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.
[Illustration: FIG. 13.--A Centipede, _Lithobius elongatus_, from Tunis,
slightly reduced in size.]
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.
[Illustration: FIG. 14.--Shell of the Bell Barnacle, _Balanus
tintinnabulum_, one-half the natural size. The figure shows several
successive generations, perched one upon another.]
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).
[Illustration: FIG. 15.--Shells of a Barnacle, _Balanus hameri_, found
in European and North American Seas, natural size.]
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.
[Illustration: FIG. 16.--Hermit Crabs. _A_, _Aniculus typicus_, from
the Indo-Pacific Seas, one-half of the natural size. _B_, _Caternus
tibicen_, from the Indo Pacific Seas, slightly enlarged.]
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.
[Illustration: FIG. 17.--Land Crab, _Gecarcinus ruricola_, from the West
Indies, one-half of the natural size.]
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."
[Illustration: FIG. 18.--A Sand-hopper, _Pallasea Cancellus_, from
Siberia, natural size.]
[Illustration: FIG. 19.--A South American Spider, _Ctenus ferus_, from
the Amazon region, natural size.]
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.
[Illustration: 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
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.
[Illustration: 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 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.
[Illustration: 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 "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.
[Illustration: FIG. 23. A Scorpion, _Buthus Kochii_, from India.]
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.
[Illustration: FIG. 24.--Larvæ 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
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.
[Illustration: FIG. 25.--_A_, Larva of the Bee, _Apis mellifica_; _B_,
Section of Honeycomb.]
[Illustration: 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 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.
[Illustration: 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.
[Illustration: FIG. 28.--Cocoons of Moths. _A_, Compound Cocoon of
_Coenodomuc hockingi_, from India, one-half natural size; _B_, of a
Silkworm, _Bombyx Japonica_, 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 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).
[Illustration: FIG. 29.--A Moth, _Saturnia pyri_ (S. Europe), with its
Caterpillar, _A_; its Cocoon, _B_; Cocoon cut open to show Chrysalis,
_C_; Adult insect, _D_.]
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.
[Illustration: FIG. 30.--Nest of gregarious Caterpillar of a Moth,
_Hypsoides_.]
[Illustration: 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 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.
[Illustration: FIG. 32. Insect pests. _A_, Locust, _Acridium
peregrinum_, one-fourth natural size; _B_, Caterpillar of Wood Leopard
Moth, _Zeutzera Æsculi_, boring in wood, about one-thirtieth of natural
size; _C_, Colorado Beetle, one-fourth natural size; _D_, Leaf-rolling
Weevil of the Oak.]