373 
-191 6 — 17.1 The Family Cliermesidse. 
and East Europe, where larch is either absent or the European species is re- 
placed by Larix siberica, but are present in Britain, where larch and spruce 
grow side by side. On his experimental area the author estimates that 
90 per cent, of the species of Chermes s. str. and Cnaphalodes on spruce 
were the non- migrating, parthenogenetic species, although the branches 
of the spruce and the larch were often interlocked. This fact suggests 
that the non-migrating species have not arisen because the intermediate 
host larch was absent, but because they are the more successful species 
even when spruce and larch are both present. 
The question whether the above cycles should be considered as those 
of separate species or of biological races of one species can only be 
determined by further research, both morphological and biological. Statis- 
tical research, such as that done by Philiptschenko (48), will be important 
in determining this question. 
PART III.— RELATION TO FORESTRY. 
1. Methods of Infection. 
The methods of infection are the same in both the genera Chermes s. str. 
and Cnaphalodes. Conditions favouring infection are proximity of the 
hosts and favourable weather conditions. These explain the rapid spread 
of the pest in forest nurseries, which are usually sheltered, with the trees 
crowded together. 
(a) Spruce. 
Spruce is infected from two sources — (a) By sexuparm. The partheno- 
genetic adaptations on larch are made at the expense of the sexupara 
generation. Thus on the area under observation, although colonici were 
very numerous on larch, the number of sexuparse attaining maturity, and 
still more those reaching spruce and laying eggs, was small. Thus infec- 
tion from that source was not serious. (f$) By gallicolae non-migrantes. 
The majority of such gallicolse settled on the tree on which they were 
born. This, together with the high fertility of the two generations 
constituting that cycle, caused the rapid increase on the host. Thus 
infection from this source was serious. 
(b) Larch. 
Larch is infected by gallicolse migrantes. This is the source of the 
first infection, but the species are thereafter principally carried on on 
this host by the parthenogenetic adaptations. 
