The phylum Arthropoda embraces an immense assemblage of small animals, inhabiting salt and fresh waters, the land, and the air above it. The typical members of this group have a body divided into segments, jointed limbs, some of which are modified into jaws, and a more or less firm external skeleton. The general organization is complex, with the nervous system and senses well developed, in some divisions showing powers of perception and brainwork of a very high order. The chief divisions, or classes, of the Arthropoda are given below in the order of rank, from those simplest in organization to the most complex. Members of the first three classes breathe by gills, and are termed Branchiata, the remainder are air breathers or Tracheata.

As several of these classes contain many subdivisions, and thousands or even tens of thousands of species, all that is possible is to give the reader such an account of each important group, as will enable him to assign to their proper place such arthropods as he may encounter in his rambles, or in his reading, and to learn something of the manner of life in the various groups.

CRABS AND THEIR SMALL RELATIVES

"Everyone," says Dr. Calman, "has some acquaintance with the animals that are grouped by naturalists under the name Crustacea. The edible crabs, lobsters, prawns, and shrimps are at least superficially familiar, either as brought to the table, or as displayed in the fishmonger's.... Many, however, will be surprised that the barnacles coating the rocks on the seashore, the sand hoppers of the beach, and the wood lice of our gardens, are members of the same class. Still less is it suspected that the living species of the group number many thousands, presenting strange diversities of structure and habit, and playing an important part in the general economy of nature."

The great majority of crustaceans are aquatic animals, breathing by gills or by the general surface of the body, having two pairs of "feelers," or antennÆ, on the front part of the head, and at least three pairs of jaws. Most crustaceans are hatched from eggs, usually in a form very different from their parents; and they reach the adult state only after passing through a series of transformations quite as remarkable as those that a caterpillar undergoes in becoming a butterfly. All crustaceans, except a few much modified land forms, breathe by means of feathery or platelike gills which are always an appendage of the legs, where they appear as one or more lobes. Colorless blood propelled by the heart wanders into spaces in these lobes, and there lies separated from the water by a mere film of tissue, through which oxygen is absorbed from the water. Most crustaceans are covered, at least in part, by some sort of shelly coat composed of a combination of the horny substance "chitin" with lime, which reaches its highest state in the big lobsters and crabs. This not only protects and gives support to the internal organs, but also to the muscles by which the animal moves. In other words it plays the part of a skeleton. As it does not increase in size after it is once formed, and cannot stretch much, the crab must cast its shell at intervals as it grows. The new covering, which had been formed underneath the old, before molting, is at first quite soft, and the animal rapidly increases in size owing to the absorption of water. The shell then gradually hardens by the deposition of lime salt.

The reader who may not hitherto have understood the difference between "hard" and "soft-shelled" crabs is now instructed; and it is observable that the figurative expression "a hard-shell," when applied to a man, signifies that he must undergo a complete change before his ideas will be enlarged.

The simplest of the crustaceans are those small creatures of the subclass Branchiopoda (gill-footed) that swarm in our waters, both salt and fresh. Lakes, ponds and ditches abound in a variety of minute or even microscopic species that, in gathering food from equally small bits of dead organic matter, as well as from living plants and animalcules, perform an important service as scavengers—a service, in fact, performed by all crustaceans in a greater degree than by any other single group of animals. They also furnish the basis of food for the whole body of aquatic life, since it is upon these minute crustaceans that fish fry, tadpoles, insect larvÆ, caddis flies, and so on, must mainly depend. One of them is Daphnia, familiar to keepers of aquariums. Another is Cyclops, a favorite with microscopists and abundant in stagnant ponds, which is a member of the group called copepods that form an important part of the oceanic plankton, where they are the chief consumers of the minute algÆ; but they also occur at all depths. In arctic waters the copepods are so abundant that they form the principal part of the food of certain fishes and of the whalebone whales. These, and their minute relatives, the ostracods, produce a large part of the phosphorescence of the sea, and some of them exhibit bright colors.

All these are free swimmers, but nearly related to them are the barnacles (Cirripedia) whose larvÆ float about for a time near shore, and then settle down and attach themselves by their hinder parts to a rock or some other support, and begin to secrete an armature of limy overlapping plates that forms a strong cup in which they sit, often in a crowd that whitens a big rock. When the tide is low these sessile "acorn shells" are tightly closed, but when the water returns, bringing its load of invisible food, the animal stands up, as it were, and thrusting out its feathery legs sweeps the water to capture a meal—a beautiful sight to watch. The relation of the plates in the barnacle's cup to those in the coat of the higher Crustacea is more easily seen in the more pelagic "goose barnacle," whose hinder part is extended into a tough, flexible stalk, while the fore part is covered by plates. This kind is fond of attaching itself to floating timber, to ships' bottoms, or even to the surface of whales, and thus floats or is carried all over the watery globe. To it belongs the ridiculous myth of the barnacle geese.

Great numbers of crustaceans of more advanced types live in the open sea, and at all depths; and many of them assume extraordinary shapes. The space between tide marks, and the mud of salt marshes and tidal creeks abound in a wide variety of species, some of which are familiar to everyone who lives at or visits the seashore. Thus the sand and rows of drifted seaweed on all our eastern beaches are likely to harbor flocks of amphipods, well called "sea fleas" or "sand hoppers," which sometimes jump away before you in hundreds as you walk along.

Here, too, are to be found the pretty, burrowing "mole crabs," or "ivory crabs," so called from their shining white jackets; and a host of other species with strange forms and habits haunt the margins of tropical and Oriental seas. All these are bandits, preying on whatever they can catch, and between times guarding themselves from capture by fishes, bigger crabs, and other enemies, by lying in mud burrows, to the bottom of which they are quick to retreat. The big arm of the fiddler crab, held across its face, closes its burrow like a door. One sort, the hermit crab, has all its hinder parts naked, and so backs into an empty snail shell, curling its taillike soft abdomen around the central column of the shell and so dragging it about with it, with its armored head and thorax sticking out of the mouth of the shell. As it grows it becomes too large for its first shell, and from time to time must leave it and find a larger tenement in which to ensconce itself—a perilous transfer. Let me quote some notes I made on a New England shore to give a picture of crustacean life there in summer.

"The lady crabs were plentiful, always alert, and inclined to be pugnacious at our intrusion. The first one I met instantly rose upright at the surface of the water, and when I made an advance it sprang half way out of the water and cracked its pincer claws together as if supposing it would reach, or at any rate frighten me. Perhaps it was my shadow it clutched at so viciously. If so, the crab probably concluded its huge antagonist to be an intangible ghost upon which the most powerful claws could have no effect, for an instant later it backed down—literally and swiftly—to the bottom, and in a twinkling had wriggled tailwise into the mud and out of sight. When with my shovel I routed madame out of that retreat, she indignantly scuttled off too briskly to be followed, and will have great tales to tell of her adventure.

"The stone and fiddler crabs were as common and comical as usual; and I made the acquaintance of a new one called Gebia, which was a small, semi-transparent, bluish white, washed-out, bloodless specimen, shaped somewhat like a crawfish and carrying bunches of roe beneath its abdomen. It looked like a miniature lobster made of glass and filled with milk. Then in the eelgrass there was a funny isopod, called Caprella. It was half an inch or so long, and clung by its hinder feet to the grass, waving its body up and down in search of minute prey. Other isopods and amphipods were exposed by turning over stones or digging in the sand at the edge of the water—small, pale, shapeless crustacea, which are flattened laterally so that they must lie on their sides, and when uncovered will kick about with feet and tail in laughable anxiety to get under something. Under the stones we found the tubes made by a certain species; and when we captured the active little architect and put him in a bucket of clean water, he instantly began to gather grains of sand and stone and to join them together Into a shield under which he might hide. We found that these grains were joined together by spiderlike threads, which the amphipod spins from two pairs of small legs under the middle of his body, secreting a fluid that hardens in the water. Another (Hippa) about the size and shape of a robin's egg, but with a thin shell of mother-of-pearl (so to speak), gave us great amusement by its extraordinary celerity in burrowing, so that we could hardly seize it before it had squirmed down out of reach into the wet sand."

The edible crabs (Cancer) live in the shallow region just below ebb tide, for they cannot endure exposure to air as well as other species, and live by scavenging. The lobsters are inhabitants of still deeper water, especially where it is somewhat rocky, and devour more carrion than living fish. That miniature of the lobster, the fresh-water crawfish, which is also edible, dwells in deep burrows in wet lands—burrows that are really wells half filled with water. Various species of these and other edible forms of Crustacea are found all over the world.

MILLIPEDES AND CENTIPEDES

The myriapods (class Myriapoda) are those unpleasant creatures more commonly known as centipedes, millipedes, or thousand-legged worms. They have a wormlike form, with the body divided into segments, a distinct head with antennÆ, jaws and several single eyes, and a varying number of air tubes, or tracheÆ; two sexes exist, and eggs are laid in the ground within cases formed by the mother of pellets of mud. They vary in size from an almost invisible minuteness to a length in some tropical species of six or more inches. The centipedes (Chilopoda) are those flattened forms so often seen in and about rotting wood and vegetation or in moist ground, their bodies looking like a chain of plates joined together by flexible skin, each section having a single pair of legs, usually very short, but in one sort (Cermatia) each leg is longer than the body, and the hinder pair twice as long, matched by two very long feelers. Most of them are predacious, feeding on anything they can catch, and their strong jaws exude poison. The larger ones may inflict a very painful bite if incautiously handled.

Another group, the Diploda, are known as galley worms, or millipedes, and have two pairs of bristlelike legs on each segment. Here the body is as round as that of an earthworm, and is incased in a hard, chitinous shell, usually red-brown in color; and when disturbed they coil up and emit an acrid, unpleasant odor as a defense.

WEAVERS OF SILKEN TRAPS

The class Arachnida, which contains the scorpions, spiders, mites and their allies, connects the Crustacea with the Insects; and some naturalists include within it the eurypterids and king crabs, classified in this book with the Crustacea. All live on land and breathe air except a small group of allies (Pycnogonida) which are marine, and may be found on the rocks, and clinging to wharf piles, etc., on our coasts as well as elsewhere; they appear to be all legs, and are known to New England fishermen as "no-body crabs." The class includes seven orders, the lowest in rank of which is that of the scorpions (Scorpionida).

Scorpions are inhabitants of warm countries, and some tropical American species are six inches in length, but those of our Southern States are smaller. They have slender bodies consisting of a cephalothorax and a long abdomen ending in a sharp sting through which two poison glands inject poison into the wound made by it, the effect of which may be very severe on a man, and is fatal to the insects and other small creatures on which scorpions prey; this "tail" with the sting is usually carried curled up over the back. The body is protected by chitinous plates above and below. The legs are four. From the head spring two great, crablike, pincer claws. When these seize an insect they hand it back to two small but powerful appendages at their base which act as jaws. Between them is a small mouth. Scorpions are nocturnal in habit, hiding by day in crevices, and wandering about at night; thus they are likely to seek such dark retreats toward morning as a person's boots; and in hot, dry regions travelers must be cautious about examining their clothing and baggage to avoid being stung. The scorpions retain their eggs until hatched. The young when born differ little except in size from their parents, and are cared for with much solicitude by the mother, who carries them around with her for some time, hanging by their pincers to her body. The race is ancient, fossil remains occurring as early, at least, as the Carboniferous age.

The second order, Pseudoscorpionida, includes the "book scorpions," a series of minute, stingless, scorpion-shaped creatures found in moss, under the bark of trees, or more often on flies. A third order, Pedipalpida, is that of the scorpion spiders, or "whip scorpions" of the tropics; the fourth, Solpugida, contains certain ugly creatures intermediate between scorpions and spiders; and the fifth order, Phalangida, is that of the small-bodied, vastly long-legged things called "harvestmen" in England and daddy longlegs by us, which run about in the summer heat, and feed on minute insects. They abound in all the warmer parts of the world, and in great variety, South America showing some very bizarre forms. This brings us to the sixth order, Araneida—the spiders.

THE SPIDERS AND THEIR WEBS

Spiders are usually thought and spoken of as "insects" by the layman. Many persons call almost every creature an insect that is small and supposed to be useless, or suspected of harmfulness. But spiders are different from insects properly so called in many important particulars of structure and habits. Spiders have four pairs of legs, while insects have six legs. The spherical abdomen, which is cut off from the head by a deep constriction, shows no segmentation, and on its floor are large glands (the arachnidium) producing the silk which is exuded from three pairs of tubes with sievelike openings, at the end of the abdomen, called the spinnerets. Their nervous system is highly developed, and they show much intelligence. Spiders are of two sexes, but the male is usually much smaller than his mate.

When egg-laying time comes the female forms a little silken bed attached to grass, or underneath a stone, or stuck to some object, or placed in a burrow, or hung like a hammock by long guy lines, and deposits in it eggs like drops of jelly. One sort places this under water, forming a nest like an inverted cup and filling it with bubbles of air, and spending much of its time in this real diving bell. A common garden spider (Lycosa) forms globular cocoons, and drags them around attached to the spinnerets, regardless of jars and bumps. In a large section of the tribe this is all the use that is made of the silk, which differs from that of insects (caterpillars) in being made up of a great number of finer threads laid together while soft enough to unite into one.

It is a common habit with spiders to draw out a thread behind them as they walk, and in this way they make the great quantities of threads that sometimes cover a field of grass. This is the gossamer often so annoying to us in late summer, but a thing of beauty when glistening with dew.

The gossamer of autumn, however, is made by the very small spiders of the genus Erigones, which hide in the herbage, but in the fine weather that comes after the first frosts climb to the tops of posts, fences and tall weeds, in company with the young of larger kinds, and "turning their spinnerets upward allow threads to be drawn out by ascending currents of air, until sometimes the spiders are lifted off their feet and carried long distances." These are the "ballooning spiders" of which one hears. In this way the whole country is overspread with lines and tangles of trailing silken threads that whiten our clothes and stick to our faces.

Three or four hundred species of spiders might be obtained in almost any locality in this country by diligent search, and thousands of foreign species are known; hence only a few conspicuous examples may be mentioned here. The tribe may be divided according to habits into two groups of families: 1. The hunting spiders, which run on the ground or on plants, catching insects by chase or by strategy; and 2. The cobweb spiders, which make webs to catch insects, and live all the time in the web or in a nest near it.

In the former group are the DrassidÆ, a family of small, varicolored spiders that run about on the ground or in bushes, one large genus of which (Clubiana) includes pale, or purely white species; their cocoons are baglike or tubular. The most conspicuous genus is Misumena, in which the species are white or brightly colored, and which spend their days among flowers, waiting in rigid attitude for an insect to alight near them on which they may pounce. Spiders can see well for four or five inches, but not much beyond that. The AttidÆ are small, hairy, or scaly jumping spiders, often brightly colored, that are found in open places and on the tops of low plants, whence they leap on their prey, or make long jumps to escape danger. To the next family, LycosidÆ, belong the large spiders most often seen in fields and pastures. They are fond of dry, sandy places, where the females live in silk-lined holes. These lycosids are long-legged, rapid runners, and capture their game by running it down. To this family belongs the famous tarantula of southern Europe, fabled to produce a madness (tarantism) in a person bitten that could be cured only by dancing to music of a certain lively measure called "tarantella." (The so-called "tarantula" of our southwestern desert region, is, however, another species.) A common northern spider (Lycosa carolinensis) is its equal in size, (the longest legs covering a spread of three inches), and in color, black with gray legs. Still larger is another North American lycosid (Dolomedes tenebrosus), gray with spiny legs ringed with dark and light gray, which spreads four inches.

These big ugly creatures, and the bites of spiders generally, are regarded with unnecessary dread by most persons. The jaws (mandibles) are close together at the front of the head. They are two-jointed, the basal joint stout, and the end joint or claw slender and sharp-pointed. The claw has near its point a small hole, which is the outlet of the poison gland. "The poison kills or disables the insects which are captured by the spider. Its effect on the human skin varies in different persons. Sometimes it has no effect at all; oftener it causes some soreness and itching ... and cases have been known in which it caused serious inflammation which lasted a long time. Spiders seldom bite and only in self-defense, the bites so commonly charged to them being often the work of other animals."

In the family AgalenidÆ we meet with the first of the web makers. These are spiders of moderate size, characterized by a big head marked off from the thorax by converging grooves. Their natural home is in the grass, where their flat, closely woven sheets of silk, almost invisible by reason of their transparency, but brought into plain view when coated with dew or dust, are spread everywhere. They also are fond of getting into cellars and old buildings, and constructing webs across corners, bracketwise. Somewhere the web sinks like a narrow funnel into a short tube in which the owner hides, watching hungrily until a fly alights on his silken platform.

"The TherididÆ," says Emerton, "are the builders of the loose and apparently irregular webs in the upper corners of rooms, in fences and among rocks, and between the leaves and branches of low trees and bushes. They are generally small, soft and light-colored spiders, with the abdomen large and round and the legs slender and usually without spines.... Most of the TherididÆ live always in their webs, hanging by their feet, back downward. The webs have in some part a more closely woven space under which the spider stands." These spiders are quick to avail themselves of any chance to spin their shapeless meshes of almost invisible silk, which few regard as real "webs," in closets, cellars, and all over the house or barn. Many of them are adorned with gay colors or striking patterns, and some are much feared, especially Latrodectus mactans, about half an inch long, which is black with scarlet spots. It is common from Canada to Chile, and everywhere is considered fatally poisonous—why, it is difficult to say.

Last of our list, and highest in rank, are the EpeiridÆ, the "orb weavers," as they are often called, who make those regular spiral nets which are in our mind's eye when we think of cobwebs. Most of the moderately large and handsome house and garden spiders are of this family, and everyone can easily examine their work, although it is less easy to watch them at it, as the webs are built and repaired at night. Among the obscurer and foreign species the abdomen often shows humps, points and long forward-reaching horns that make them exceedingly grotesque, and doubtless difficult to handle by birds and other creatures that seize them as food.

One of the round webs of the EpeiridÆ consists of several radiating lines, varying in different species from a dozen to seventy, crossed by two spirals—an inner spiral that begins in the center and winds outward, and an outer spiral that begins at the edge of the web and winds inward. The inner spiral is made of smooth thread, like that of the rays, to which dust will not cling; the outer spiral is made of more elastic thread which, when fresh, is covered with fine drops of sticky liquid.

"In beginning a web, after the radiating threads are finished, the spider fastens them more firmly at the center and corrects the distances between them by [inserting] several short, irregular threads, and then begins the inner spiral, with the turns at first close together and then widening
... until they are as far apart as the spider can
reach with the spinnerets [resting] on one and the front feet on the next, and so goes on nearly to the outside of the web, where it stops abruptly. The spider usually rests a moment, and then begins, sometimes at another part of the web, the outer sticky spiral.... As soon as the inner spiral is found in the way a part of it is cut out, and by the time the outer spiral is finished the inner is reduced to the small and close portion near the center.... The whole making of the web seems to be done entirely by feeling, and is done as well in the dark as in daylight. When the spider is active and the food supply good, a fresh web is made every day, the old one being torn down and thrown away."

As a rule these orb weavers do not stay in the web in the daytime, but hide away in their nests made in some near-by but concealed place; and their egg cocoons are hidden in all sorts of places.

All of the spiders that have been considered so far belong to the division of the class that has but a single pair of lungs. A second division has been made for those having two pairs of lungs, composed of a single family, the MygalidÆ, consisting of the so-called "bird-catching" spiders and the trapdoor spiders. The great mygale of Guiana has a body sometimes two inches long, and its legs will span eight or nine inches of space. It is hairy all over, intensely black, and a terror to all small creatures, even catching small birds, according to tradition; but proof of this is wanting.

The trapdoor spiders are those of the genera Cteniza and Atypus which dig and inhabit vertical holes in the soil, lined with silk and closed at the top by a hinged stopper or "trapdoor." Several species occur in southern Europe, one of which has a second door hanging by a silken hinge half way down the shaft; and in case of trouble the spider goes below it and pushes it above its head, so that the intruder is deceived into thinking it has opened an empty nest. Cteniza californica is the common species of our Southwest. The cover of the hole is made of dirt fastened together with threads, and is lined, like the tube, with silk, and fastened by a thick hinge of silk. The spider holds the door shut from inside. These underground homes are safe retreats for the spiders during the day, and nesting places in which their eggs are deposited and young reared; at night the spiders go forth in search of prey.

MITES AND TICKS

Mites and ticks are classified with the spiders as degenerate relatives of arachnoid stock. Ticks are large enough to be seen without a magnifying glass, and some become half an inch long. Ticks are wholly parasitic. The female lays several thousand eggs at one time on the ground or just beneath the surface. "The young 'seed ticks' that hatch from these in a few days soon crawl up on some near-by blade of grass, or on a bush or shrub, and wait quietly until some animal comes along. If the animal comes close enough they leave the grass or other support and cling to their new-found host." These parasites are the agents of the spread of several infectious diseases of cattle, the worst of which is the destructive Texas fever, and of mankind, as spotted fever and other ills resulting from the presence of blood parasites.