There are also many insects which live as parasites on plants, and demand from them both a resting-place and their food. Almost all the Hemiptera are among these; we have already mentioned them. The hemiptera, which live on the sap of vegetables, are parasites in the same manner as those which live at the expense of animals. We ought not to make a difference between the manner of life of the bugs of plants and those of animals. It may be said that Providence has placed these beings as riders on both the vegetable and animal kingdoms to 263 restrain them with a bridle. What the gardener does to plants, the aphis has often done before in order to arrest a too vigorous and rapid growth. [Illustration: Fig. 80.--Cochineal insect, male (Coccus cacti), natural size and magnified.] The cochineal insect (_Coccus cacti_) Figs. 80 and 81, originally 264 from Mexico, lives on the cactus nopal as a true parasite, and furnishes a precious colouring matter, carmine. This insect has been introduced into the Antilles, Spain, the Canary Isles, Algeria, and Java. [Illustration: Fig. 81.--Cochineal insect, female.] [Illustration: Fig. 82.--Aphis.] Lake is produced by a species of the same genus, originally a native of India (_Coccus lacca_). Aphides (Fig. 82) feed on the sap of plants; they multiply rapidly without the male insect. Rose-trees, and more especially their buds, are attacked by a species of a green colour, of which we give a representation (Fig. 83). An aphis, the _Phylloxera vastatrix_, has, a short time since, invaded the vineyards, and small as it is, it is dreaded as a plague which scatters ruin in its path. According to recent observations this insect has a double series of generations which precede each other: the mother type and the tubercular type. But this polymorphism seems to be more apparent than real, although there is a considerable 265 difference in their manner of life and of procuring nourishment. Is this difference the result of the different kinds of food taken from the roots and the leaves? There is one thing which may reassure us as to the future attacks of the phylloxera, that Mons. Planchon has 266 just discovered in America the cat of the phylloxera, one of the acaridæ, its mortal enemy; and it is only necessary to multiply these in order to destroy this terrible pest of the vineyards. We thus see that we have only to imitate this so-called blind Nature, in order that we may arrest a misfortune against which man is unable to protect himself by his own powers. [Illustration: Fig. 83.--Rose-Aphis. Male and Female.]