430 AN INTRODUCTION TO ENTOMOLOGY 



This indicating of a typical life-cycle is an effort to outline as 

 simply as possible the life-history of these insects. In some species 

 it is much more complicated; thus, for example, Borner ('08) in his 

 account of the life-history of Cnaphalodes strohilohius recognizes seven 

 parallel series of forms. 



The distinctive characters of the five differing generations in the 

 typical life-cycle are indicated below. 



A. GENERATIONS ON SPRUCE {PtCeo) 



A one-year cycle or the first year of a two-year cycle. 



First generation. — This consists of the true stem-mother (fundatrix vera), 

 a wingless agamic female. In the case of those supposed parthenogenetic species 

 which do not migrate to another host-plant and which complete their life-cycle in 

 one year, this form is the offspring of the second generation, an agamic fortn; 

 in the case of species that migrate to a secondary host-plant, and where there are 

 two parallel series, the stem-mother is the offspring of either the second generation 

 or the fifth generation, the sexual forms. 



The stem-mothers hatch in the autumn; they hibernate immature in crevices 

 at the bases of buds, complete their growth in the spring, and by their attack upon 

 the buds cause the beginning of the growth of galls. Each stem-mother lays a 

 large number of eggs. 



Second generation. — The members of this generation hatch from the eggs laid 

 by the stem-mothers, and by their attack upon the buds cause the completion of 

 the growth of the galls. The galls are formed by the hypertrophy and coalescence 

 of the spruce-needles. The members of this generation have been termed the 

 gallicolce, because they inhabit the galls. They reach the last nymphal instar 

 within the galls. When this stage is reached, the galls open and the nymphs 

 emerge and soon molt, becoming winged agamic females. 



As to their habits, there are two types of gallicolae: first, the non-migrants, 

 which remain on the spruce and lay the eggs from which the stem-mothers of the 

 one-year cycle are hatched; and second, tiie migrants, which fly to a secondary 

 host-plant, which is not spruce, and where they lay many eggs, but not so many as 

 are laid by the stem-mothers. 



B. GENERATIONS ON A SECONDARY HOST 



Part of the second year of a two-year cycle. 



The secondary host may be a species of either larch (Larix), pine {Pinus), 

 or fir {A bies) ; but no galls are produced on any of these. 



Third generation. — The members of this generation hatch from eggs laid by 

 migrants of the second generation that have flown from spruce to larch or other 

 secondary host and laid their eggs there. The young that hatch from these eggs 

 hibernate in crevices in the bark and comolete their growth in the spring, becom- 

 ing wingless agamic females. The members of this generation and of similar 

 generations which follow immediately but which are not numbered here, are 

 termed colonici, because they are settlers in a new region, or exsules, that is, 

 exiles. Some writers term the first of this series of generations false stem-mothers 

 {fundatrices spunce) to distinguish them from the true stem-mother, which is the 

 beginning of the two-year cycle. The members of the third generation resemble 

 those of the first generation, but usually lay fewer eggs and do not cause the 

 growth of galls. 



The offspring of the third generation are all wingless agamic females, which 

 reproduce their kind. Of these there may be a series of generations, which are not 

 numbered in this generalized statement ; and there may be among these several 

 parallel series of generations, differing in the life-cycle but all reproducing 

 parthenogenetically on the secondary host. The secondary host may be thus 

 infested throughout the year; while the primary host, if there is not an annual 

 series, will be free during the interval between the migration of the second genera- 

 tion and the return migration of the fourth generation. 



