confine our thought almost exclusively to the most common 
form—the green plant louse, or aphis. 
As you gather the insects, you will observe that they differ 
in size from a minute object scarcely visible to the naked eye 
to the size of a pin head, and 
you will assume at once that the 
small ones are the young and 
the large ones the old individuals 
(see Fig. 2i). That is a correct 
assumption. 
Instead of taking the insects 
from the plants, take both for 
inspection, that the insects may 
be observed upon the branches 
where they occur. If you have a 
hand magnifying glass, observe 
next, by a side view, that the in¬ 
sects which are feeding have their 
mouths inserted into the tender 
stem of the plant, and remain at 
one spot as long as they are feed¬ 
ing. The mouth in this insect is 
a piercing proboscis. You will remember that in the house fly 
we observed a proboscis that took up food by lapping sweet 
liquids from the surface of objects. In the case of the green 
plant lice, the proboscis pierces the tissue of the plant, and draws 
the sap from the interior of the plant for its food. 
Let us now observe 
the full-grown insect 
closely. First, as to 
color: the very young 
are almost white, as 
they increase in size 
become yellowish green, 
and, when full grown> 
black markings appear 
upon the body and at 
the joints of the appen¬ 
dages. There are a few individuals with wings. Look carefully, 
therefore, over all the material at hand, and see if you cannot 
Fig. 21. Agamic female. 
With young at birth and two successive 
moults. Magnified 15 times. 
View from above. 
Fig. 22. Winged female. Magnified 7 J 4 times. 
