How the Harm done by Insects is Controlled.--The combating of insects is directed by several bodies of men, all of which have the same end in view.
These are the Bureau of Entomology of the United States Department of Agriculture, the various state experiment stations, and medical and civic organizations.
The Bureau of Entomology works in harmony with the other divisions of the Department of Agriculture, giving the time of its experts to the problems of controlling insects which, for good or ill, influence man's welfare in this country. The destruction of the malarial mosquito and control of the typhoid fly; the destruction of harmful insects by the introduction of their natural enemies, plant or animal; the perfecting of the honeybee (see Hodge, _Nature Study and Life_, page 240), and the introduction of new species of insects to pollinate flowers not native to this country (see _Blastophaga_, page 43), are some of the problems to which these men are now devoting their time.
All the states and territories have, since 1888, established state experiment stations, which work in cooperation with the government in the war upon injurious insects. These stations are often connected with colleges, so that young men who are interested in this kind of natural science may have opportunity to learn and to help.
The good done by these means directly and indirectly is very great.
Bulletins are published by the various state stations and by the Department of Agriculture, most of which may be obtained free. The most interesting of these from the high school standpoint are the Farmers' Bulletins, issued by the Department of Agriculture, and the Nature Study pamphlets issued by the Cornell University in New York state.
[Illustration: This diagram shows how bubonic plague is carried to man.
Explain the diagram.]
Animals Other than Insects may be Disease Carriers.--The common brown rat is an example of a mammal, harmful to civilized man, which has followed in his footsteps all over the world. Starting from China, it spread to eastern Europe, thence to western Europe, and in 1775 it had obtained a lodgment in this country. In seventy-five years it reached the Pacific coast, and is now fairly common all over the United States, being one of the most prolific of all mammals. Rats are believed to carry bubonic plague, the "Black Death" of the Middle Ages, a disease estimated to have killed 25,000,000 people during the fourteenth century. The rat, like man, is susceptible to plague; fleas bite the rat and then biting man transmit the disease to him. A determined effort is now being made to exterminate the rat because of its connection with bubonic plague.
Other Parasitic Animals cause Disease.--Besides parasitic protozoans other forms of animals have been found that _cause_ disease. Chief among these are certain round and flat worms, which have come to live as parasites on man and other animals. A one-sided relationship has thus come into existence where the worm receives its living from the host, as the animal is called on which the parasite lives. Consequently the parasite frequently becomes fastened to its host during adult life and often is reduced to a mere bag through which the fluid food prepared by its host is absorbed.
Sometimes a complicated life history has arisen from their parasitic habits. Such is seen in the life history of the liver fluke, a flatworm which kills sheep, and in the tapeworm.
[Illustration: The life cycle of a tapeworm. (1) The eggs are taken in with filthy food by the pig; (2) man eats undercooked pork by means of which the bladder worm (3) is transferred to his own intestine (4).]
Cestodes or Tapeworms.--These parasites infest man and many other vertebrate animals. The tapeworm (_Taenia solium_) passes through two stages in its life history, the first within a pig, the second within the intestine of man. The developing eggs are passed off with wastes from the intestine of man. The pig, an animal with dirty habits, may take in the worm embryos with its food. The worm develops within the intestine of the pig, but soon makes its way into the muscle or other tissues. It is here known as a bladderworm. If man eats raw or undercooked pork containing these worms, _he_ may become a host for the tapeworm. Thus during its complete life history it has two hosts. Another common tapeworm parasitic on man lives part of its life as an embryo within the muscles of cattle.
The adult worm consists of a round headlike part provided with hooks, by means of which it fastens itself to the wall of the intestine. This head now buds off a series of segmentlike structures, which are practically bags full of sperms and eggs. These structures, called _proglottids_, break off from time to time, thus allowing the developing eggs to escape. The proglottids have no separate digestive systems, but the whole body surface, bathed in digested food, absorbs it and is thus enabled to grow rapidly.
[Illustration: _Trichinella spiralis_ imbedded in human muscle. (After Leuckart.)]
Roundworms.--Still other wormlike creatures called roundworms are of importance to man. Some, as the vinegar eel found in vinegar, or the pinworms parasitic in the lower intestine, particularly of children, do little or no harm. The pork worm or _trichina_, however, is a parasite which may cause serious injury. It passes through the first part of its existence as a parasite in a pig or other vertebrate (cat, rat, or rabbit), where it lies, covered within a tiny sac or _cyst_, in the muscles of its hosts. If raw pork containing these worms is eaten by man, the cyst is dissolved off by the action of the digestive fluids, and the living trichina becomes free in the intestine of man. Here it reproduces and the young bore their way through the intestine walls and enter the muscles, causing inflammation there. This causes a painful and often fatal disease known as _trichinosis_.
The Hookworm.--The discovery by Dr. C. W. Stiles of the Bureau of Animal Industry, that the laziness and shiftlessness of the "poor whites" of the South is partly due to a parasite called the _hookworm_, reads like a fairy tale.
The people, largely farmers, become infected with a larval stage of the hookworm, which develops in moist earth. It enters the body usually through the skin of the feet, for children and adults alike, in certain localities where the disease is common, go barefoot to a considerable extent.
A complicated journey from the skin to the intestine now follows, the larvae passing through the veins to the heart, from there to the lungs; here they bore into the air passages and eventually work their way by way of the windpipe into the intestine. One result of the injury of the lungs is that many thus infected are subject to tuberculosis. The adult worms, once in the food tube, fasten themselves and feed upon the blood of their host by puncturing the intestine wall. The loss of blood from this cause is not sufficient to account for the bloodlessness of the person infected, but it has been discovered that the hookworm pours out a poison into the wound which prevents the blood from clotting rapidly (see page 315); hence a considerable loss of blood occurs from the wound after the worm has finished its meal and gone to another part of the intestine.
[Illustration: A family suffering from hookworm.]
The cure of the disease is very easy; thymol is given, which weakens the hold of the worm, this being followed by Epsom salts. For years a large area in the South undoubtedly has been retarded in its development by this parasite; hundreds of millions of dollars and thousands of lives have been needlessly sacrificed.
"The hookworm is not a bit spectacular: it doesn't get itself discussed in legislative halls or furiously debated in political campaigns. Modest and unassuming, it does not aspire to such dignity.
It is satisfied simply with (1) lowering the working efficiency and the pleasure of living in something like two hundred thousand persons in Georgia and all other Southern states in proportion; with (2) amassing a death rate higher than tuberculosis, pneumonia, or typhoid fever; with (3) stubbornly and quite effectually retarding the agricultural and industrial development of the section; with (4) nullifying the benefit of thousands of dollars spent upon education; with (5) costing the South, in the course of a few decades, several hundred millions of dollars. More serious and closer at hand than the tariff; more costly, threatening, and tangible than the Negro problem; making the menace of the boll weevil laughable in comparison--it is preeminently the problem of the South."--_Atlanta Constitution._
Animals that prey upon Man.--The toll of death from animals which prey upon or harm man directly is relatively small. Snakes in tropical countries kill many cattle and not a few people.
The bite of the rattlesnake of our own country, although dangerous, seldom kills. The dreaded cobra of India has a record of over two hundred and fifty thousand persons killed in the last thirty-five years. The Indian government yearly pays out large sums for the extermination of venomous snakes, over two hundred thousand of which have been killed during a single year.
[Illustration: A flesh-eating reptile, the alligator.]
Alligators and Crocodiles.--These feed on fishes, but often attack large animals, as horses, cows, and even man. They seek their prey chiefly at night, and spend the day basking in the sun. The crocodiles of the Ganges River in India levy a yearly tribute of many hundred lives from the natives.
Carnivorous animals such as lions and tigers still inflict damage in certain parts of the world, but as the tide of civilization advances, their numbers are slowly but surely decreasing so that as important factors in man's welfare they may be considered almost negligible.
REFERENCE BOOKS
ELEMENTARY
Hunter, _Laboratory Problems in Civic Biology_. American Book Company.
Beebe, _The Bird_. Henry Holt and Company.
Bigelow, _Applied Biology_. Macmillan and Company.
Davison, _Practical Zoology_. American Book Company.
Herrick, _Household Insects and Methods of Control_. Cornell Reading Courses.
Hornaday, _Our Vanishing Wild Life_. New York Zoological Society.
Hodge, _Nature Study and Life_. Ginn and Company.
Kipling, _Captains Courageous_. Charles Scribner's Sons.
Sharpe, _Laboratory Manual_, pp. 157-158, 182-203, 320-341.
American Book Company.
Stone and Cram, _American Animals_. Doubleday, Page and Company.
Toothaker, _Commercial Raw Materials_. Ginn and Company.
ADVANCED
Flower, _The Horse_. D. Appleton and Company.
Hornaday, _The American Natural History_. Macmillan and Company.
Jordan, _Fishes_. Henry Holt and Company.
Jordan and Evermann, _American Food and Game Fishes_.
Doubleday, Page and Company.
Schaler, _Domesticated Animals, their Relations to Man and to His Advancement in Civilization_. Charles Scribner's Sons.
XVI. THE FISH AND FROG, AN INTRODUCTORY STUDY OF VERTEBRATES
_Problems._--_To determine how a fish and a frog are fitted for the life they lead._ _To determine some methods of development in vertebrate animals._ _(a) Fishes._ _(b) Frogs._ _(c) Other animals._
LABORATORY SUGGESTIONS
_Laboratory exercise._--Study of a living fish--adaptations for protection, locomotion, food getting, etc.
_Laboratory demonstration._--The development of the fish or frog egg.
_Visit to the aquarium._--Study of adaptations, economic uses of fishes, artificial propagation of fishes.
Two Methods of Breathing in Vertebrates.--Vertebrate animals have at least two methods of getting their oxygen. In other respects their life processes are nearly similar. Of all vertebrates fishes are the only ones fitted to breathe all their lives under water. Other vertebrates are provided with lungs and take their oxygen directly from the air.[32] We will next take up the study of a fish to see how it is fitted for its life in the water.
Footnote 32: With the exception of a few lungless salamanders. Most salamanders get much of their supply of oxygen through their moist skins.
STUDY OF A FISH
The Body.--One of our common fresh-water fish is the bream, or golden shiner. The body of the bream runs insensibly into the head, the neck being absent. The long, narrow body with its smooth surface fits the fish admirably for its life in the water. Certain cells in the skin secrete mucus or slime, another adaptation. The position of the scales, overlapping in a backward direction, is yet another adaptation which aids in passing through the water. Its color, olive above and bright silver and gold below, is protective. Can you see how?