A Civic Biology Part 24

Food of Snakes.--Probably the most disliked and feared of all animals are the snakes. This feeling, however, is rarely deserved, for, on the whole, our common snakes are beneficial to man. The black snake and the milk snake feed largely on injurious rodents (rats, mice, etc.), the pretty green snake eats injurious insects, and the little DeKay snake feeds partially on slugs. If it were not that the rattlesnake and the copperhead are venomous, they also could be said to be useful, for they live on English sparrows, rats, mice, moles, and rabbits.

Food of Herbivorous Animals.--We must not forget that other animals besides insects and birds help to keep down the rapidly growing weeds. Herbivorous animals the world over destroy, besides the gra.s.s which they eat, untold mult.i.tudes of weeds, which, if unchecked, would drive out the useful occupants of the pasture, the gra.s.ses and grains.

HARM DONE BY ANIMALS

Economic Loss from Insects.--The money value of crops, forest trees, stored foods, and other material destroyed annually by insects is beyond belief.

It is estimated that they get one tenth of the country's crops, at the lowest estimate a matter of some $300,000,000 yearly. "The common schools of the country cost in 1902 the sum of $235,000,000, and all higher inst.i.tutions of learning cost less than $50,000,000, making the total cost of education in the United States considerably less than the farmers lost from insect ravages.

"Furthermore, the yearly losses from insect ravages aggregate nearly twice as much as it costs to maintain our army and navy; more than twice the loss by fire; twice the capital invested in manufacturing agricultural implements; and nearly three times the estimated value of the products of all the fruit orchards, vineyards, and small fruit farms in the country."--SLINGERLAND.

The total yearly value of all farm and forest products in New York is perhaps $150,000,000, and the one tenth that the insects get is worth $15,000,000.

Insects which damage Garden and Other Crops.--The gra.s.shoppers and the larvae of various moths do considerable harm here, especially the "cabbage worm," the cutworm, a feeder on all kinds of garden truck, and the corn worm, a pest on corn, cotton, tomatoes, peas, and beans.

Among the beetles which are found in gardens is the potato beetle, which destroys the potato plant. This beetle formerly lived in Mexico upon a wild plant of the same family as the potato, and came north upon the introduction of the potato into Colorado, evidently preferring cultivated forms to wild forms of this family.

[Ill.u.s.tration: Cotton-boll weevil. _a_, larva; _b_, pupa; _c_, adult.

Enlarged about four times. (Photographed by Davison.)]

The one beetle doing by far the greatest harm in this country is the cotton-boll weevil. Imported from Mexico, since 1892 it has spread over eastern Texas and into Louisiana. The beetle lays its eggs in the young cotton fruit or boll, and the larvae feed upon the substance within the boll. It is estimated that if unchecked this pest would destroy yearly one half of the cotton crop, causing a loss of $250,000,000. Fortunately, the United States Department of Agriculture is at work on the problem, and, while it has not found any way of exterminating the beetle as yet, it has been found that, by planting more hardy varieties of cotton, the crop matures earlier and ripens before the weevils have increased in sufficient numbers to destroy the crop (see page 126).

The bugs are among our most destructive insects. The most familiar examples of our garden pests are the squash bug; the chinch bug, which yearly does damage estimated at $20,000,000, by sucking the juice from the leaves of grain; and the plant lice, or aphids. One, living on the grape, yearly destroys immense numbers of vines in the vineyards of France, Germany, and California.

[Ill.u.s.tration: Female tussock moth which has just emerged from the coc.o.o.n at the left, upon which it has deposited over two hundred eggs. (Photograph by Davison.)]

[Ill.u.s.tration: Caterpillar of tussock moth. (Photograph by Davison.)]

Insects which harm Fruit and Forest Trees.--Great damage is annually done trees by the larvae of moths. Ma.s.sachusetts has already spent over $3,000,000 in trying to exterminate the imported gypsy moth. The codling moth, which bores into apples and pears, is estimated to ruin yearly $3,000,000 worth of fruit in New York alone, which is by no means the most important apple region of the United States. Among these pests, the most important to the dweller in a large city is the tussock moth, which destroys our shade trees. The caterpillar may easily be recognized by its hairy, tufted red head. The eggs are laid on the bark of shade trees in what look like ma.s.ses of foam. (See figure on page 215.) By collecting and burning the egg ma.s.ses in the fall, we may save many shade trees the following year.

The larvae 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 larvae 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 Jose 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 larvae feeding upon all sorts of woolen material. The larvae 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]

Footnote 31: Directions for the treatment of these pests may be found in pamphlets issued by the U. S. Department of Agriculture.

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 sh.o.r.e to die or to be used as fertilizer.

THE RELATIONS OF ANIMALS TO DISEASE

[Ill.u.s.tration: 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 s.e.xual stage. Development within the body of the mosquito will only take place when the parasite is taken into its body at this s.e.xual 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 amoeba is called _Plasmodium malariae_. It causes the disease known as malaria. When a mosquito (the _anopheles_) sucks the blood from a person having malaria this parasite pa.s.ses 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.

[Ill.u.s.tration: How to distinguish the harmless mosquito (_culex_), _a_, from the malarial mosquito (_anopheles_), _b_, when at rest. Notice the position of legs and body.]

The Malarial Mosquito.--Fortunately for mankind, not all mosquitoes harbor the parasite which causes malaria. The harmless mosquito (_culex_) may be usually distinguished from the mosquito which carries malaria (_anopheles_) by the position taken when at rest. Culex lays eggs in tiny rafts of one hundred or more eggs in any standing water; thus the eggs are distinguished from those of anopheles, which are not in rafts. Rain barrels, gutters, or old cans may breed in a short time enough mosquitoes to stock a neighborhood. The larvae are known as wigglers. They breathe through a tube in the posterior end of the body, and may be recognized by their peculiar movement when on their way to the surface to breathe. The pupa, distinguished by a large thoracic region, breathes through a pair of tubes on the thorax. The fact that both larvae and pupae take air from the surface of the water makes it possible to kill the mosquito during these stages by pouring oil on the surface of the water where they breed. The introduction of minnows, gold fish, or other small fish which feed upon the larvae in the water where the mosquitoes breed will do much to free a neighborhood from this pest. Draining swamps or low land which holds water after a rain is another method of extermination. Some of the mosquito-infested districts around New York City have been almost freed from mosquitoes by draining the salt marshes where they breed. Long shallow trenches are so built as to tap and drain off any standing water in which the eggs might be laid. In this way the mosquito has been almost exterminated along some parts of our New England coast.

[Ill.u.s.tration: Swamps are drained and all standing water covered with a film of oil in order to exterminate mosquitoes. Why is the oil placed on the surface of the water?]

Since the beginning of historical times, malaria has been prevalent in regions infested by mosquitoes. The ancient city of Rome was so greatly troubled by periodic outbreaks of malarial fever that a G.o.ddess of fever came to be worshiped in order to lessen the severity of what the inhabitants believed to be a divine visitation. At the present time the malaria of Italy is being successfully fought and conquered by the draining of the mosquito-breeding marshes. By a little carefully directed oiling of water a few boys may make an almost uninhabitable region absolutely safe to live in. Why not try it if there are mosquitoes in your neighborhood?

Yellow Fever and Mosquitoes.--Another disease carried by mosquitoes is yellow fever. In the year 1878 there were 125,000 cases and 12,000 deaths in the United States, mostly in Alabama, Louisiana, and Mississippi. During the French occupation of the Panama Ca.n.a.l zone the work was at a standstill part of the time because of the ravages of yellow fever. Before the war with Spain thousands of people were ill in Cuba. But to-day this is changed, and yellow fever is under almost complete control, both here and in the Ca.n.a.l zone, where the mosquito (_stegomyia_) which carries yellow fever exists.

[Ill.u.s.tration: Notice the difference in the number of yearly deaths from yellow fever before and _after_ the American occupation of Havana.]

This is due to the experiments during the summer of 1900 of a Commission of United States army officers, headed by Dr. Walter Reed. Of these men one, Dr. Jesse Lazear, gave up his life to prove experimentally that yellow fever was caused by mosquitoes. He allowed himself to be bitten by a mosquito that was known to have bitten a yellow fever patient, contracted the disease, and died a martyr to science. Others, soldiers, volunteered to further test by experiment how the disease was spread, so that in the end Dr. Reed was able to prove to the world that if mosquitoes could be prevented from biting people who had yellow fever the disease could not be spread. The accompanying ill.u.s.tration shows the result of this knowledge for the city of Havana. For years Havana was considered one of the pest spots of the West Indies. Visitors shunned this port and commerce was much affected by the constant menace of yellow fever. At the time of the American occupation after the war with Spain, the experiments referred to above were undertaken. The city was cleaned up, proper sanitation introduced, screens placed in most buildings, and the breeding places of the mosquitoes were so nearly destroyed that the city was practically free from mosquitoes. The result, so far as yellow fever was concerned, was startling, as you can see by reference to the chart. Notice also the rise in the death rate when the young Cuban Republic took control. How do you account for that? We all know what American scientific medicine and sanitation is doing in Panama and in the Philippines.

[Ill.u.s.tration: Stegomyia, the carrier of yellow fever. (After Howard.)]

Other Protozoan Diseases.--Many other diseases of man are probably caused by parasitic protozoans. Dysentery of one kind appears to be caused by the presence of an amoeba-like animal in the digestive tract which comes usually through an impure water supply. Smallpox, rabies, and possibly other diseases are caused by protozoans. Smallpox, which was once the most dreaded disease known to man, because of its spread in epidemics, has been conquered by _vaccination_, of which we shall learn more later. The death rate from rabies or hydrophobia has in a like manner been greatly reduced by a treatment founded on the same principles as vaccination and invented by Louis Pasteur.

Another group of protozoan parasites are called _trypanosomes_. These are parasitic in insects, fish, reptiles, birds, and mammals in various parts of the world. They cause various diseases of cattle and other domestic animals, being carried to the animal in most cases by flies. One of this family is believed to live in the blood of native African zebras and antelopes; seemingly it does them no harm. But if one of these parasites is transferred by the dreaded tsetse fly to one of the domesticated horses or cattle of the colonist of that region, death of the animal results.

Another fly carries a species of trypanosome to the natives of Central Africa, which causes "the dreaded and incurable sleeping sickness." This disease carries off more than fifty thousand natives yearly, and many Europeans have succ.u.mbed to it. Its ravages are now largely confined to an area near the large Central African lakes and the Upper Nile, for the fly which carries the disease lives near water, seldom going more than 150 feet from the banks of streams or lakes. The British government is now trying to control the disease in Uganda by moving all the villages at least two miles from the lakes and rivers. Among other diseases that may be due to protozoans is kala-agar, a fever in hot Asiatic countries which is probably carried by the bedbug, and African tick fever, probably carried by a small insect called the tick. Bubonic plague, one of the most dreaded of all infectious diseases, is carried to man by fleas from rats. In this country many fatal diseases of cattle, as "tick," or Texas cattle fever, are probably caused by protozoans.

[Ill.u.s.tration: Life history of house flies, showing from left to right the eggs, larvae, pupae, and adult flies. (Photograph, about natural size, by Overton.)]

The Fly a Disease Carrier.--We have already seen that mosquitoes of different species carry malaria and yellow fever. Another rather recent addition to the black list is the house fly or typhoid fly. We shall see later with what reason this name is given. The development of the typhoid fly is extremely rapid. A female may lay from one hundred to two hundred eggs. These are usually deposited in filth or manure. Dung heaps about stables, privy vaults, ash heaps, uncared-for garbage cans, and fermenting vegetable refuse form the best breeding places for flies. In warm weather, the eggs hatch a day or so after they are laid and become larvae, called maggots. After about one week of active feeding, these wormlike maggots become quiet and go into the pupal stage, whence under favorable conditions they emerge within less than another week as adult flies. The adults breed at once, and in a short summer there may be over ten generations of flies.

This accounts for the great number. Fortunately relatively few flies survive the winter. The membranous wings of the adult fly appear to be two in number, a second pair being reduced to tiny k.n.o.bbed hairs called balancers. The head is freely movable, with large compound eyes. The mouth parts form a proboscis, which is tonguelike, the animal obtaining its food by lapping and sucking. The foot shows a wonderful adaptation for clinging to smooth surfaces. Two or three pads, each of which bears tubelike hairs that secrete a sticky fluid, are found on its under surface. It is by this means that the fly is able to walk upside down, and carry bacteria on its feet.

[Ill.u.s.tration: The foot of a fly, showing the hooks, hairs, and pads which collect and carry bacteria. The fly doesn't wipe his feet.]

[Ill.u.s.tration: Colonies of bacteria which have developed in a culture medium upon which a fly was allowed to walk.]

The Typhoid Fly a Pest.--The common fly is recognized as a pest the world over. Flies have long been known to spoil food through their filthy habits, but it is more recently that the very serious charge of spread of diseases, caused by bacteria, has been laid at their door. In a recent experiment two young men from the Connecticut Agricultural Station found that a single fly might carry on its feet anywhere from 500 to 6,600,000 bacteria, the average number being over 1,200,000. Not all of these germs are harmful, but they might easily include those of typhoid fever, tuberculosis, summer complaint, and possibly other diseases. A recent pamphlet published by the Merchants' a.s.sociation in New York City shows that the rapid increase of flies during the summer months has a definite correlation with the increase in the number of cases of summer complaint. Observations in other cities seem to show the increase in number of typhoid cases in the early fall is due, in part at least, to the same cause. A terrible toll of disease and death may be laid at the door of the typhoid fly.

[Ill.u.s.tration: Showing how flies may spread disease by means of contaminating food.]

Recently the stable fly has been found to carry the dread disease known as infantile paralysis.

[Ill.u.s.tration: There were 329 typhoid cases in Jacksonville, Florida, in 1910, 158 in 1911, 87 in first 10 months of 1912. 80 to 85 per cent of outdoor toilets were made fly proof during winter of 1910. Account for the decrease in typhoid after the flies were kept out of the toilets.]

Remedies.--Cleanliness which destroys the breeding place of flies, the frequent removal and destruction of garbage, rubbish, and manure, covering of all food when not in use and especially the _careful_ screening of windows and doors during the breeding season, will all play a part in the reduction of flies. To the motto "swat the fly" should be added, "remove their breeding places!"

[Ill.u.s.tration: Flea which transmits Bubonic plague from rat to man.]

Other Insect Disease Carriers.--Fleas and bedbugs have been recently added to those insects proven to carry disease to man. Bubonic plague, which is primarily a disease of rats, is undoubtedly transmitted from the infected rats to man by the fleas. Fleas are also believed to transmit leprosy although this is not proven.

To rid a house of fleas we must first find their breeding places. Old carpets, the sleeping places of cats or dogs or any dirty unswept corner may hold the eggs of the flea. The young breed in cracks and crevices, feeding upon organic matter there. Eventually they come to live as adults on their warm-blooded hosts, cats, dogs, or man. Evidently destruction of the breeding places, careful washing of all infected areas, the use of benzine or gasoline in crevices where the larvae may be hid are the most effective methods of extermination. Pets which might harbor fleas should be washed frequently with a weak (two to three per cent) solution of creolin.

Bedbugs are difficult to prove as an agent in the transmission of disease but their disgusting habits are sufficient reason for their extermination.

It has been proven by experiment that they may spread typhoid and relapsing fevers. They prefer human blood to other food and have come to live in bedrooms and beds because this food can be obtained there. They are extremely difficult to exterminate because their flat body allows them to hide in cracks out of sight. Wooden beds are thus better protection for them than iron or bra.s.s beds. Boiling water poured over the cracks when they breed or a mixture of strong corrosive sublimate four parts, alcohol four parts and spirits of turpentine one part, are effective remedies.