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INSECTS. 
giving rise to the latter often consists of winged individuals, which leave the plant 
on which they were born and fly to some other. In the genus Phylloxera, the 
males are wingless and each of the sexual females lays but a single egg, known as 
the winter eew ; but in other forms the number is often much greater. Each of 
the parthenogenetic females of Phylloxera may in the course of its life lay as 
many as two hundred eggs, and each of the viviparous females of other species may 
give birth before they die to forty or fifty young. When we consider that there 
are several generations every year, it can be easily understood how it is that these 
insects spread with such rapidity; and a sum in geometrical progression would 
show that the individuals which might arise in the course of a year from a single 
winter egg of Phylloxera, are not to be counted by hundreds or thousands, but by 
millions. Other species are capable of multiplying as rapidly. Fortunately, 
plant-lice have many enemies, such as the larvae of lady-bird beetles, of lace-wing 
flies, and of the flies of the family Syrphidce. These larvae devour great numbers, 
and ichneumon-flies also help to keep them in check. Plant-lice are divided into 
a number of subfamilies, of which the first is represented by the genus Aphis. In 
this genus the antennae are seven-jointed and about as long as the body; the two 
horny tubes called cornicles, which project from the back of the abdomen, are also 
characteristic. Through these tubes the lice secrete a sweet kind of liquid much 
sought after by ants, who, in an affectionate way, come and caress the aphides in 
order to obtain it. The sticky substance known as honey-dew, which is often 
spread in a shiny layer over the upper surface of leaves, is, in most cases, nothing 
but the liquid dropped by the crowds of plant-lice living above on the under side of 
other leaves. The members of the allied subfamily Lachnince have six-jointed 
antennae, and instead of cornicles possess prominent grandular structures placed on 
the back of the abdomen. The figured Lachnus punctatus is found on the willow. 
The apple-blight insect ( Schizoneura lanigera), 
which may be recognised by the white fluff 
covering in the wingless individuals the back 
of the abdomen, belongs to another subfamily. 
The winged individuals of this species are 
black, whereas those devoid of wings are of a 
yellowish or reddish brown colour, and live in 
the crevices of bark. The species is supposed 
to have been introduced from America, and was 
consequently at first known as American blight. In the genus Phylloxera —- 
distinguished among other characters by the three-jointed antennae—one species 
lives on the leaves of the oak-tree, while a second (P. vastatrix ) is the dreaded 
insect so destructive to the leaves and roots of the vine. These, like many other 
species of the family, cause the formation of galls on the leaves and roots which 
they attack. The curious galls with the appearance of small fir cones, so often 
seen on young shoots of the spruce-fir, are caused by a species ( Chernies ctbietis ) 
remarkable for its complicated life-history. 
The scale-insects ( Coccidcc ), which owe their name to the fact that the larvse 
and females of many species look like oval or rounded scales attached to the bark 
and leaves of plants, are very dissimilar in the two sexes. The adult males are 
Lachnus •punctatus (six times nat. size). 
