The larvæ of some moths damage the trees by boring into the wood of the tree on which they live. Such are the peach, apple, and other fruit-tree borers common in our orchards. Many beetle larvæ also live in trees and kill annually thousands of forest and shade trees. The hickory borer threatens to kill all the hickory trees in the Eastern states.
Among the bugs most destructive to trees are the scale insect and the plant lice. The San José scale, a native of China, was introduced into the fruit groves of California about 1870 and has spread all over the country. A ladybird beetle, which has also been imported, is the most effective agent in keeping this pest in check.
Insects of the House or Storehouse.—Weevils are the greatest pests, frequently ruining tons of stored corn, wheat, and other cereals. Roaches will eat almost anything, even clothing; they are especially fond of all kinds of breadstuffs. The carpet beetle is a recognized foe of the housekeeper, the larvæ feeding upon all sorts of woolen material. The larvæ of the clothes moth do an immense amount of damage, especially to stored clothing. Fleas, lice, and particularly bedbugs are among man's personal foes. Besides being unpleasant they are believed to be disease carriers and as such should be exterminated.[31]
Food of Starfish.—Starfish are enormously destructive to young clams and oysters, as the following evidence, collected by Professor A. D. Mead, of Brown University, shows. A single starfish was confined in an aquarium with fifty-six young clams. The largest clam was about the length of one arm of the starfish, the smallest about ten millimeters in length. In six days every clam in the aquarium was devoured. Hundreds of thousands of dollars' damage is done annually to the oysters in Connecticut alone by the ravages of starfish. During the breeding season of the clam and oyster the boats dredge up tons of starfish which are thrown on shore to die or to be used as fertilizer.
the relations of animals to disease
The life history of the malarial parasite. This cut of the malarial parasite shows parts of the body of the mosquito and of man. To understand the life history begin at the point where the mosquito injects the crescent-shaped bodies into the blood of man. Notice that after the spores are released from the corpuscles of man two kinds of cells may be formed. These are probably a sexual stage. Development within the body of the mosquito will only take place when the parasite is taken into its body at this sexual stage.
The Cause of Malaria.—The study of the life history and habits of the Protozoa has resulted in the finding of many parasitic forms, and the consequent explanation of some kinds of disease. One parasitic protozoan like an amœba is called Plasmodium malariæ. It causes the disease known as malaria. When a mosquito (the anopheles) sucks the blood from a person having malaria this parasite passes into the stomach of the mosquito. After completing a part of its life history within the mosquito's body the parasite establishes itself within the glands which secrete the saliva of the mosquito. After about eight days, if the infected mosquito bites a person, some of the parasites are introduced into the blood along with the saliva. These parasites enter the corpuscles of the blood, increase in size, and then form spores. The rapid process of spore formation results in the breaking down of the blood corpuscles and the release of the spores, and the poisons they manufacture, into the blood. This causes the chill followed by the fever so characteristic of malaria. The spores may again enter the blood corpuscles and in forty-eight or seventy-two hours repeat the process thus described, depending on the kind of malaria they cause. The only cure for the disease is quinine in rather large doses. This kills the parasites in the blood. But quinine should not be taken except under a physician's directions.
How to distinguish the harmless mosquito (culex), a, from the malarial mosquito (anopheles), b, when at rest. Notice the position of legs and body.