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XX. GENERAL CONSIDERATIONS FROM THE STUDY OF

INSECTS

Problem XXXI. How insects became winners in life's race. (Laboratory Manual, Prob. XXXI.)

(a) Protective resemblance.

(b) Aggressive resemblance. (c) Mimicry.

(d) Communal life.

(e) Symbiosis.
(f) Parasitism.

Insects are by far the most numerous of all animals. It is estimated that there are more species of insects than of all other species of animals upon the globe. Why should insects come to have existed in so much greater numbers than other animals? We cannot explain this, but some light is thrown on the problem when we consider

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some of the ways in which insects have become winners in life's

race.

Protective Resemblance.

When we re

member that the

chief enemies of insects are birds and other ani

mals which use

them as food, we

The walking stick on a twig, showing protective re

semblance.

can see that the

insect's power of rapid flight must have been of considerable importance in escaping from enemies. But other means of pro

tection are seen when we examine insects in their native haunts. We have noted that various animals, such as the earthworm and crayfish, escape observation because they have the color of their surroundings. Insects give many interesting examples of protective coloration or protective resemblance. The grasshopper is colored like the grass on which it lives. The katydid, with its green body and wings, can scarcely be distinguished from the leaves on which it rests. The walking stick, which resembles the twigs on which it is found, and the walking-leaf insect of the tropics, are other examples.

One example frequently quoted is the dead-leaf butterfly of India. This insect at rest resembles a dead leaf attached to a limb; in flight, because of its vivid colors, it is conspicuous. The underwing moth is another example of a wonderful simulation of the background of bark on which the animal rests in the daytime. At night the brightly colored underwings probably give a signal to others of the same species. The beautiful luna moth, in color a delicate green, rests by day among the leaves of the hickory. The small measuring worms stand out stiff upon the branches on which they crawl, thus simulating lateral twigs. Hundreds of other examples might be given.

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The underwing moth; above, flying; below, at rest on bark.

This likeness of an animal to its immediate surroundings has already been noted as protective resemblance.

Aggressive Resemblance. Sometimes animals which resemble their surroundings are thus better able to catch their prey. The polar bear is a notable example. Some insects are thus colored. The mantis, shown in the figure, has strongly built forelegs, with which it seizes and holds insects on which it preys. The mantis

has the color of its immediate sur-
roundings, and is thus enabled to
seize its prey before the latter is
aware of its presence.
Many other
examples could be given.

Warning Coloration and Protective

Mimicry. Some insects

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are ex

tremely unpleasant, both to smell or to taste, while others are provided with means of defense such as poison hairs or stings. Such animals are almost always brightly colored or marked as if to warn animals to keep off or take the consequences. Examples of insects which show warning by color may be seen in many examples of beetles, especially the spotted ladybirds, potato beetles, and the like. Wasps show yellow bands, while many forms of caterpillars are conspicuously marked or colored. Some insects, especially caterpillars, are brightly colored and protrude horns, or pretend to sting when threatened with attack. These animals evidently mimic animals which really are protected

Monarch and viceroy butterflies: the latter (at the

right) is a mimic.

Mantis, showing aggressive resemblance.

terfly (Anosia plexippus) is an example

by a sting or by poison, although this mimicry is not voluntary on the part of the insect. One of the best-known cases of insect mimicry is seen in the case of the imitation of the monarch butterfly by the viceroy.

The monarch butof a race which has

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received protection from enemies in the struggle for life, because of its nauseous taste and, perhaps, because its caterpillar feeds on plants of no commercial value.

Another butterfly, less favored by nature, resembles the monarch in outward appearance. This is the viceroy (Basilarchia archippus). It seems probable that in the early history of the species called viceroy some of this edible form escaped from the birds because they resembled in color and form the species of inedible monarchs. These favored individuals produced new butterflies which resembled the monarch more closely. So for generation after generation the ones which were most like the inedible species were left, the others becoming the food of birds. Ultimately a species of butterflies was formed that owed its existence to the fact that it resembled another more favored species. In this way nature selects the animals which can exist upon the earth. Many other examples

Hornet mimicked by locust borer, a beetle.

of mimicry may be found among insects; one of the easiest to find is the locust borer, shown in the Figure. Some flies imitate bees, and thus escape capture.

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The chief insect enemies are the birds, and from these the most effective protection seems to be hairs on the body. Few birds eat hairy caterpillars of any species; fortunately, however, the hairy larvæ of the gypsy moth, a serious pest, are eaten by no less than thirty-one species of birds. The odors or ill flavors of insects seem to be generally protective, but stinging insects do not appear to be protected from all birds, flycatchers and swallows habitually feeding on the bees and wasps.1

Communal Life among Insects. - Insects are of especial interest to man because among certain species a system of social life has arisen comparable to that which exists among men. In connection with this communal life, nature has worked out a division of labor which is very remarkable. This can be seen in tracing out the lives of several of the insects which live in communities.

1 See Judd, J. S., "The Efficiency of Some Protective Adaptations in securing Insects from Birds," American Naturalist, Vol. 33, pages 461-484.

Solitary Wasps. Some bees and wasps lead a solitary existence. The solitary and digger wasps do not live in communities. Each female constructs a burrow in which she lays eggs and rears her young. The young are fed upon spiders and insects previously caught and then stung into insensibility. The nest is closed up after food is supplied, and the young later gnaw their way out. In the life history of such an insect there is no communal life.

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Bumblebee. - In the life history of the big bumblebee we see the beginning of the community instinct. Some of the female bees (known as queens) survive the winter and lay their eggs the following spring in a mass of pollen, which has been previously gathered and placed in a hole in the ground. The young hatch as larvæ, then pupate, and finally become workers, or females. In the working bee the egg-laying apparatus, or ovipositor, is modified to be used as a sting. The workers bring in pollen to the queen, in which she lays more eggs. Several broods of workers are thus hatched during a summer. In the early fall a brood of males or drones, and egg-laying females or queens, are produced instead of workers. By means of these egg-producing females the brood is started the following year.

The Honeybee. The most wonderful communal life is seen among the honeybees.1

The honeybee in a wild state makes its home in a hollow tree; hence the term "bee tree." In the hive the colony usually consists of a queen, or egg-laying female, a few hundred drones, or males, and several thousand working females, or workers. The colonies vary greatly in numbers, in a wild state there being fewer in the colony. The division of labor is well seen in a hive in which the bees have been living for some weeks. The queen does nothing except lay eggs, sometimes laying three thousand eggs a day and keeping this up, during the warm weather, for several years. She may lay one million eggs during her life. She does not, as is popularly believed, rule the hive, but is on the contrary a captive most of her life. Most of the eggs are fertilized by the sperm cells of the males; the unfertilized eggs develop into males or drones.

1 Their daily life may be easily watched in the schoolroom, by means of one of the many good and cheap observation hives now made to be placed in a window frame. Directions for making a small observation hive for school work can be found in Hodge, Nature Study and Life, Chap. XIV. Bulletin No. 1, U.S. Department of Agriculture, entitled The Honey Bee, by Frank Benton, is valuable for the amateur beekeeper. It may be obtained for twenty-five cents from the Superintendent of Documents, Union Building, Washington, D.C.

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