292 NATURAL HISTORY. 
* 496. On the back of these insects there project behind 
two tubes, from which issues a sweet fluid. Ants are 
very fond of this, and take it from the tubes as it exudes, 
or from the surface of the plants, where it is known as 
honey-dew. The Aphides are, therefore, appropriately 
called the milch-cows of the Ants. Some species of Ants 
even gather them into flocks, and keep them in a sort 
of pasture, as we do cows. 
497. The Scale- -insects, though very small, are, like the 
Aphides, greatly injurious to plants. files fae they 
are abundantly prolific, and when they once get posses- 
sion of a plant or young tree, it is almost certain to die, 
the minute size of the larve of the insect rendering it 
almost impossible to find and exterminate them. The 
name Shield-louse, so often given to these insects, is de- 
rived from the appearance of the female, which, with its 
shield-shape, clings tightly to the plant, logkice more 
like a wart than an animal. It lives on the sap, which it 
sucks with its beak or snout. It deposits eggs on the 
bark, covering them with a sort of cottony secretion. 
It then dies, and its dried body forms another covering 
for the eggs. The cochineal, so valuable to commerce, 
is a scale-insect. It is found chiefly in Mexico and Cen- 
tral America. It is estimated that the export of coch- 
ineal from these countries is to the amount annually of 
two and a half millions of dollars. This rich dyeing ma- 
terial was used for a long time without its being known 
what it was; and a French naturalist, in 1792, was uni- 
versally ridiculed for asserting that cochineal was an in- 
sect. It is gathered from cactus plants, which are large- 
ly cultivated in plantations for the purpose of raising this 
insect for the market. The lac of the East Indies, so ex- 
tensively employed in the composition of varnishes, seal- 
ing-wax, etc., is the product of another species of these 
sects. 
498. There are various bugs belonging to this order, 
in some of which the wings are entirely absent. They 
