162 ANNUAL REPORT 



as laid,by having a harder shell and by being abundantly covered with 

 a gu i my substance, like many of the winter buds of trees, notably 

 that of the poplars themselves. This gummy substance not only 

 cements the egg firmly to the twig but acts also like a coat of var- 

 nish to the egg against the moisture and inclemency of the weather. 



The eggs of the species in question do not hatch Avith the first 

 appearance of the buds in spring, as is the case with most of the 

 tree-feeding species, but they appear first about the middle of May, 

 when the young twigs have already made a growth of some two 

 to four inches in length. The young aphis ascends immediately 

 to the very top of the twig and stations itself on one of the tender 

 leaves just expanding, thrusts its set* into it here and there causes 

 it to contract over the insect and form a hood-like covering which 

 soon develops into a gall. After the gall has thus been started 

 and affords a protection to the young aphis it is ready to undergo 

 its first moult, four of which are passed through before maturity 

 is reached. 



As some of you may not quite understand what is meant by a 

 moult as applied to insects, a few words of explanation may not be 

 out of place. Insects are soft-bodied creatures without any inter- 

 nal skeleton or bones as found in all higher animals, but their 

 body derives its firmness from the hardening of the outer skin, 

 forming a crust like the shell of an egg, to give a familiar example, 

 which corresponds to the skeleton as commonly understood. Now 

 if we should suppose an egg to grow we know it could not do so 

 without breaking the shell, and would need to acquire a second one 

 to take its place. This is what takss place among insects when 

 they pass through a moult. Our little aphis just hatched is rather 

 soft and allows of some growth, but soon on exposure to the air 

 the outer skin hardens and all further growth would be stopped 

 if provision was not made for the growth of a new and soft skin 

 under the hardened one which is now thrown off and allows the 

 insect to expand and materially increase in size again for a short 

 period of time. Thus the outer skin is cast off four successive 

 times in the course of about two weeks, and the insect increasing 

 in size after each, is mature soon after the last, when it begins to 

 bring forth young aphids of the second generation. This first in- 

 dividual starting the gall has been called the stem-mother by Prof. 

 Riley, a translation of the same word used by German entomol- 

 ogists, and may well be appropriate as she continues to bring forth 

 young aphids for more than a month, when from one to two hun- 

 dred individuals are found in the gall, which are all her issue. 

 Soon after this she dies without ever acquiring wings or leaving 

 the gall. 



The second generation differs from the corresponding stages of 

 the stem-mother by a longer and somewhat narrower body; in the 

 antennae at first four jointed, soon becoming five jointed and much 

 longer in proportion to the body. They are also more active, wan- 

 dering about in the gall from which deriving their nourish- 

 ment and acquiring full growth in it just like the stem-mother. 

 They also pass through four moults, increasing in size after each. 



