132 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



(3) Colonici Generation. — The first-stage larvae migrate from 

 the needles to the bark of the intermediate host and there pass 

 the winter. In spring they wake up, feed, moult, and become 

 adult. All stages are wingless, and the generation consists 

 entirely of females. From their eggs two types of larvae 

 may hatch. 



{a) Sistens. — These are identical with the larvae of their 

 parent, and remain undeveloped for a longer or shorter 

 period. 



{b) Progrediens. — These larvae develop immediately, and may 

 become wingless females which produce a series of 

 similar generations on the needles of the intermediate 

 host. 



(4) Sexupara Generation. — The Progrediens larvse may 

 become nymphs, and then winged adults which are all females. 

 They migrate back to spruce and lay a small number of eggs on 

 the needles. 



(5) Sexuales Generation. — Larvae hatch from these eggs and 

 develop into males and females, wingless in all stages. When 

 adult the female lays a single, large fertilised egg under a bark 

 scale. This egg gives a first-stage Fundatrix larva. 



{D) Sexual reproduction is restricted in this family, for 

 females only are found in Generations 1-4 inclusive. Stalked 

 eggs are laid by the females of the Chermes species. 



Many of the great forest biologists, such as Hartig and 

 Ratzeburg, studied these insects, but little progress was made 

 until it was discovered that two hosts were necessary for the 

 complete development of the species. This discovery was made 

 independently by Blochmann, Cholodkovsky, and Dreyfus during 

 1889. Cholodkovsky has continued his researches until the 

 present day, while during the last ten years Borner and Nusslin 

 in Germany and Marchal in France have increased our know- 

 ledge of this difficult group. Although these pests are widely 

 distributed in Britain, little or no research has been done on 

 the family in this country. Buckton's descriptions in British 

 Aphides., vol, iv., were made before the discovery of the 

 migrations between the primary and intermediate hosts, and 

 are therefore incomplete and misleading. In 1908 Burden 

 published several papers, including a critical review of 

 Cholodkovsky's first monograph {Jour, of Econ. Biol.., vol. ii., 



