378 Proceedings of the Royal Society of Edinburgh. [Sess. 
2. The collective damage to larch by the colonici of the species of 
Chermes s. str. and Cnaphalodes and the progredientes of 
Cnaphalodes is serious in Britain. 
3. The fumigation of coniferous nursery stock before dispatch to 
the planting area has proved a practical method of limiting the 
further distribution of these pests, and of ensuring that the plants 
get a reasonable chance of establishing themselves in their new 
environment. 
PART IV.— GENERAL. 
1. Theories as to the Origin of Migration. 
All authors, from Blochmann to Niisslin, have considered that spruce 
was the original host of the Chermesidae ; for instance, Cholodkovsky (29) 
thinks that the cycle was an annual one and on spruce only. Sexuales 
were produced towards the end of the summer. The winged forms were 
transported by wind to trees of other genera, and there adapted themselves 
to feeding and breeding. The migration back to spruce took place in a 
similar way to the first migration. 
In 1907 Borner (3) introduced a new theory which reversed the above 
theory. He held that the Picea host was intermediate, and that Pine was 
the primary host. Niisslin (46) pointed out the many difficulties which 
this new theory raised, and Borner (9) abandoned it for that of Mordwilko. 
Mordwilko (44) considers that all aphids were originally polyphagous, 
and that the present migrations are remnants of that ancestral polyphagia. 
The relative suitability of the hosts as regards food and breeding is the 
impulse inducing any particular migration. He divides modern aphids 
into groups on these lines : — 
A. A. group in which there are no real migrations ; two different 
host plants are not necessary, but the species are widely poly- 
phagous. Here come numerous Aphidinae, some Lachninae and 
Schizoneurinae. 
B. A group in which there is facultative migration ; two host plants 
may be utilised, while the polyphagia of the species is limited ; 
e.g. Siphocoryne xylostei, Schr., according to Mordwilko (44) 
can complete uninterruptedly its life-cycle on honeysuckle. The 
part from the first winged parthenogenetic females to the 
Sexuparae and the winged males can, however, be passed on an 
umbelliferous host. 
