The injuries occasioned by plant-lice, are much greater than would at first be expected, from an observation of the small size and extreme weakness of the insects; but these make up by their numbers what they lack in strength individually, and thus become formidable enemies to vegetation. By their punctures and the quantity of sap they draw from the leaves, the functions of these important organs are deranged, or interrupted, the sap is withdrawn or contaminated, and unfitted to supply the wants of vegetation. Plants are differently affected; some wither and cease to grow, their leaves and stems become sickly, and die from exhaustion. Others, not killed, are greatly impeded in their growth; the tender parts, which are attacked, become stunted and curled. The punctures of the lice appear to poison some plants, producing warts or swellings, which are sometimes solid, sometimes hollow, containing within them a swarm of lice, descendants of a single individual.[28] These last are often seen upon the leaves of the Elm, and upon some Poplars, and other trees; but I have not found any upon the foliage of our cultivated fruits, unless it be those on the grape. =Aphis mali=, or the Apple Leaf-louse, is a small, green insect without wings, accompanied by a few black and green ones having wings. These are all crowded together upon the green tips of twigs, and under the leaves, sucking the sap. The eggs remain in deep cracks of the bark during the winter, and hatch as soon as the buds expand in the spring. The most successful treatment is to scrape off the loose bark, and to apply to the stems of the trees alkaline or lime washes. Many of our familiar little winter birds consume these eggs. In the spring and summer, alkaline solutions may be used with advantage, syringed or sprinkled upon the affected shoots and foliage. The smell of these insects is peculiar, which, indeed, is generally characteristic with bugs. Each sort seems to derive a special flavor from the tree or plant upon which it feeds. Most insects of this family secrete copiously a sweetish fluid, called the honey dew, which is ejected from two little horns or nectaries, that project, one on each side of their bodies. This sweet material attracts a great many flies, and other insects, particularly ants, which are the constant attendants of these creatures, and are said to protect them from their enemies in order to obtain their sweet secretion. Some entomologists have called _Aphides_ the Ants' cows.