BRANCH ARTHROPODA: CRUSTACEANS, CENTIPEDS, INSECTS, AND SPIDERS

The great branch Arthropoda includes a host of familiar animals. It contains more species than any other branch of the animal kingdom. To it belong the crayfishes, shrimps, crabs, lobsters, water-fleas, and other animals which compose the cla.s.s Crustacea; the centipeds and thousand-legged worms which compose the cla.s.s Myriapoda; the true or six-footed insects forming the cla.s.s Insecta, which includes nearly two-thirds of all the known species of animals; and the scorpions, mites, ticks, and spiders which const.i.tute the cla.s.s Arachnida. There is also a fifth cla.s.s in the branch Arthropoda which includes a few species of animals unfamiliar to us but of great interest to zoologists.

All these varied kinds of animals have a body on the annulate or segmented type-plan, like that shown by most worms, but they differ from the worms in possessing jointed appendages, used for locomotion or food taking. There is typically or racially one pair of these jointed or segmented appendages on each segment of the body, but in all of the Arthropoda some of the segments have lost their appendages. The body is covered by a firm cuticle or outer body-wall called the exoskeleton.

This exoskeleton serves not only to enclose and protect the soft parts of the body but also for the attachment of the body muscles. It may be flexible as in the sutures between the body-segments in most insects, or hard and rigid as in the sclerites of the segments. The firmness is due primarily, and in the insects usually solely, to a deposit in the cuticle of _chitin_, a substance probably secreted by the underlying cells of the true skin, or it may be due chiefly, as in the crabs, to a calcareous deposit. In such cases it becomes a veritable armor. The internal organs of the Arthropods show a more or less obvious segmentation corresponding with the segmentation of the body-wall. The alimentary ca.n.a.l runs longitudinally through the center of the body from mouth to a.n.a.l opening. The nervous system consists of a brain lying above the sophagus and a double nerve-chain running backward from beneath the sophagus, along the median line of the ventral wall, to the posterior extremity of the body. This ventral nerve-chain consists of a pair of longitudinal commissures or cords and a series of pairs of ganglia, arranged segmentally. The two ganglia of each pair are fused more or less nearly completely to form a single ganglion, and the nerve-cords are partially fused, or at least lie close together. In addition there is a smaller sympathetic system composed of a few small ganglia and certain nerves running from them to the viscera, this system being connected with the main or central nervous system. In this group the organs of special sense reach for the first time a high stage of development. Compound eyes are peculiar to Arthropoda. The heart lies above the alimentary ca.n.a.l. Respiration is carried on by gills in the aquatic forms, and by a remarkable system of air-tubes or tracheae in the land forms (insects). The s.e.xes are usually distinct, and reproduction is almost universally s.e.xual. Most of the species lay eggs.

The Arthropods are animals of a high degree of organization. The extremely diverse life-habits of the various kinds among them have led to much modification and to great specialization of structure. The course of development, too, is made very complicated by the elaborate metamorphosis undergone by many of the members of the branch.

We shall study the Arthropoda by getting acquainted with a few examples of each cla.s.s and thus learning the special cla.s.s characteristics.

CLa.s.s CRUSTACEA: CRAYFISHES, CRABS, LOBSTERS, ETC.

THE CRAYFISH (_Cambarus_ sp.)

=Structure.=--The structure of the crayfish has been already studied (see Chapter IV and figs. 3 and 4).

=Life-history and habits.=--Crayfish frequent fresh-water lakes, rivers, and springs in most parts of the United States. Many of them perish whenever the small prairie ponds dry up. But some burrow into the earth when the dry season comes. There may be noticed in meadows where water stands for certain seasons of the year many scattered holes with slight elevations of mud about them. These are mostly the burrows of crayfish.

During the dry season the crayfish digs down until it reaches water, or at least a damp place, where it rests until wet weather brings it to the surface once more. One of these burrows, followed in digging a mining shaft, extended vertically down to a distance of twenty-six feet, where the crayfish was found tucked snugly away.

The eggs are carried by the female on her abdominal appendages. Previous to the laying of the eggs the female rubs off all foreign matter from the appendages, thus preparing them for the reception of the eggs. This cleaning is done with the fifth pair of legs. When the eggs are ready to be laid, which is during the last of March or in April in the Central States, a sticky secretion pa.s.ses out of the openings at the base of the walking legs and smears the pleopods of the abdomen. The eggs as they pa.s.s out are fertilized and caught on the pleopods, where they remain attached in cl.u.s.ters. After some weeks the young crayfishes issue from the eggs. In general appearance they are not very unlike the adults.

They grow very rapidly at this stage. As the animal is enclosed in a hard sh.e.l.l, growth can only take place during the period just following the molt, for the crayfish casts its skin periodically, and it is while the new sh.e.l.l is forming that the animal does its growing. The crayfish when it molts casts not only the exoskeleton, but also the lining of part of the alimentary ca.n.a.l. After the females have hatched their young many die in the shallow pools, in which places the dried-up skeletons are noticeable during the summer months.

OTHER CRUSTACEANS.

Most of the crustaceans live in water, a few being found in damp soil or in other moist places. Some are fresh-water animals and some marine. They vary in size from the tiny water-fleas, a millimeter long, to crabs two feet across the sh.e.l.l or sixteen feet from tip to tip of legs. They present great differences in form and general appearance of body, being adapted for various conditions of life. Some crustaceans live as parasites on other animals, in some cases on other crustaceans. Such parasitic species have the body much modified and are hardly to be recognized as members of the cla.s.s.

=Body form and structure.=--In structural character and body organization the Crustaceans show, of course, the general characteristics already attributed to the Arthropoda, the branch to which they belong. The characteristics which distinguish them from other Arthropods are the possession of gills for respiration (some insects have gills, but of a very different kind as will be seen later), and the bi-ramose condition of the body appendages, each appendage (excepting the antennules) consisting of a single basal segment from which arise two branches made up of one or more segments.

Of the form of the crustacean body few generalizations can be made.

"There is no [other] cla.s.s in the animal kingdom which presents so wide a range of organization as the Crustacea, or in which the deviations in structure from the 'type form' are so striking and so interesting from their obvious adaptation to the mode of life." For this reason no attempt will be made to discuss in general terms the form of the crustacean body, but brief accounts will be given of a few of the more familiar kinds of Crustacea which will serve to ill.u.s.trate this remarkable diversity of body form.

Similarly impossible is it also to give a general account of the development of the crustaceans. The s.e.xes are distinct in most Crustacea, and there is often great difference in form between the male and female. A certain amount of metamorphosis takes place in the development of all crustaceans; that is, the young when hatched from the egg differs, often decidedly, in appearance and structure from the parent, and in the course of its post-embryonic development undergoes more or less striking change or metamorphosis. This metamorphosis is often very marked.

=Water-fleas (Cyclops).=--TECHNICAL NOTE.--The water-fleas are common in the water of ponds or of slow streams; they may often be found in the school aquarium. They are, though small (about 1 mm.

long), readily seen with the unaided eye; they are white, rather elongate, and have a rapid jerky movement. Examine specimens alive in water in a watch gla.s.s. Note the "split pear" shape, broadest near the front, tapering posteriorly, flat beneath, convex above; note the forked stylets at tip of abdomen; also the two pairs of antennae, the single median eye, the mandibles, two pairs of maxillae, and five pairs of legs (last pair very small). There are no gills. Some of the specimens, females, may have attached to the first abdominal segment on either side an egg sac. Make drawings showing all these structural details. Watch the _Cyclops_ capturing and feeding on _Paramcium_ or other small animals.

[Ill.u.s.tration: FIG. 35.--A water-flea, _Cyclops_ sp. Female with egg-ma.s.ses. (From living specimen.)]

The water-fleas (_Cyclops_) (fig. 35) are among the smallest of the Crustacea. They are extremely abundant, having great power of multiplication. "An old _Cyclops_ may produce forty or fifty eggs at once, and may give birth to eight or ten broods of children living five to six months. As the young begin to reproduce at an early age, the rate of multiplication is astonishing. The descendants of one _Cyclops_ may number in one year nearly 4,500,000,000, or more than three times the total population of the earth, provided that all the young reach maturity and produce the full number of offspring." The _Cyclops_ feed on smaller aquatic animals such as Protozoa, Rotifera, etc. They in turn serve as food for fishes; and because of their immense numbers and occurrence in all except the swiftest fresh waters "they form the main food of most of our fresh-water fishes while young." Many aquatic insect larvae feed almost exclusively on them.

Related to the _Cyclops_ are a host of other kinds of minute Crustaceans. Among these the so-called fish-lice are specially interesting because of their parasitic habits and greatly modified and degenerate structure. There are many kinds of these parasitic crustaceans infesting fishes, whales, molluscs, and worms. "As on land almost every species of bird or mammal has its own parasitic insects, so in the water almost every species of fish or larger invertebrate has its parasitic crustaceans." Some of the most common of these parasites attach themselves to the gills of fishes. Here they cling, sucking the blood or animal juices from the host. In form of body they do not at all resemble other Crustaceans, but are strangely misshapen. They are often worm-like, or sac-like, without legs or other locomotory appendages. As with other parasites (see Chapter x.x.x) an inactive dependent life results in the atrophy and loss by degeneration of the body-parts concerned with locomotion and orientation.

=Wood lice (Isopoda).=--TECHNICAL NOTE.--Specimens of wood lice, pill bugs, or damp bugs, as they are variously called, may be readily found in concealed moist places, as under stones or boards on damp soil. They are often common in houses, near drains or in dark, damp places. Examine some live wood lice, and some dead specimens (killed by chloroform or in an insect-killing bottle).

Note the division of the body into the head, thorax, and abdomen; find the eyes, the antennae and the mouthparts (mandibles and maxillae are usually pressed closely together). All the locomotory appendages are adapted for walking or running, not swimming. Note the number of pairs of legs; the structure of a leg; find gills and gill-covers. Some females may be found with eggs on the under side of the thorax near the bases of the legs, the eggs being covered by thin membranous plates. Make drawings showing the general form and character of body and details of legs, gills, etc. Compare with the crayfish and _Cyclops_.

The wood-lice (fig. 36) are among the few Crustacea which have a wholly terrestrial life. They run about quickly and feed chiefly on decaying vegetable matter. They are night scavengers. They have the body oval and convex above, rather purplish or grayish brown, and smooth. Although they do not live in the water they breathe partly at least by means of gills (though they may breathe partly through the skin). It is therefore necessary for them to live in a damp atmosphere so that the gill membranes may be kept damp. If not kept moist they could not serve as osmotic membranes.

[Ill.u.s.tration: FIG. 36.--A damp bug, Isopod, species not determined.

(From specimen.)]

=Lobsters, Shrimps and Crabs (Decapoda).=--TECHNICAL NOTE.--Teachers living near the sea-sh.o.r.e can get specimens of live and dead lobsters, shrimps, and crabs in the markets. Schools in the interior should have a few preserved specimens for examination. These specimens should be compared with the crayfish; although differences in shape of body are evident, the character and arrangement of body parts will be found to be very similar.

The largest and most familiar Crustaceans, as the crayfishes, lobsters, shrimps, prawns and crabs, all belong to the order Decapoda, or ten-legged Crustacea. The members of this order have, including the large claws, ten walking feet; they all have eyes on movable stalks, and the front portion of the body is covered by a h.o.r.n.y fold of the body-wall called the carapace.

The lobsters are large ocean-inhabiting crustaceans which are very like the fresh-water crayfish in all structural characters. They live on the rocky or sandy ocean-bottom at shallow depths. They feed largely on decaying animal matter. They are caught in great numbers in so-called "lobster pots," a kind of wooden trap baited with refuse. "The number thus taken upon the sh.o.r.es of New England and Canada amounts to between twenty and thirty million annually." Live lobsters are brownish or greenish with bluish mottling; they turn red when boiled. A single female will lay several thousand eggs. The eggs are greenish and are carried about by the mother until the young hatch. The young are free-swimming larvae, until they reach a length of half an inch.

The shrimps and prawns are mostly marine, though some species live in fresh water. They are, like the lobsters, used for food. Some of the species are gregarious in habit, occurring in great "schools" of individuals. Like the lobsters they crawl about on the sea-bottom feeding on decaying animal matter. Shrimps are very abundant near San Francisco, where extensive "shrimp fishing" is done by the Chinese.

[Ill.u.s.tration: FIG. 37.--Some crabs and barnacles of the Pacific coast; the short sessile acorn barnacles in the upper left-hand corner belong to the genus _Bala.n.u.s_; the stalked barnacles in the upper right-hand corner are of the species _Pollicipes polymenus_; the largest crab (upper left-hand) is _Brachynotus nudus_; the one in left-hand lower corner is a young rock-crab, _Cancer productus_; the crab in the sea-weed at the right is a kelp-crab, _Epialtus productus_, while the two in snail-sh.e.l.ls in lower corner are hermit-crabs, _Pagurus samuelis_. (From living specimens in a tide-pool on the Bay of Monterey, California.)]

The crabs (fig. 37) differ from the lobsters and crayfishes and shrimps in having the body short and broad, instead of elongate. This is due to the special widening of the carapace and the marked shortening of the abdomen. The abdomen, moreover, is permanently bent underneath the body, so that but little of it is visible from the dorsal aspect. The number of abdominal legs or appendages is reduced.

When the tide is out the rocks and tide-pools of the ocean sh.o.r.e are alive with crabs. They "scuttle" about noisily over the rocks, withdrawing into crevices or sinking to the bottom of the pools when disturbed. They move as readily backward or sidewise, "crab-fashion,"

as forward. They are of various colors and markings, often so patterned as to harmonize very perfectly with the general color and appearance of the rocks and sea-weeds among which they live. The spider-crabs are especially strange-looking creatures with unusually long and slender legs and a comparatively small body-trunk. They include the _Macrocheira_ of j.a.pan, the largest of the crustaceans.

Specimens of this crab are known measuring twelve to sixteen feet from tip to tip of extended legs; the carapace is only as many inches in width or length. The soft-sh.e.l.led crab is a species common along our Atlantic coast. It is "soft-sh.e.l.led" only at the time of molting, and has to be caught in the few days intervening between the shedding of the old hard sh.e.l.l and the hardening of the new body-wall. The little oyster-crabs (_Pinnotheres_) which live with the live oyster in the cavity enclosed by the oyster sh.e.l.l are well-known and interesting crabs. They are not parasites preying on the body of the oyster, but are simply messmates feeding on particles of food brought into the sh.e.l.l by the currents of water created by the oysters.

Among the most interesting crabs are the hermit crabs (fig. 37), familiar to all who know the seash.o.r.e. There are numerous species of these crabs, all of which have the habit of carrying about with them, as a protective covering into which to withdraw, the spiral sh.e.l.l of some gastropod mollusc. The abdomen of the crab remains always in the cavity of the sh.e.l.l; the head and thorax and legs project from the opening of the sh.e.l.l, to be withdrawn into it when the animal is alarmed or at rest. The abdomen being always in the sh.e.l.l and thus protected loses the hard body-wall, and is soft, often curiously shaped and twisted to correspond to the cavity of the sh.e.l.l. It has on it no legs or appendages except a pair for the hindmost segment which are modified into hooks for holding fast to the interior of the sh.e.l.l. As the hermit crab grows it takes up its abode in larger and larger sh.e.l.ls, sometimes killing and removing piece-meal the original inhabitant. Some hermit crabs always have attached to the sh.e.l.l certain kinds of sea-anemones.

It is believed that both crab and sea-anemone derive advantage from this arrangement. The sea-anemone, which otherwise cannot move, is carried from place to place by the crab and so may get a larger supply of food, while the crab is protected from its enemies, the predaceous fishes, by the stinging threads of the sea-anemone, and also perhaps by the concealment of the sh.e.l.l its presence affords. This living together by two kinds of animals to their mutual advantage is called commensalism or symbiosis (see Chapter x.x.x). The hermit crabs are not true crabs, but are more nearly related to the crayfishes and shrimps than to the true broad-bodied, short-tailed crabs.

=Barnacles.=--TECHNICAL NOTE.--Specimens of barnacles may be got readily from the tide rocks or from piles in a harbor. Interior schools should have, if possible, specimens preserved in alcohol or formalin for examination. The "sh.e.l.ls" of acorn (sessile) barnacles may often be found on oyster sh.e.l.ls (get at restaurants).

Crustaceans which at first glance are hardly recognizable as such are the stalked or sessile barnacles (fig. 37) which live fixed in great numbers on the rocks between the tide lines, or on the piles supporting wharves, or on the bottom of ships or even on the body-wall of whales and other ocean animals. In the stalked forms the stalk is a flexible stem or peduncle covered with a blackish finely-wrinkled skin bearing at its free end the greatly modified body of the barnacle.

This body is enclosed in a sort of bivalved sh.e.l.l or carapace formed by a fold of the skin and stiffened by five calcareous plates. Within this curious sh.e.l.l is the compact, rather worm-like body-ma.s.s, showing little or no indication of segmentation. The legs, of which there are usually six pairs, are much modified, being long, feathery, and divided nearly to the base. These feathery feet project from the opened sh.e.l.l when the animal is undisturbed, and waving about in the water catch small animals which serve as the barnacle's food. When disturbed the barnacle withdraws its feet and closes tightly its strong protecting sh.e.l.l. The acorn-barnacles have no stalk, but look like a low bluntly-pointed pyramid, this appearance being due to the converging arrangement of six calcareous plates in its body-wall.

The barnacles present several unusual conditions with regard to the internal organs. They have no heart nor any blood-vessels; most of the species are hermaphroditic; and there are other indications of a degenerate condition. This degeneration of the barnacles is due to their fixed life, the results of which are like those of a parasitic life. The young barnacles when hatched from the egg are free-swimming larvae as with the other Crustacea. They finally attach themselves and undergo the changes, some of them of degenerative nature, which produce the body-structure of the adult. It was long a belief among many people that the barnacle produced the barnacle goose. Pictures in ancient books show the young barnacle geese issuing from the opened sh.e.l.l of the barnacle. The early naturalists believed barnacles, on account of the sh.e.l.l, to be a kind of sh.e.l.l-fish or mollusc, but when their development was thoroughly worked out, it became evident that they belong to the Crustacea.

CHAPTER XXI

BRANCH ARTHROPODA (_continued_); CLa.s.s INSECTA: THE INSECTS

THE LOCUST (_Melanoplus_ sp.)

TECHNICAL NOTE.--Locusts or gra.s.shoppers are common and familiar insects all over the country. The genus _Melanoplus_ includes numerous species, one or more of which are to be found in almost any locality. The common red-legged locust (_M. femur-rubrum_) of the East, the Rocky Mountain migratory locust (_M. spretus_), of the West, the large differential (_M. differentialis_) and two-striped (_M. bivittatus_) locusts of the Southwest, are especially common species. All the members of the genus have their hind wings uncolored, and the front wings marked with a longitudinal series of small dots more or less distinct, or with a longitudinal line. There is a small blunt spine or process projecting from the ventral aspect of the prothorax. If a species of _Melanoplus_ cannot be found, any other locust may be used, although there are some slight variations in the external structure of the various species. Fresh specimens killed in a cyanide bottle (for preparing see p. 463) are preferable in the study of the external structure, but specimens preserved in alcohol will do.

=External structure= (fig. 38).--Note that the body of the gra.s.s-hopper is composed of successive rings or _segments_ grouped into three regions, the _head_ (anterior), _thorax_ (median), and _abdomen_ (posterior). In which region of the body are the segments most readily distinguished? Of how many segments does the head appear to be composed?

The thorax is composed of three segments of which the most anterior, to which is attached the front pair of legs, differs from the succeeding two, being freely movable and bearing a large hood- or saddle-shaped piece on its dorsal aspect. To the other two thoracic segments the second and third pair of legs are attached, as are also the two pairs of wings. The remaining segments of the body compose the abdomen.

[Ill.u.s.tration: FIG. 38.--The red-legged locust, _Melanoplus femur-rubrum_, to show external structure.]

Note the smooth, rather firm and h.o.r.n.y character of the body. This is due to the fact that the skin is everywhere covered with a cuticle in which is deposited a h.o.r.n.y substance called _chitin_. The cuticle is not uniformly firm over the body. At the junction of the body segments in the abdomen, in the neck and between the segments of the legs, in fact, wherever motion is desirable, the cuticle is flexible, thus making bending of the body-wall possible. Elsewhere, however, it is hard and stiff, serving not only as a protective coat or armor over the body, but also affording firm places for the attachment of muscles.

Insects (and all other Arthropods) have no[9] internal skeleton, but, in this firm cuticle, an _exoskeleton_.