CHAPTER VIII

ARTHROPODA, THE LOBSTERS, SPIDERS AND INSECTS

The above is a very descriptive name for a division which includes the Crabs and Lobsters and the Insects. Formerly they were included, along with the worms, under the name Annulosa, the Ringed Animals. They resemble these as possessing what is termed metameric symmetry, but they are distinguished from them as the Leggy Animals, a fact which is explained in the name, Arthropoda, joint-footed. Worms, as we have seen, have no true legs, but the Arthropods, theoretically, have a pair of legs to every ring. In some of the lower members of the group this is literally the case, the Centipedes, or hundred-footed animals, for example (Fig. 13). In higher forms the number of legs is greatly reduced; several successive rings may become merged with one another, losing, along with their independence, their legs. The true Insects, thus, have only three pairs of legs and the Spiders four.

What are theoretically regarded as legs, however, may practically be turned to many other uses, according to the position of the particular body-ring to which they are attached. Thus, in the case of a body-ring near the mouth, we find such things as "jaw-feet," maxillipedes--that is to say, legs used for jaws. It consequently results that zoologists are sometimes driven to speak of "walking legs," or, hiding the tautology under a Latin phrase, "ambulatory legs"; and absurd although this may seem, it is sometimes quite necessary for the sake of accuracy. It is therefore more convenient to speak of the "appendages" of a body-ring than of its legs. For this vague term can be applied equally to all the row, whatever their uses. Among the different forms taken by the "appendages" are those of "antennæ," long, hair-like feelers attached to the head; "chelæ," or claws, such as the large claws of the lobster; "cheliceræ," or "claw-horns," tearing appendages attached to the head; "mandibles," mouth appendages used for biting, etc., etc. The reader who wishes to attain a clear idea of the structure of a segmented animal, and of the ways in which its parts are modified, should consult Huxley's classical study of "The Crayfish" (International Science Series).

The Arthropoda include two main groups--the Crustacea, or Jointed Animals of the water, which breathe by gills; the Insects, or Jointed Animals of the land, which breathe through tubes in their sides, called tracheæ.

The Crustacea include the familiar Crabs and Lobsters. These are among their highest forms as well as their largest, and if we begin at the beginning we must seek much smaller forms. The group called Entomostraca include the so-called Freshwater Flea, a very active little thing found in English ditches, and a great many other freshwater forms: also the little Cypris, which has a shield forming a sort of bivalve-shell, and is interesting from its wide occurrence as a fossil form. Most of the Entomostraca have a larval form called a Nauplius; but this larva refuses to tell us anything about the past history of the Arthropods. It is itself already a jointed animal with legs. So we see that the Arthropods, unlike the worms and the Chordata, have obliterated all record of their poor relations. The parasitic "fish-lice," so-called, are entomostracous Crustacea, often greatly degenerated in consequence of their habit of life. Some live in the gill-chambers of a fish, some on, or even embedded in the skin.

Among the most curiously modified forms of the Crustacea are the Barnacles or Cirripedia. These creatures, like the sponges, have a free-swimming larvæ, which eventually fixes itself by its anterior end, so that the adult animal passes its existence upside down. The young is an ordinary little creature with jointed legs, but the adult protects itself by a strange armour of shell. An intermediate stage exists in which the creature eats no food; it has therefore been compared with the chrysalis of insects. At the top of the adult shell two little valves open and shut, allowing the legs to dart out and seize upon prey. These legs, gathered into a bunch, and extended and retracted together, remind one of the fingers of a hand opening and closing. They are clothed with a fringe of "cirrhi" or small processes; hence the name of the group. The Common Barnacle of our own shores, sometimes called the Acorn-Shell, is found on shells and stones, and often on those that are left uncovered between tides. In places where the rocks of the coast are very steep, a belt of white, several feet or yards deep, may often be seen above low-water mark. This white zone, when examined more nearly, is found to consist of barnacles, so crowded together that they obscure the natural colour of the rock. The Common Barnacle is one of the smaller species of the genus: in warmer seas barnacles attain to a much greater size (Figs. 14 and 15).

The higher Crustacea, Malacostraca, include the familiar Crabs and Lobsters, Decapoda. The lobsters receive the name of Macrura or Big-tails; associated with them are the Shrimps and the Hermit-Crabs (Fig. 16). The latter are therefore not crabs at all, but somewhat divergent lobsters. Their tails are soft, and they thus require protection: they choose the dried shell of some univalve mollusc and live in it (Fig. 16). How far the case is that they need a house because their tails are soft, and how far the contrary is true that their tails are soft because they live in a house, it would be difficult to say. Readers of another volume in this series, Professor Hickson's "Story of Animal Life in the Sea," will remember that the hermit-crab often offers a curious instance of "commensalism" or partnership with other animals. The hermit-crab was, in fact, one of the earliest instances in which such a partnership was observed, the companion being in this case a sea-anemone perched on the shell in which the crab lives.

The true Crabs are called Brachyura, or Short-tails; for obvious reasons, the tail of a crab being very curiously modified and tucked in under the carapace or "shell." A form exceptional in the fact that frequents the land is the Land-Crab of the West Indies (Fig. 17). Another land crustacean, _Birgus latro_, the Robber Crab, belongs to the previous group.

In addition to the above the Malacostraca include the Arthrostraca, or crustaceans which have the front of the body jointed as well as the tail, so that there is no large shield formed by the fused armour of several segments (cephalo-thoracic shield, _cf._ Figs. 16 and 17), as in crabs and lobsters. The Amphipoda, or Sand-hoppers, sometimes called Sand-fleas, are familiar examples of these. There are several common kinds found on our English shores, and sometimes they appear in such numbers, hopping above sand or seaweed left by the tide, that they seem to form a sort of cloud, every unit of which, however, is but in the air an instant, falling and giving place to some other, while it prepares for a fresh hop. The so-called Freshwater Shrimp, _Gammarus_, is another common member of the Amphipoda. Fig. 18 shows the general form of a Sand-hopper. Nearly allied are the Isopoda or Wood-lice, interesting because they are among the few terrestrial forms of the crustacea; they live, however, in damp places, and are but too well-known in gardens, where the gardener often mis-names them "insects."

The mention of terrestrial forms would naturally bring us to the discussion of the true Insects. In the Arthropoda we for the first time meet with terrestrial animals except in scattered instances, and the true Insects are the largest and most important group of these. There are, however, various creatures belonging to the Arthropoda which are neither Crustacea nor yet Insects. Among these is the familiar spider, an "insect" in popular language, but not so described by the zoologist. Among other differences, the true spiders have eight legs, whereas the true insects have only six. Fig. 19 shows a typical spider; the eight jointed legs are attached to the thorax ("breastplate"); with the latter the head is united. The abdomen, as in insects, is formed by the fusion of several segments, and has no legs, but it has, however, out of sight, the spinning legs or "spinnerets," out of which the thread of the spider's web is spun. The venom of the spider is not a fable; spiders have poison-glands with ducts which open on the tops of the cheliceræ. They dispose of their prey by sucking it; they do not swallow solid food. The habits and webs of spiders are familiar to every one: their nests, as a rule, are only noticed by close observers. The nest is made of spun threads closely felted together to form a round hollow ball. This the house-spiders hang on a wall or among the rafters of a roof. There are, however, spiders which build their nests under ground; and in this case the nest may be conveniently furnished with a lid, which can be pushed up when the animal wishes to come out. Fig. 20 shows the nest of the Trap-door Spider, so called from the construction of its nest.

Fig. 21 shows a spider-like animal which, at first sight, seems to have five pairs of legs. In fact, however, it has only three pairs, thus approaching the insects in structure. These three pairs of legs are attached to the thorax, while the head, which is separate from the thorax, unlike that of the true spider, bears two pairs of leg-like appendages. This is the chief of a group which are sometimes placed in a class by themselves, on account of their great differences from real spiders. Their head is separated from the thorax; and the thorax is divided into three segments; these, however, do not come out clearly in the diagram. The head bears, posteriorly, a pair of appendages which are practically legs; in front of these a pair of long "pedipalps" or "foot-feelers"; and quite in front the comparatively short "cheliceræ." These creatures are very venomous; they move about by night to seek their prey.

Another kind of spider-like animal is familiar in English fields and waysides--the long-legged spiders, called Harvestmen or Phalangidæ, which spin no web, but jump upon their prey. Unlike the last group, the body differs from that of true spiders, in being more, not less, compact: for not only is the head joined to the thorax, but also the thorax is joined to the abdomen, the outline of the body being therefore almost globular. They receive the name Phalangidæ, Joint-Spiders, from the sharp joints in their long legs.

Allied also to the spiders are the Mites, Acarina, so destructive to cheese, flour, and other eatables; and the Ticks, which infest the skins of various animals Fig. 22 shows a specimen of the latter. They are practically blood-sucking Mites. It is the female which attacks animals, while the males live among vegetation.

The Scorpions, also, are relatives of the Spiders. They are inhabitants of hot countries, and highly venomous. They possess a jointed tail, instead of an abdomen with fused segments, and a lobster-like pair of appendages in front; these are the second pair of appendages, the "pedipalps," while the short "cheliceræ" lie in front. In the living animal the tail is often carried curled up over the back. The Mites, Ticks, and Scorpions all agree with the true spiders in possessing eight legs. The King-Crab, Limulus, has not hitherto been named, because, though living in the sea, it is not a crab at all. It has been shown by Professor Ray Lankester to be related to the spiders. It is a large crab-like creature, which may be seen in museums and aquaria, and is brought from the tropical seas.

Before passing to consider the true Insects, or Hexapoda, something must be said about the discovery of _Peripatus_, a creature which comes from Cape Colony. It has been called caterpillar-like in appearance, but its structure is in many respects so peculiar, that it has been described as a link between insects and the higher worms. Its legs, for instance, although jointed, and much resembling those of insects in appearance, are hollow, like the "parapodia" of worms.

The Centipedes have been already referred to. These, with the Millipedes, form the group Myriapoda. In outward form, at any rate, these suggest an intermediate position between Peripatus and Insects.

The true Insects have a definite head, separated from the thorax, and a constriction between the thorax and the abdomen; this is why they are called insects, "cut in two." The thorax bears three pairs of legs, the mouth has typically three pairs of appendages, which may be altered and modified in many different ways, according to the nature of the animal's way of feeding. While the Crustacea are typically adapted for breathing in water by means of their gills, the Insects are adapted for breathing air. This they do by means of their air-tubes or tracheæ, the inlets of which open on their sides. These are divided into fine branches, which diffuse air through the body of the Insect. Two interesting points must be noticed about insects. The first is that they were the first group in which zoologists were able to study the nature of larval forms, long before the microscope had revealed the larval forms of marine animals. The changes undergone by insects are known as metamorphosis, or change of form; and are typically represented by the life-history of a caterpillar, which assumes during the winter a resting form called a Chrysalis or Pupa, and finally emerges as a Butterfly. Insects have sometimes been classified according to the greater or less completeness of the metamorphosis they undergo, which in some cases is comparatively slight. It has been mentioned elsewhere that larval forms usually exist where the young animal is placed under very different conditions from the adult. Fig. 24 shows two well-known instances of insect larvæ in which this is strikingly the case, the larval form being a water-dweller, and the adult a winged fly. Of these, one, the larvæ of the Dragon-fly, crawls about free; while the other, the so-called caddis-"worm," builds itself a case of grains of stone and shell cemented together.

The second point of interest is the wonderful part which has been played by insects in modifying the world we live in. We owe the bright colours and the sweet honey of flowers to the selection exercised by insects; they carry the pollen of flowers from one plant to its neighbouring kindred, thus securing cross-fertilization for the advantage of the plant, and thereby perpetuating any quality, such as colour or sweetness, which has originally attracted the insect to the flower. While a few plants only are fertilised by means of the wind, a vast majority depend entirely upon insects for the cross-fertilisation which is so necessary for the production of healthy seeds. We have already alluded to the part played by the earthworm in preparing the soil. If the earthworm has been the ploughman the insect has been the more intelligent gardener, who has filled the world with bright flowers. The earlier forms of plant life had green and inconspicuous flowers (Cryptogamia); the Phanerogamia, or showy-flowered plants, including all those that bear what are popularly termed flowers, have been produced by the artificial selection exercised by insects long before man was here to admire the result, and to carry on the same work in his gardens. The insect owes its food to the plant world; the plant world owes health and beauty to the constant ministration of the insect; so marvellous is the inter-connexion of one form of life with another.

The number of different kinds of insect is enormous; the number of named species has been estimated at nearly a quarter of a million. It is therefore no wonder that entomology, the study of insects, has claimed the rank of a special science. We cannot here do more than refer in passing to a few of the more familiar types. First of all, by right of its work in fertilising flowers, let us take the Bee. Fig. 25 shows its honeycomb and its larvæ. The bee-grub differs from the caterpillar in its comparative helplessness. It is fed like a child by the worker bees, which are undeveloped females; and it does not leave the cell in which the egg is originally placed until it is ready to take on the adult form. The metamorphosis is complete; that is to say there is a grub stage and a pupa stage before the adult stage. There are three kinds of bees--the workers, which are sexless; the drones, which are males, and the queen, who is the sole female of the hive. The bee-grub may develop into a worker or a queen, according to the food it receives as a grub, the grubs that are intended to become queens being placed in a larger cell. The bee-grub differs from the caterpillar in having no feet.

The ants are nearly allied to the bees, and also have a complete metamorphosis. Fig. 26 shows the English red ant, female and neuter. The wings of the female drop off after the pairing season, a fact which has given a name, Hymenoptera, to the whole group to which the ant belongs, although the name is often quite inapplicable. A recent discovery in entomology is the fact that ants have a voice. Dr. D. Sharp of Cambridge has described their "stridulating," _i.e._ noise-producing, organs. These consist of parallel ridges present on the sides of certain segments. By working the body up and down, the insect scrapes these ridges with the edge of the preceding segment, so that a musical note is produced, intelligible to other ants. The question has also been investigated by French observers. The principle involved will readily be recognised by those who in childhood were guilty of trying to extract music from a comb.

The white ants, so destructive in tropical climates, are not true ants, but belong to a different order. These also live in colonies; like the bees, they have an egg-laying queen. She has a partner, the king. There are neuter soldiers and neuter workers, both wingless, while the male and female have wings, afterwards lost.

The Lepidoptera or butterflies and moths receive their name, Scaly-winged, from the beautiful microscopic scales with which their wings are covered. Fig. 28 shows the cocoons which the larvæ of some of the moths make for themselves in which to pass their pupa stage. Some are made wholly of silk, others of dried leaves woven together. Fig. 29 shows a Moth with its caterpillar, cocoon, and chrysalis. The threads of which a caterpillar weaves its cocoon are familiarly exemplified in the silk of commerce. The caterpillar, in some cases, is gregarious, and builds a common nest (Fig. 30).

The beetles, Coleoptera, are, like the butterflies, endlessly numerous. They are characterised by the striking difference in their two pairs of wings, of which the anterior pair is strong and horny, and forms, when at rest, a sheath which covers the thinner posterior pair of wings. The metamorphosis is complete in this group also. Fig. 31 shows an example which is typical except in one respect--the adult form, namely, is one of the comparatively few instances of adult insects that live in water.

Much has been said above in praise of insects and their wonderful work in selecting flowers. There is, however, another side to this, as the gardener and farmer know too well. While the winged honey seekers help the plants, their larvæ devour them, and so do many other forms of insect. Fig. 32 gives us in miniature some of the most notorious insect pests. The work of the locust has been dreaded since the days of the Pharaohs and before: the Colorado beetle which infests the potato, is a plague as terrible, if more modern. The weevils and caterpillars that destroy trees, though not directly dangerous to our food supply, are sufficiently destructive. The terror of insect pests lies in their vast numbers, which may render an otherwise harmless creature dangerous. I read last year of a curious railway mishap in the United States. A train was brought to a standstill by the wheels sliding on something greasy that covered the track. It proved to be a flock of the so-called "Army worm," a variety of caterpillar which travels long distances in crowds, when its numbers have become too many for the supply of food, or when it is about to enter into the pupa stage. These covered the railway track, and the whole country for a long distance; and the "greasiness" of the rails was produced by the crushed bodies of the unfortunate caterpillars. The train was delayed for hours, while a gang of men with brooms cleared the way in front of it.