pairs) is definite in the insects; in the crustaceans the number sometimes varies (as in the Entomostraca), but is always more than three pairs. The exoskeleton, composed wholly of chitin in the insects, is usually strengthened with lime in the crustaceans. Both groups have compound eyes, but those of the Crustacea are staked and movable. The other sense organs do not differ greatly. The most marked differences are physiological. The crustaceans take in oxygen from the water by means of gills, while the insects are air breathers, using for this purpose air tubes called trachea. The young of both insects and crustaceans usually undergo several changes in form before the adult stage is reached. They are thus said to pass through a metamorphosis. Both insects and crustaceans, because of their exoskeleton, must molt in order to increase in bulk.

CLASSIFICATION OF ARTHROPODA

PHYLUM ARTHROPODA

CLASS, Crustacea. Arthropods with limy and chitinous exoskeleton, rarely more than 20 body segments, usually breathing by gills, and having two pairs of antennæ.

SUBCLASS I.

Entomostraca. Crustacea with a variable number of segments, chiefly small forms with simple appendages. Some degenerate or parasitic. Examples: barnacles, water flea (Daphnia), and copepod (Cyclops). SUBCLASS II.

Malacostraca. Usually large Crustacea having nineteen pairs of appendages. Examples: American lobster (Homarus Americanus), crab (Cancer), and shrimp (Palamonetes).

CLASS, Hexapoda (insects). Arthropoda having chitinous exoskeleton, breathing by air tubes (trachea), and having three distinct body regions.

Order, Aptera (without wings). Several wingless forms. Examples: springtails. Order, Orthoptera (straight wings). Example: Rocky Mountain locust.

Order, Lepidoptera (scale wings). Examples: cabbage butterfly, cecropia moth. Order, Diptera (two wings). Examples: fly, mosquito.

Order, Hemiptera (half wing).

Order, Neuroptera (nerve wings).

Examples: all true bugs, plant lice, and cicada.
Examples: May fly, dragon fly.

Order, Coleoptera (shield wings). Examples: beetles.

Order, Hymenoptera (membrane wings). Examples: bees, wasps, ants.

CLASS, Arachnida. Arthropoda with head and thorax fused. Six pairs of appendages. No antennæ. Breathing by both lung sacs (spiders) or tracheæ. Examples spiders and scorpions.

:

CLASS, Myriapoda. Arthropoda, having long bodies with many segments; one or two pairs of appendages to each segment. Breathing by means of trachea. Example: centipede.

An exercise for field work with a simple key for identification of orders will be found in the Labratory Manual, Prob. XXX.

REFERENCE BOOKS

ELEMENTARY

Sharpe, A Laboratory Manual for the Solution of Problems in Biology. American

Book Company.

Comstock, J. H., Insect Life. D. Appleton and Company.

Davison, Practical Zoology. American Book Company.

Howard, L. O., The Insect Book. Doubleday, Page, and Company.
Hunter, S. J., Elementary Studies in Insect Life. Crane and Company.
Needham, Outdoor Studies. American Book Company.

ADVANCED

Comstock, J. H., An Introduction to Entomology.

Comstock Publishing Company.

Emerton, The Structure and Habits of Spiders. Knight and Millett.
Kellogg, V. L., American Insects. Henry Holt and Company.

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.

It is esti

Insects are by far the most numerous of all animals. mated 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 some of the ways in which insects have become winners in life's race.

Protective Re

semblance.

When we re

member that the

chief enemies of insects are birds and other animals which use

them as food, we

can see that the

semblance.

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.

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 surroundings, 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 are extremely 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

right) is a mimic.

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 but

terfly (Anosia plexippus) is an example of a race which has 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.