196 
INSECTS. 
feed on the sap, their puncture being often followed by the formation of gall-like 
swellings. The figured Psylla genistas, 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 amongst 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 composed of from 
three to seven joints, on some of which are 
a number of 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-jointecl 
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, consisting in 
1, Ledra aurita ; 2, The same seen from the side 
(both enlarged); 3, Aphrophora spumaria; 4, 
Larva of the same. 
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 com¬ 
monly known; others are black, red, 
or some other colour. 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 
Psylla genistas (six times nat. size), rose (Aphis rosce ); the green aphis of the apple (A. 
Centrotus cornutus (slightly enlarged). 
