2B 
become roughened, thereby rendering the plant liable to disease and 
injury by climatic influences. Any one who has watched the opera¬ 
tions of the wooly aphis, of the apple tree, is aware how rapidly a 
little colony causes a minute crevice in the bark to expand. 
Some aphides limit their attacks to timber trees exclusively; others 
feed on soft’ succulent vegetables and low herbs; while others infest 
the roots of grasses and other plants, or hide under stones and the 
rotten mortar of old walls. 
Although some plants, especially trees and shrubs, appear to be at¬ 
tacked only by their peculiar aphis, other trees are infested by several 
species, each appearing to select only a certain part as its point of at¬ 
tack, a few changing during the season from one portion to another; 
for example, the Grape Phylloxera passes from the leaves to the roots; 
the Wooly aphis from the bark to the roots; the Corn louse from the 
tassel to the ear stalk and thence to the root. Some, by a peculiar 
property not fully understood, are enabled to form galls upon the 
leaves, in which they pass their entire existence through numerous gen¬ 
erations, and until the time arrives for them to seek their winter quar¬ 
ters, or homes for new colonies. As these insects are not furnished 
with jaws by means of which they can cut their way out of their 
prisons, by a wise provision of nature these galls always form so that 
a minute passage remains open as a means of exit. This fact enables 
the entomologist to decide at once whether a gall is the work of an 
Aphis or some other insect. 
Some families of plants appear to be exempt from the attacks 
of these insects. For example none have so far been observed to 
feed upon the Fumariaceae , the Genticinae, or the Iridae. The L<th¬ 
in tae are almost wholly exempt, as are the Ferns and other Cryptoga- 
mic plants. Acridity and poisonous qualities do not appear to prevent 
their attacks, as some of the most virulent plants are infested. 
How those species which inhabit annuals, pass the remainder of the 
year after these plants die, is not well understood, and is one of the 
points which needs further investigation, as it may possibly furnish 
one method of counteracting such as are injurious to useful annuals. 
These insects, as others, undergo, in their process of growth, several moult- 
ings, that is, cast off their outer or external integument; the number 
of moults through which they pass is supposed to be four, but in re¬ 
gard to this there is some difference of opinion. 
One of the strangest facts connected with the history of the plant- 
lice is their method of reproduction. Butin order to understand this 
it is necessary to give a brief account of their life history through 
one season as is generally observed. 
These insects, as likewise all other species belonging to the order 
Homoptera, undergo only a partial -metamorphosis or transformation, 
that is to say, they never are worms or grubs, and never undergo those 
remarkable changes which transform the grub into a beetle, and the 
caterpillar into a butterfly. The larvae and pupa 1 are similar to the 
perfect insect in form and habits; and although they frequently cast 
their skins, and the winged specimens gradually acquire these organs, 
there is no true dormant pupae or chrysalis state, as we find in many 
other insects. Their whole lives are therefore devoted to imbibing 
food and in producing new broods. 
