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. Like them, 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 Iarva3 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, looking 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 

 insects. 



498. There are various bugs belonging to this order, 

 in some of which the wings are entirely absent. They 



