MIGRATION. 
73 
pupse never give birth to young ; this operation is 
deferred to the appearance of the imago. 
Migration seems to take place entirely through the 
agency of the winged females, just as we find to be the 
case amongst the ants. Probably through them alone 
a change in the description of food for the new colony 
is effected. They alone can select the plants best 
suited for depositing their young, which, immediately 
after being dropped by their mothers, commence 
sucking the sap nearest at hand. My experiments 
lead me to believe that when once an Aphis has 
accustomed itself to feed on a particular juice, it will 
prefer starvation to any change of food-plant, notwith- 
standing that, on the same spot perhaps. Aphides of the 
same species may be feeding on plants which do not 
belong to the same natural family. I have repeatedly 
observed the effect of stinted food in hastening the 
development of wings, whilst keeping the larvae in 
confinement under bell-glasses. 
Some naturalists have thought that the often sudden 
appearance of swarms of winged Aphides in early spring 
may be caused by the action of the nipping easterly 
winds, which, by checking the flow of sap in the vege- 
tation, remotely produces the same effect on the in- 
sects as the stinted food above noticed. To this 
atmospheric condition, which is usually accompanied 
by insect swarms and a peculiar haze, the popular voice 
gives the name of blight. Similar conditions of food 
and climate probably operate to produce the second 
large migratory flights of early autumn. 
Mr. Walker has shown that certain Aphides have a 
true migration from one species of plant to another. 
Without some such action in their economy we can 
scarcely see how the insect could prolong its existence 
beyond the duration of the plant on which it feeds. 
He affirms that the hop Aphis, Phorodon humuli, makes 
its first appearance on the common sloe, Prunus spinosa^ 
and that Siphonophora granaria migrates in autumn 
from the wheat to several kinds of grass. Examples 
