3164 THE JOINTED ANIMALS 



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 ladybird beetles, of lace-winged 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 antennas 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 honeydew, which is often spread in a shiny layer over the surface of leaves, is, in 

 most cases, nothing but the liquid dropped by the crowns of plant lice living above 

 on the under side of other leaves. The members of the allied subfamily Lachnince have 

 six-jointed antennas, 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 recognized 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 color, and live in 

 Lachnus punctatus. the crevices of bark. The species is supposed 



(Six times natural size.) to have been introduced from America, and was 



consequently at first known as American blight. In the genus Phylloxera dis- 

 tinguished among other characteristics by the three-jointed antennas 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 ( Coctidce) , 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 

 provided with one pair of wings; the hind-wings being rudimentary or altogether 

 absent; they have rather long antennae, distinct eyes, and, in some cases, are 

 furnished with two long bristle-like tails. These winged males are very rarely 

 seen, which is accounted for by the fact that their mouth parts are atrophied, 

 so that they are incapable of taking nourishment, and live only a short time. 

 The females are always wingless, and usually remain fixed to one spot, with their 

 beak buried in the tissues of the plant, and their back often spread out in the 

 form of a shield covering the head and body. The beak is generally three jointed, 



