198 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 egg ; 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 Syrphidai. 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 Lachninai 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 



Lachnus punctatus (six times nat. size). 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 {Chermes abietis) 

 remarkable for its complicated life-history. 



The scale-insects (Coccidai), which owe their name to the fact that the larvae 

 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 



