HOMOPTERA 431 



Among the offspring of the third generation two types are recognized by 

 Marchal ('13): first, nymphs which remain undeveloped for a time, the sistens 

 type; and second, nymphs which develop at once into wingless agamic females, 

 the progrediens type.* 



Fourth generation. — The members of this generation are produced by indi- 

 viduals of the progrediens type of the third generation. They develop into winged 

 agamic females. The adults migrate to spruce and there lay a small number of 

 eggs. Since their offspring are the sexual forms, this generation is known as the 

 sexuparce. 



C. A GENERATION ON SPRUCE 



The completion of the second year of a two-year cycle. 



Fifth generation. — From eggs laid by the sexuparae that have migrated from 

 the secondary host to spruce, there are developed males and sexually perfect 

 females, termed the sexuales; both of these forms are wingless. They pair and 

 each female lays a single egg. These eggs hatch in the autumn; the young 

 hibernate and become the true stem-mothers. Thus is completed the two- 

 year life-cycle. 



Omitting the annual series, the typical two-year life-cycle includes the follow- 

 ing series of generations, which are described above. 



First. — The wingless agamic stem-mother. 



Second. — The winged agamic migrants. 



Third. — The wingless agamic colonici or exsules. 



(a) The sistentes, several generations. 



(b) The progredientes, several generations. 

 Fourth. — The winged agamic sexuparas. - 



Fifth. — The wingless sexuales, males and sexually perfect females. Each 

 female produces a single fertilized egg, from which hatches a stem-mother, thus 

 completing the life- cycle. 



In the case of some species, which have been studied very carefully 

 by different observers, only an annual series, consisting of the first 

 and second generations described above, is known. It should be 

 noted that in a life-cycle of this kind there are no sexual forms and that 

 although a winged form appears it is not known to migrate. These 

 facts indicate that either some members of the winged generation 

 migrate to a secondary host-plant which has not been discovered, or 

 that the species in question have become, by adaptation, purely par- 

 thenogenetic. Which of these alternatives is true has been much dis- 

 cussed. 



The following species are some of the more common of our repre- 

 sentatives of this subfamily. 



The pine-leaf adelges, Adelges pinifdlicB. — Our knowledge of the 

 life-history of this species is still fragmentary. In one part of its life- 

 cycle it infests the leaves of white pine {Pinus strobus) . The genera- 

 tions found here are winged agamic females. These attach them- 

 selves firmly to the pine-needles, each with its head directed towards 

 the base of the needle. Within each there are developed about one 

 hundred eggs, which are not extruded. After the death of the female, 

 the mass of eggs remains adhering to the leaf, covered over and 



*Si.stens, Latin sislo, to stand; progrediens, Latin pro, forth, gradior, to go. 



