3162 



THE JOINTED ANIMALS 



these insects, they infest the leaves and buds of plants. They prick the leaves to 

 feed on the sap, their puncture being often followed by the formation of gall-like 

 swellings. The figured Psylla genistce feeds on the broom, but other species are 

 found on apple and pear trees. The plant lice (Aphidce*) are small insects, which 

 make up in numbers what they lack in size, and owing to the injury they inflict on 

 plants, must be ranked among the greatest pests with which the gardener and 



horticulturist have to contend. 

 They are those soft pulpy little 

 creatures, with rather long antennae 

 and conspicuous round eyes, so 

 commonly seen crowded together 

 on the under side of leaves, in buds 

 and flowers, in clefts in the bark 

 of trees, and sometimes even on 

 the roots. The antennae are com- 

 posed of from three to seven joints, 



Centrotus cornutus. on some of which are a number of 



(Slightly enlarged.) curious rounded pits, probably of a 



sensory nature. The eyes are placed 



on the sides of the head, and each has often a sort of supplementary eye attached 

 to its hind border; while in the winged aphides there are three ocelli on the 

 crown of the head. The beak is composed of three joints; and the tarsi are two 

 jointed and terminate in two claws. Wings, as a rule, are found only in the adult 

 males and in some of those generations of asexual individuals to be mentioned 

 presently. The fore-wings are longer than the hind pair, and placed in repose like 

 a roof over the hind part of the body. Both pairs have a scanty venation, con- 

 sisting in each wing of a single longitudinal vein, and of some simple or forked 

 branches given off obliquely from it. The number 

 of species is considerable, and there is scarcely a single 

 kind of plant that does not suffer as the special host 

 of some one or more. Many are green, whence the 

 name of green fly by which they are commonly known; 

 others are black, red, or some other color. They are 

 usually named after the plants on which they more 



particularly live, though each species is not necessarily confined to one kind of plant. 

 Thus we have the plant louse of the rose (Aphif rosce] ; the green aphis of the 

 apple {A. mail}, which is found also on the pear and sloe tree; the cherry aphis 

 {A. cerasi), and a host of others named in the same manner. The life history 

 of plant lice is very complicated; and although differing somewhat in different 

 species is always characterized by what is known as an alternation of genera- 

 tions. There are several broods or generations of these insects in the course of 

 a year, but it is only in the last autumn brood that true sexual individuals are 

 found. The males are generally provided with wings, but the females are larger 

 and wingless; they lay fertilized eggs, from which, in the following spring, 

 the first brood of the year is produced. The insects of this brood are usually wing- 



Psylla genistce. 

 (Six times natural size.) 



