150 PLANT LICE. 
spicuous, because the louse is covered with a white woolly mat- 
ter. In consequence of their work the bark ceases to grow 
wherever such clusters occur. This species has also a root-in- 
habiting form, which, like the phylloxera, causes gall-like growths 
on the smaller roots. This makes this pest a very difficult one 
to combat. 
Fig. 136, Plate X, shows a plant-louse in the act of sucking; 
it will be seen that the beak does not enter into the tissue at all, 
but that the four lancets laid together to form a single long tube 
is the only organ that is used to obtain the liquid sap of the 
c d. e 
Fig. 139a—Venation of wings of plant lice: A, upper wing of Sipphon- 
ophora; B,. lower wing of same; a, sub-costal vein or cubitus; b, poster- 
ior margin; c, costa or costal vein; d, third discoidal or cubital vein; e, first 
fork or first furcal vein; f second fork or second furcal vein; g, second discoidal 
vein; h, first discoidal vein; i, the base or point of insertion; k, apex or apical 
margin; J, sigmatic or fourtii vein; p, hooklet; 1, costal cell; 2, basal cell; 3, 
first discoidal cell; 4, second discoidal cell; 5, infra-marginal cell; 6, margina, 
cell; 7, second cubital cell; 8, first cubital cell; 9, stigma; b, wing of Aphis, /, 
of Schizoneura; d, of Tetraneura; e, of chermes. After Thomas. 
plants. The structure of the beak of a plant-louse is also shown, 
in Fig. 137, greatly enlarged. It also shows the very peculiar 
feelers, covered with an immense number of sense-organs (Fig. 
138). The arrangements and distribution of these organs are of 
great importance in classification. 
The Aphididz comprise four sub-families; the following 
table for separating these sub-families is published by Dr. 
Thomas, in the “Eighth report of the State Entomologist of IIli- 
nois.” It is necessary here to give the names applied to the veins 
of the wings, as some of them are used in this table (see Fig. 
139). The principal vein of the front wing, that which extends 
—_—e 
