172 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



cycle of the same insect Gillettea cooleyi. Fi^e years later 

 Chrystal studied the chermes in Vancouver, where he unwittingly 

 confirmed Borner's suggestion, and traced Gillettea cooleyi from 

 the Sitka spruce to the Douglas fir and back again. Both 

 Gillette and Chrystal, however, failed to trace the sexual stages 

 of the chermes on its primary host, and although Chrystal's 

 work clearly showed that the Sitka spruce chermes and the 

 Douglas fir chermes were merely different stages in the one 

 insect's life-cycle, the necessary connecting link, the sexual 

 generation, had not been found. 



When Chrystal began his researches under the Forestry Com- 

 mission, Chermes {Gillettea) cooleyi yi^.% known in this country only 

 on the Douglas fir, and the first question to be settled in regard 

 to it was to ascertain whether it had a primary or spruce host 

 in Britain, and if not what factors contributed towards the 

 reduction of the life-cycle. Chrystal's research has gone a 

 considerable way towards solving the problem. In the first 

 place, both laboratory experiments and field work have shown 

 that, so far at least, the Douglas fir is the only host on which 

 C. cooleyi persists in this country. Further, it has been shown 

 that the full life-cycle of the insect, which evidently exists in 

 America, has, as it were, been short circuited, and that C. cooleyi 

 exists in this country only as a parthenogenetic race incapable 

 of producing successful sex elements, although a sexual genera- 

 tion on the Sitka spruce may be and is, in fact, frequently 

 produced. 



In addition to new information on the biology of the 

 Douglas fir chermes, the Forestry Commission bulletin 

 contains important discussions on the chermes problem 

 generally. The researches of Cholodkovsky, Borner, Marchal, 

 and Gillette among foreign workers, and of Burdon and Steven 

 in our own country, have been summarised and correlated. 

 A particularly important section deals with the sex generations 

 in the Chermesidae. The relations of the various chermes 

 species to their hosts are discussed, important new observations 

 being given as regards C. cooleyi, while an interesting account 

 is given of some of the known natural enemies of the 

 Chermesidae. 



Lack of time — Chrystal's work was unfortunately suspended 

 owing to economy restrictions — prevented any proper study of 

 the important question of the modes of dispersal of the chermes. 



