27 
ing them even when they exist, there is considerable difference in 
opinion, some contending they are usually absent, while one or 
two of the latest authorities contend they are generally present. 
The honey tubes also form a characteristic of the family, though 
subject to numerous exceptions where they are wanting. 
The character and veining of the wings as heretofore described form 
tolerably good distinguishing features of most species; some of the 
different forms and modes of venation being represented in figures 4, 
5, 6 and 7. The chief difficulty in using the wing characters arises 
from the fact that the very large majority of the individuals we see 
are without these organs, and do not acquire them during life; yet, 
as will be seen hereafter, there are few, if any, species without winged 
individuals, of one or both sexes, during some portion of the year, al¬ 
though brood after brood lives and dies with most of its members 
wingless. The very great similarity of the wingless individuals of the 
different groups, renders any specific characters derived from their col¬ 
or, markings, etc., often of doubtful value; and this fact has so im¬ 
pressed itself upon the casual and ordinary observers that they usually 
designate them, no matter where found or what their habits, by the 
common name Plant-lice, as though all belonged to one species. 
Naturalists have therefore sought some more positive method of dis¬ 
tinguishing species from each other; this has been found in their 
habits. 
It was the belief of the immortal Linnaeus that every plant support¬ 
ed a distinct species of Aphis, or, in other words, that a species of 
Aphis inhabited but one species of plant; and Curtis, the great English 
(entomologist, tells us that, after careful and extensive examination of 
this subject, he is inclined to subscribe generally to this opinion, add¬ 
ing that, although a species of Aphis is limited to a certain plant, yet 
there may be more than one species of plant-louse to one plant. 
Although most species have the bodies naked, there are many which 
are more or less covered when young or during life with a cottony or 
downy substance, on which account they are often called “Wooly plant- 
lice.” This substance is secreted by certain glandular organs situated 
in the thorax and abdomen. It is not, as is supposed by many, a waxy 
secretion, but when held over the lamp scorches brown, giving off an 
jodor like burnt feathers. It is composed of fibers or reduced to par¬ 
ticles giving the insect the appearance of being covered with fine 
meal. 
Life History and Habits. 
As a general rule plant-lice prefer warm and somewhat moist situa¬ 
tions, which are sheltered from the direct rays of the sun and strong 
drafts of air. Some species feed exposed on the upper surface of 
leaves, but many more select the underside where they are less ex¬ 
posed to the sun and their numerous enemies. By constantly with¬ 
drawing the sap they cause the leaves to curl and blister, and gradually 
losing their vitality, to die. Others select the young twigs of trees 
and tender shoots or stems of plants; while others reside upon the 
trunks and branches, where by constantly drawing the sap from the 
bark they not only lessen its vitality, but cause it to split, wrinkle or 
