METAMORPHOSIS. 
71 
burst of a thunder-storm, will often cause the almost 
entire extermination of swarms, and wash them, never 
to return, from their native plants. Nevertheless, the 
close and hot atmosphere before thunder seems to be 
peculiarly suited to their propagation. At such times 
the winged forms occur in great number and take 
flight on the gentle winds, which transport them many 
miles to other feeding-grounds, to become the found- 
resses of other colonies. 
Although certain trees and shrubs, as the lime, the 
privet, the honeysuckle, &c., appear to be attacked 
exclusively by their peculiar Aphis, other trees give 
shelter indiscriminately to numerous species. Thus, 
the oak is infested by at least six, the birch and willow 
by eight, the elm by four, the fir tribe by eight, and 
the currant bush by three different species. 
Some botanical families appear to be exempt from 
their attack. For example, no Aphides have yet been 
observed to feed on the Fumariacise, the Gentianse or 
the Iridiae. Professor Kaltenbach considered that no 
Aphides attacked the Felices or ferns, but the family 
is not wholly exempt, for I have found, in the green- 
house, a new species which clusters on a foreign Cys- 
tojpteris. As far as I know, our indigenous ferns are 
not injured in the open air. From the silence of Lin- 
naeus, Fabricius, and Schrank on the subject, we 
might be led to think that the Labiatae and all crypto- 
gamous plants are free, but Kaltenbach with reference 
to the first, and Passerini with reference to the last, 
have proved that both families are injured by at least 
one species of plant-louse. 
The active principles contained in those saps which 
powerfully affect the functions of warm-blooded animals 
seem to have no deleterious action on Aphides. On 
the contrary, the sustenance of some kinds is entirely 
drawn from such irritant and poisonous juices as are 
elaborated by Digitalis purpurea^ Pap aver rhoea, Eu- 
phorbia peplus. Even the tobacco plant is laid under 
contribution by some Aphides. 
