to ascertain the life history of the particular 

 bark beetle w^e are studying, but also at the same time that of one or 

 more of its insect enemies. For instance, you may find one or more 

 Niponius beetles easily recognizable from the bark beetle in the galleries ; 

 or at the end of one or more larval galleries you may see a small papery 

 cocoon, which on being opened is seen to contain a tiny Ichneumon, 

 Chalcid, or Bracon fly, perfectly developed and just ready to issue. For 

 it appears to be a general rule that many of these bark- and wood- 

 boring insect parasites pass through a similar number of generations to 

 that of their insect hosts ; a fact which would seem to be within the bounds 

 of probability. 



We have not yet done with the first series of trees, however ; they still 

 have in them the buprestid and longicorn grubs, whose growth is much 



