Stories of the Universe: Animal Life Part 5

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 chelicerae.

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.

[Ill.u.s.tration: FIG. 20.--Nest of the Trap-door Spider, from the South of France, three-quarters of the natural size.]

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 cla.s.s 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 "chelicerae."

These creatures are very venomous; they move about by night to seek their prey.

[Ill.u.s.tration: FIG. 21.--A venomous spider-like animal, _Galeodes araneoides_, from North Africa, natural size (Diagrammatic).]

Another kind of spider-like animal is familiar in English fields and waysides--the long-legged spiders, called Harvestmen or Phalangidae, 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 Phalangidae, 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.

[Ill.u.s.tration: FIG. 22.--The Tick which infests the Hippopotamus, from South Africa, twice natural size.]

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 "chelicerae" 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.

[Ill.u.s.tration: FIG. 23. A Scorpion, _Buthus Kochii_, from India.]

Before pa.s.sing 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 tracheae, 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 a.s.sumes during the winter a resting form called a Chrysalis or Pupa, and finally emerges as a b.u.t.terfly. Insects have sometimes been cla.s.sified 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 larvae in which this is strikingly the case, the larval form being a water-dweller, and the adult a winged fly. Of these, one, the larvae of the Dragon-fly, crawls about free; while the other, the so-called caddis-"worm," builds itself a case of grains of stone and sh.e.l.l cemented together.

[Ill.u.s.tration: FIG. 24.--Larvae of insects. _A_, of a Dragon-Fly, enlarged; _B_, House of the larva of the Caddis Fly, natural size; _C_, the Caddis Larva itself, enlarged.]

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 pa.s.sing 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 larvae. 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 s.e.xless; 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.

[Ill.u.s.tration: FIG. 25.--_A_, Larva of the Bee, _Apis mellifica_; _B_, Section of Honeycomb.]

[Ill.u.s.tration: FIG. 26.--Ants, _Formica rufa_, English, enlarged. _A_, Female; _B_, Neuter, or Worker.]

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 sc.r.a.pes 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.

[Ill.u.s.tration: FIG. 27.--White Ants, _Eutermes morio_, from Pernambuco, twice the natural size. _A_, Soldier; _B_, Worker; _C_, Young male; _D_, Female.]

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.

[Ill.u.s.tration: FIG. 28.--Coc.o.o.ns of Moths. _A_, Compound Coc.o.o.n of _Coenodomuc hockingi_, from India, one-half natural size; _B_, of a Silkworm, _Bombyx j.a.ponica_, one-half natural size; _C_, of Green-shaded Honey Moth; _D_, of Death's Head Moth, one-quarter natural size; _E_, of _Metura Savendersii_, from New South Wales, natural size; _F_, of _Castnia Endesmia_, from Chili, one-sixth of the natural size; _G_, of _Attacus attas_ from Bombay, one-fourth of the natural size.]

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

[Ill.u.s.tration: FIG. 29.--A Moth, _Saturnia pyri_ (S. Europe), with its Caterpillar, _A_; its Coc.o.o.n, _B_; Coc.o.o.n cut open to show Chrysalis, _C_; Adult insect, _D_.]

The beetles, Coleoptera, are, like the b.u.t.terflies, endlessly numerous.

They are characterised by the striking difference in their two pairs of wings, of which the anterior pair is strong and h.o.r.n.y, 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.

[Ill.u.s.tration: FIG. 30.--Nest of gregarious Caterpillar of a Moth, _Hypsoides_.]

[Ill.u.s.tration: FIG. 31.--Development of an English Water-Beetle, _Dytiscus_. Grub; Pupa; Adult insect.]

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 larvae 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.

[Ill.u.s.tration: FIG. 32. Insect pests. _A_, Locust, _Acridium peregrinum_, one-fourth natural size; _B_, Caterpillar of Wood Leopard Moth, _Zeutzera aesculi_, boring in wood, about one-thirtieth of natural size; _C_, Colorado Beetle, one-fourth natural size; _D_, Leaf-rolling Weevil of the Oak.]

CHAPTER IX

MOLLUSCA, THE Sh.e.l.l-FISH

The sh.e.l.l-fish are called Mollusca, the soft-bodied animals. It will easily be seen that this name was intended to point out the distinction between them and the Arthropoda, as regards the way in which the skin is protected. In the latter, as we have seen, the skin itself is hardened.

In the sh.e.l.l-fish, the skin secretes a covering which lies outside it.

Just as our skins pa.s.s out superfluous moisture to the outside, in the form of perspiration, so the skin of the mollusc continually pa.s.ses to the outside the solid substances which the body has taken in from the sea-water; and by the continual acc.u.mulation of these, the sh.e.l.l is formed. This, at least, is the view taken by modern authorities of the formation of the sh.e.l.l in most instances.

The juvenile sh.e.l.l-collector usually begins his knowledge of the cla.s.sification of the Mollusca, by learning that sh.e.l.ls are cla.s.sified as Univalves and Bivalves. This distinction is useful as a beginning.

Univalves, that is to say sh.e.l.ls which consist of one piece, are those of the snail-like animals, Gasteropoda, or Gastropoda, as some prefer to spell it. Bivalves, or sh.e.l.ls which consist of two flaps, are those of the Lamellibranchiata or animals with plate-like gills, such as the mussel or oyster.

Let us begin with the former. Everybody knows the snail. The snail proper bears a typical univalve sh.e.l.l: though in its relatives (the slugs), the sh.e.l.l is more or less suppressed. The name, Gasteropoda (stomach-footed animals), is supposed to be descriptive of the way in which a snail crawls. Half getting out of its sh.e.l.l, so to speak, it does its best to lay its body to the ground, and its so-called "foot" is an extensive muscular expansion underlying its body, not just a muscular organ thrust out of the sh.e.l.l, as in some other groups. The sh.e.l.l, the mode of crawling, and the "horns," tipped with eye-specks, and directed, intelligently and inquisitively, towards things of interest--these make up, for most people, the idea of Snail. But the most distinctive feature of the cla.s.s is a less obvious feature, namely, the structure of the tongue. We may see, on any damp day or dewy evening, the snail working away with its tongue at some tender leaf. Its tongue is practically a file with which it files away the substance of the leaf, the resulting green mash being thus made ready in minute quant.i.ties for the snail to swallow. Thus are made the too familiar holes which disfigure the leaves of plants in our garden. When seen under the microscope, the file-like structure of the tongue is visible; indeed, in large tongues, it may, to some extent, be made out with the naked eye. Across the tongue, which is a flat ribbon-like structure, there runs a pattern of small teeth, bilaterally symmetrical, and this pattern is repeated over and over again throughout the whole length of the tongue. It might be thought that snails' tongues, being so much alike in their mode of use, would not need to be very various in pattern: but far from this, they vary in appearance as much as the sh.e.l.l. Not only is there a different pattern for every different order of the cla.s.s, but a different pattern for every genus; nay, there are even distinctions between the tongues of different species in the same genus. Consequently some authorities on sh.e.l.l-fish prefer to cla.s.sify them by their tongues, a cla.s.sification which for the most part holds good. So characteristic is the tongue of the Gasteropod, that when new animals have turned up which were difficult to cla.s.sify by means of the structure of the body, they have been finally recognised as Molluscs, somewhat related to the snails, by the tongue. This file-like tongue-ribbon of the snails is often called the Odontoph.o.r.e or Tooth-Carrier; sometimes the part which actually bears the teeth receives the name of the radula.

The snail and its relative, the slugs, belong to the Pulmonate (_i.e._ air-breathing) division of the Gasteropoda. The sea-slugs, in which, like the land slugs, the sh.e.l.l is absent or reduced, are relatives of the land snails. Some of those found on our own sh.o.r.es are handsome creatures, brilliantly coloured. Both groups fall under the division Euthyneura, while the majority of the marine univalves belong to the division Streptoneura (_i.e._ Gasteropods with twisted nerves). The Gasteropods, in the course of the evolution of their sh.e.l.l, have had the body thrown crooked by the burden of carrying it; the Streptoneura are the forms in which this crookedness is most p.r.o.nounced; in the Euthyneura it is less so. There are degrees of crookedness even among the Streptoneura; and the limpet is less crooked than the periwinkle (see Table, p. 30).

The older cla.s.sifications of the Gasteropoda were largely founded on the characters of the sh.e.l.l; but these, though in the main they hold good, have required some modifications in recent times. Conchology, the study of sh.e.l.ls, was at one time the hobby of many collectors whose knowledge of the animals possessing the sh.e.l.ls was not of a very extensive kind; and consequently the very name of conchology is often enough to ruffle the feelings of the zoologist of the present day. Yet many interesting problems of variation may be studied from sh.e.l.ls alone, by those whose circ.u.mstances forbid them to study the living animal. Nor is there any branch of zoology which is more useful to the teacher who wishes to catch the eye and the attention of the beginner in the study of natural history, especially if the beginner is young, as beginners ought to be. Therefore we must by no means undervalue the past labours of conchologists, or the valuable collections which their industry has brought together and set in order for the benefit of the world.

For example of the most crooked, or Azygobranchiate division of the Streptoneura, turn now to Fig. 33, in which we see a typical Gasteropod sh.e.l.l, _Murex ramosus_, the Branchy Murex, aptly enough named from the many p.r.i.c.kly branches which beset it. These rough points are probably a.s.sumed for protective purposes; any animal that might wish to dine upon the _Murex ramosus_ would think twice before trying to swallow it--the morsel of sh.e.l.l-fish is so small, its sh.e.l.ly case so large and so p.r.i.c.kly. If we look for its nearest English relative, that is _Murex erinaceus_, the Hedgehog Murex, or Sting-winkle. This, though a comparatively plain sh.e.l.l, has still enough rough ridges upon it to have secured it a comparison to the p.r.i.c.kly hedgehog. Perhaps the most p.r.i.c.kly member of the genus, however, is _Murex tenuispina_, sometimes called Venus' Comb, because the crowded parallel spines which decorate the elongated front of the sh.e.l.l somewhat resemble the parallel teeth of a comb.

[Ill.u.s.tration: FIG. 33.--The Branchy Murex, _M. ramosus_, a typical specimen of the sh.e.l.l of the Carnivorous Gasteropods. _Sp._, spire or posterior end of the sh.e.l.l; _S_, siphon or anterior end of the sh.e.l.l.

Fig. A, shows the mouth of the sh.e.l.l; Fig. B, the exterior only. Less than one-half the natural size.]

How does the _Murex_ get its living? Let us notice the shape of the sh.e.l.l, drawn out to a point, at the end opposite to the spire. According to the older cla.s.sification of the Mollusca, now somewhat fallen out of use, this point marks the sh.e.l.l as belonging to one of the Siphonostomata (sh.e.l.l-fish with a siphon at the mouth of the sh.e.l.l, _i.e._). These sh.e.l.l-fish are, with few exceptions, carnivorous; not that the siphon shape of the sh.e.l.l has any direct connection with the animal's way of feeding. Just as the snail files among soft vegetable substances, so the Murex and many of its relations file away much harder things. A Sting-winkle, or a Dog-whelk, can sit down over a helpless bivalve sh.e.l.l-fish, and patiently file away, until it has worked a neat round hole in the protecting sh.e.l.l of the latter. You may find, among the dead sh.e.l.ls on any sandy part of the English coast, any number of bivalve half-sh.e.l.ls with a neat little round hole in them, indicating unmistakeably how the tenant came to its death. There is some controversy as to the spot chosen by the a.s.sailant for its attack. Some authorities have stated that the predatory mollusc is so wise that it knows where to find a weak spot, and makes a hole just over some vital organ of the bivalve, or else above its adductor muscles, so that, when these are cut, the half-sh.e.l.ls cannot be drawn tightly together and kept shut. Recently this has been denied, and statistics of the attacks of _Purpura_, the common small whelk, a relation of the _Murex_, on _Mytilus edulis_, the Common Mussel, have shown that the perforation occurs in every part of the sh.e.l.l. It is possible, however, that the Mussel, from the peculiar shape of its sh.e.l.l, offers an exceptional case; and I am inclined to think that in the case of bivalves of a more flattened shape, the earlier statement holds true. At South s.h.i.+elds, England, perforated half-sh.e.l.ls of the Common _Venus_ (Fig. 34) are so abundant that the children string them for necklaces; yet I have never been able, by the most industrious search, to find more than one or two specimens in which the hole is at all near the lip of the sh.e.l.l. It is possible that these exceptional instances were the work of a young and inexperienced univalve mollusc, or a stupid one. It is possible, also, that the mode of attack differs somewhat according to the species of the a.s.sailant. (It should perhaps be explained, for the benefit of those who have no experience in the ways of children or of sh.e.l.l necklaces, that the hole must be moderately near the beak of the sh.e.l.l, to enable the sh.e.l.l to "sit" properly on a string. Every unit in the necklace may therefore be counted as one in favour of the older theory.) Many of the Siphonostomatous molluscs are surprisingly active and strong, so that they are well fitted for a predatory existence. In fact, they not only eat bivalves, but occasionally attack the vegetable-feeding univalves when nothing better is to be got, so that occasionally the sh.e.l.ls of these also may be found displaying the deadly little round hole we have described.

[Ill.u.s.tration: FIG. 34.--Half Sh.e.l.ls of the Common Venus, several of them perforated by carnivorous molluscs. From South s.h.i.+elds, England.]

Let us contrast with the _Murex_ one of the sh.e.l.ls which are "holostomatous," _i.e._ possessing an unindented sh.e.l.l-mouth--that is to say, one without a "siphon." The common edible periwinkle, _Littorina littorea_, may be taken as an example. No sh.e.l.l is more familiar; even the town-dweller, who has never found it on the sea-sh.o.r.e, has seen it often on stalls in the slums. The mouth of the sh.e.l.l is quite round and unindented, and in this case the character holds good as the mark of a vegetable-feeder--a non-predatory sea-snail. It is hardly necessary to remind the reader that its name (the sh.o.r.e-sh.e.l.l) is given it because it lives where the tide leaves the rocks exposed during part of the day.

Another common species of _Littorina_, which frequently lives a little lower down, where the large sea-weeds grow, has been described in Chapter II.; and another, _L. rudis_, lives a little higher up, so that it spends most of its time in a dry state, and is fast on its way to become a land-sh.e.l.l. At most of the familiar English seaside resorts one may see dozens of it baking in a hot July sun on rocks where only the highest tides can reach them: and yet under these conditions they continue to live and flourish. The periwinkles are remarkable for the great length of the tooth-ribbon, in comparison with the size of the animal. The number of separate teeth upon it has been estimated at 3500.

A familiar feature of the common periwinkle is the lid or stopper (Operculum), with which the animal can close the mouth of the sh.e.l.l.

This is developed and carried by the outside of the animal's foot. In the periwinkle and other English molluscs it is comparatively soft and semi-transparent, and reminds one of a thin slice of horn. In many tropical molluscs, however, it is hard and sh.e.l.ly. The large tropical sh.e.l.ls named _Turbo_ have ma.s.sive lids of considerable weight. These sh.e.l.ls, which are nearly allied to the pearly Top-sh.e.l.ls (_Trochus_) of the English sh.o.r.es, are sold as ornaments, the outer coat of the sh.e.l.l being partly sc.r.a.ped off to show the inner coat of pearl: it is rarely, however, that the purchaser obtains a lid, or even knows that the creature had one. The reverse is the case with some of the smaller kinds, the lids of which, being brightly coloured, are imported without the sh.e.l.l, and sometimes set as articles of jewellery. Some of these are of a bright green hue.