ANIMALS WITH JOINTED FRAMES
The phylum Arthropoda embraces an immense assemblage of small animals,
inhabiting salt and fresh waters, the land, and the air above it.
The typical members of this group have a body divided into segments,
jointed limbs, some of which are modified into jaws, and a more or
less firm external skeleton. The general organization is complex,
with the nervous system and senses well developed, in some divisions
showing powers of perception and brainwork of a very high order. The
chief divisions, or classes, of the Arthropoda are given below in the
order of rank, from those simplest in organization to the most complex.
Members of the first three classes breathe by gills, and are termed
Branchiata, the remainder are air breathers or Tracheata.
_Crustacea_--Crabs, lobsters, shrimps, barnacles, beach fleas.
_Trilobita_--Trilobites; eurypterids (fossil only).
_Xiphosura_--Horseshoe crabs.
_Onychophora_--Peripatus.
_Myriapoda_--Centipedes; millipedes.
_Arachnoidea_--Spiders, mites, ticks, scorpions.
_Insecta_--Insects.
As several of these classes contain many subdivisions, and thousands
or even tens of thousands of species, all that is possible is to give
the reader such an account of each important group, as will enable him
to assign to their proper place such arthropods as he may encounter in
his rambles, or in his reading, and to learn something of the manner of
life in the various groups.
CRABS AND THEIR SMALL RELATIVES
"Everyone," says Dr. Calman, "has some acquaintance with the animals
that are grouped by naturalists under the name Crustacea. The edible
crabs, lobsters, prawns, and shrimps are at least superficially
familiar, either as brought to the table, or as displayed in the
fishmonger's.... Many, however, will be surprised that the barnacles
coating the rocks on the seashore, the sand hoppers of the beach, and
the wood lice of our gardens, are members of the same class. Still
less is it suspected that the living species of the group number many
thousands, presenting strange diversities of structure and habit, and
playing an important part in the general economy of nature."
The great majority of crustaceans are aquatic animals, breathing by
gills or by the general surface of the body, having two pairs of
"feelers," or antennæ, on the front part of the head, and at least
three pairs of jaws. Most crustaceans are hatched from eggs, usually
in a form very different from their parents; and they reach the adult
state only after passing through a series of transformations quite
as remarkable as those that a caterpillar undergoes in becoming a
butterfly. All crustaceans, except a few much modified land forms,
breathe by means of feathery or platelike gills which are always
an appendage of the legs, where they appear as one or more lobes.
Colorless blood propelled by the heart wanders into spaces in these
lobes, and there lies separated from the water by a mere film
of tissue, through which oxygen is absorbed from the water. Most
crustaceans are covered, at least in part, by some sort of shelly coat
composed of a combination of the horny substance "chitin" with lime,
which reaches its highest state in the big lobsters and crabs. This not
only protects and gives support to the internal organs, but also to the
muscles by which the animal moves. In other words it plays the part of
a skeleton. As it does not increase in size after it is once formed,
and cannot stretch much, the crab must cast its shell at intervals
as it grows. The new covering, which had been formed underneath the
old, before molting, is at first quite soft, and the animal rapidly
increases in size owing to the absorption of water. The shell then
gradually hardens by the deposition of lime salt.
The reader who may not hitherto have understood the difference between
"hard" and "soft-shelled" crabs is now instructed; and it is observable
that the figurative expression "a hard-shell," when applied to a man,
signifies that he must undergo a complete change before his ideas will
be enlarged.
The simplest of the crustaceans are those small creatures of the
subclass Branchiopoda (gill-footed) that swarm in our waters, both
salt and fresh. Lakes, ponds and ditches abound in a variety of minute
or even microscopic species that, in gathering food from equally
small bits of dead organic matter, as well as from living plants and
animalcules, perform an important service as scavengers--a service,
in fact, performed by all crustaceans in a greater degree than by any
other single group of animals. They also furnish the basis of food
for the whole body of aquatic life, since it is upon these minute
crustaceans that fish fry, tadpoles, insect larvæ, caddis flies, and
so on, must mainly depend. One of them is Daphnia, familiar to keepers
of aquariums. Another is Cyclops, a favorite with microscopists and
abundant in stagnant ponds, which is a member of the group called
copepods that form an important part of the oceanic plankton, where
they are the chief consumers of the minute algæ; but they also occur
at all depths. In arctic waters the copepods are so abundant that
they form the principal part of the food of certain fishes and of the
whalebone whales. These, and their minute relatives, the ostracods,
produce a large part of the phosphorescence of the sea, and some of
them exhibit bright colors.
All these are free swimmers, but nearly related to them are the
barnacles (Cirripedia) whose larvæ float about for a time near shore,
and then settle down and attach themselves by their hinder parts to a
rock or some other support, and begin to secrete an armature of limy
overlapping plates that forms a strong cup in which they sit, often in
a crowd that whitens a big rock. When the tide is low these sessile
"acorn shells" are tightly closed, but when the water returns, bringing
its load of invisible food, the animal stands up, as it were, and
thrusting out its feathery legs sweeps the water to capture a meal--a
beautiful sight to watch. The relation of the plates in the barnacle's
cup to those in the coat of the higher Crustacea is more easily seen in
the more pelagic "goose barnacle," whose hinder part is extended into a
tough, flexible stalk, while the fore part is covered by plates. This
kind is fond of attaching itself to floating timber, to ships' bottoms,
or even to the surface of whales, and thus floats or is carried
all over the watery globe. To it belongs the ridiculous myth of the
barnacle geese.
Great numbers of crustaceans of more advanced types live in the open
sea, and at all depths; and many of them assume extraordinary shapes.
The space between tide marks, and the mud of salt marshes and tidal
creeks abound in a wide variety of species, some of which are familiar
to everyone who lives at or visits the seashore. Thus the sand and rows
of drifted seaweed on all our eastern beaches are likely to harbor
flocks of amphipods, well called "sea fleas" or "sand hoppers," which
sometimes jump away before you in hundreds as you walk along.
Here, too, are to be found the pretty, burrowing "mole crabs," or
"ivory crabs," so called from their shining white jackets; and a host
of other species with strange forms and habits haunt the margins of
tropical and Oriental seas. All these are bandits, preying on whatever
they can catch, and between times guarding themselves from capture
by fishes, bigger crabs, and other enemies, by lying in mud burrows,
to the bottom of which they are quick to retreat. The big arm of the
fiddler crab, held across its face, closes its burrow like a door. One
sort, the hermit crab, has all its hinder parts naked, and so backs
into an empty snail shell, curling its taillike soft abdomen around the
central column of the shell and so dragging it about with it, with its
armored head and thorax sticking out of the mouth of the shell. As it
grows it becomes too large for its first shell, and from time to time
must leave it and find a larger tenement in which to ensconce itself--a
perilous transfer. Let me quote some notes I made on a New England
shore to give a picture of crustacean life there in summer.
"The lady crabs were plentiful, always alert, and inclined to be
pugnacious at our intrusion. The first one I met instantly rose upright
at the surface of the water, and when I made an advance it sprang
half way out of the water and cracked its pincer claws together as if
supposing it would reach, or at any rate frighten me. Perhaps it was my
shadow it clutched at so viciously. If so, the crab probably concluded
its huge antagonist to be an intangible ghost upon which the most
powerful claws could have no effect, for an instant later it _backed
down_--literally and swiftly--to the bottom, and in a twinkling had
wriggled tailwise into the mud and out of sight. When with my shovel
I routed madame out of that retreat, she indignantly scuttled off
too briskly to be followed, and will have great tales to tell of her
adventure.
"The stone and fiddler crabs were as common and comical as usual;
and I made the acquaintance of a new one called Gebia, which was a
small, semi-transparent, bluish white, washed-out, bloodless specimen,
shaped somewhat like a crawfish and carrying bunches of roe beneath
its abdomen. It looked like a miniature lobster made of glass and
filled with milk. Then in the eelgrass there was a funny isopod, called
Caprella. It was half an inch or so long, and clung by its hinder feet
to the grass, waving its body up and down in search of minute prey.
Other isopods and amphipods were exposed by turning over stones or
digging in the sand at the edge of the water--small, pale, shapeless
crustacea, which are flattened laterally so that they must lie on
their sides, and when uncovered will kick about with feet and tail in
laughable anxiety to get under something. Under the stones we found
the tubes made by a certain species; and when we captured the active
little architect and put him in a bucket of clean water, he instantly
began to gather grains of sand and stone and to join them together Into
a shield under which he might hide. We found that these grains were
joined together by spiderlike threads, which the amphipod spins from
two pairs of small legs under the middle of his body, secreting a fluid
that hardens in the water. Another (Hippa) about the size and shape of
a robin's egg, but with a thin shell of mother-of-pearl (so to speak),
gave us great amusement by its extraordinary celerity in burrowing, so
that we could hardly seize it before it had squirmed down out of reach
into the wet sand."
The edible crabs (Cancer) live in the shallow region just below ebb
tide, for they cannot endure exposure to air as well as other species,
and live by scavenging. The lobsters are inhabitants of still deeper
water, especially where it is somewhat rocky, and devour more carrion
than living fish. That miniature of the lobster, the fresh-water
crawfish, which is also edible, dwells in deep burrows in wet
lands--burrows that are really wells half filled with water. Various
species of these and other edible forms of Crustacea are found all over
the world.
MILLIPEDES AND CENTIPEDES
The myriapods (class Myriapoda) are those unpleasant creatures more
commonly known as centipedes, millipedes, or thousand-legged worms.
They have a wormlike form, with the body divided into segments, a
distinct head with antennæ, jaws and several single eyes, and a
varying number of air tubes, or tracheæ; two sexes exist, and eggs are
laid in the ground within cases formed by the mother of pellets of mud.
They vary in size from an almost invisible minuteness to a length in
some tropical species of six or more inches. The centipedes (Chilopoda)
are those flattened forms so often seen in and about rotting wood and
vegetation or in moist ground, their bodies looking like a chain of
plates joined together by flexible skin, each section having a single
pair of legs, usually very short, but in one sort (Cermatia) each leg
is longer than the body, and the hinder pair twice as long, matched by
two very long feelers. Most of them are predacious, feeding on anything
they can catch, and their strong jaws exude poison. The larger ones may
inflict a very painful bite if incautiously handled.
[Illustration: GIANT CENTIPEDE
(_Scolopendra gigas_)]
Another group, the Diploda, are known as galley worms, or millipedes,
and have two pairs of bristlelike legs on each segment. Here the
body is as round as that of an earthworm, and is incased in a hard,
chitinous shell, usually red-brown in color; and when disturbed they
coil up and emit an acrid, unpleasant odor as a defense.
WEAVERS OF SILKEN TRAPS
The class Arachnida, which contains the scorpions, spiders, mites
and their allies, connects the Crustacea with the Insects; and
some naturalists include within it the eurypterids and king crabs,
classified in this book with the Crustacea. All live on land and
breathe air except a small group of allies (Pycnogonida) which are
marine, and may be found on the rocks, and clinging to wharf piles,
etc., on our coasts as well as elsewhere; they appear to be all legs,
and are known to New England fishermen as "no-body crabs." The class
includes seven orders, the lowest in rank of which is that of the
scorpions (Scorpionida).
Scorpions are inhabitants of warm countries, and some tropical American
species are six inches in length, but those of our Southern States
are smaller. They have slender bodies consisting of a cephalothorax
and a long abdomen ending in a sharp sting through which two poison
glands inject poison into the wound made by it, the effect of which
may be very severe on a man, and is fatal to the insects and other
small creatures on which scorpions prey; this "tail" with the sting
is usually carried curled up over the back. The body is protected by
chitinous plates above and below. The legs are four. From the head
spring two great, crablike, pincer claws. When these seize an insect
they hand it back to two small but powerful appendages at their base
which act as jaws. Between them is a small mouth. Scorpions are
nocturnal in habit, hiding by day in crevices, and wandering about at
night; thus they are likely to seek such dark retreats toward morning
as a person's boots; and in hot, dry regions travelers must be cautious
about examining their clothing and baggage to avoid being stung. The
scorpions retain their eggs until hatched. The young when born differ
little except in size from their parents, and are cared for with much
solicitude by the mother, who carries them around with her for some
time, hanging by their pincers to her body. The race is ancient, fossil
remains occurring as early, at least, as the Carboniferous age.
The second order, Pseudoscorpionida, includes the "book scorpions,"
a series of minute, stingless, scorpion-shaped creatures found in
moss, under the bark of trees, or more often on flies. A third order,
Pedipalpida, is that of the scorpion spiders, or "whip scorpions" of
the tropics; the fourth, Solpugida, contains certain ugly creatures
intermediate between scorpions and spiders; and the fifth order,
Phalangida, is that of the small-bodied, vastly long-legged things
called "harvestmen" in England and daddy longlegs by us, which run
about in the summer heat, and feed on minute insects. They abound in
all the warmer parts of the world, and in great variety, South America
showing some very bizarre forms. This brings us to the sixth order,
Araneida--the spiders.
THE SPIDERS AND THEIR WEBS
Spiders are usually thought and spoken of as "insects" by the layman.
Many persons call almost every creature an insect that is small and
supposed to be useless, or suspected of harmfulness. But spiders
are different from insects properly so called in many important
particulars of structure and habits. Spiders have four pairs of legs,
while insects have six legs. The spherical abdomen, which is cut off
from the head by a deep constriction, shows no segmentation, and on its
floor are large glands (the arachnidium) producing the silk which is
exuded from three pairs of tubes with sievelike openings, at the end
of the abdomen, called the spinnerets. Their nervous system is highly
developed, and they show much intelligence. Spiders are of two sexes,
but the male is usually much smaller than his mate.
When egg-laying time comes the female forms a little silken bed
attached to grass, or underneath a stone, or stuck to some object,
or placed in a burrow, or hung like a hammock by long guy lines, and
deposits in it eggs like drops of jelly. One sort places this under
water, forming a nest like an inverted cup and filling it with bubbles
of air, and spending much of its time in this real diving bell. A
common garden spider (Lycosa) forms globular cocoons, and drags them
around attached to the spinnerets, regardless of jars and bumps. In
a large section of the tribe this is all the use that is made of the
silk, which differs from that of insects (caterpillars) in being made
up of a great number of finer threads laid together while soft enough
to unite into one.
It is a common habit with spiders to draw out a thread behind them as
they walk, and in this way they make the great quantities of threads
that sometimes cover a field of grass. This is the gossamer often so
annoying to us in late summer, but a thing of beauty when glistening
with dew.
The gossamer of autumn, however, is made by the very small spiders of
the genus Erigones, which hide in the herbage, but in the fine weather
that comes after the first frosts climb to the tops of posts, fences
and tall weeds, in company with the young of larger kinds, and "turning
their spinnerets upward allow threads to be drawn out by ascending
currents of air, until sometimes the spiders are lifted off their feet
and carried long distances." These are the "ballooning spiders" of
which one hears. In this way the whole country is overspread with lines
and tangles of trailing silken threads that whiten our clothes and
stick to our faces.
Three or four hundred species of spiders might be obtained in almost
any locality in this country by diligent search, and thousands of
foreign species are known; hence only a few conspicuous examples may be
mentioned here. The tribe may be divided according to habits into two
groups of families: 1. The hunting spiders, which run on the ground or
on plants, catching insects by chase or by strategy; and 2. The cobweb
spiders, which make webs to catch insects, and live all the time in the
web or in a nest near it.
In the former group are the Drassidæ, a family of small, varicolored
spiders that run about on the ground or in bushes, one large genus of
which (Clubiana) includes pale, or purely white species; their cocoons
are baglike or tubular. The most conspicuous genus is Misumena, in
which the species are white or brightly colored, and which spend their
days among flowers, waiting in rigid attitude for an insect to alight
near them on which they may pounce. Spiders can see well for four or
five inches, but not much beyond that. The Attidæ are small, hairy, or
scaly jumping spiders, often brightly colored, that are found in open
places and on the tops of low plants, whence they leap on their prey,
or make long jumps to escape danger. To the next family, Lycosidæ,
belong the large spiders most often seen in fields and pastures. They
are fond of dry, sandy places, where the females live in silk-lined
holes. These lycosids are long-legged, rapid runners, and capture
their game by running it down. To this family belongs the famous
tarantula of southern Europe, fabled to produce a madness (tarantism)
in a person bitten that could be cured only by dancing to music of a
certain lively measure called "tarantella." (The so-called "tarantula"
of our southwestern desert region, is, however, another species.) A
common northern spider (_Lycosa carolinensis_) is its equal in size,
(the longest legs covering a spread of three inches), and in color,
black with gray legs. Still larger is another North American lycosid
(_Dolomedes tenebrosus_), gray with spiny legs ringed with dark and
light gray, which spreads four inches.
These big ugly creatures, and the bites of spiders generally, are
regarded with unnecessary dread by most persons. The jaws (mandibles)
are close together at the front of the head. They are two-jointed, the
basal joint stout, and the end joint or claw slender and sharp-pointed.
The claw has near its point a small hole, which is the outlet of
the poison gland. "The poison kills or disables the insects which
are captured by the spider. Its effect on the human skin varies in
different persons. Sometimes it has no effect at all; oftener it causes
some soreness and itching ... and cases have been known in which it
caused serious inflammation which lasted a long time. Spiders seldom
bite and only in self-defense, the bites so commonly charged to them
being often the work of other animals."
In the family Agalenidæ we meet with the first of the web makers. These
are spiders of moderate size, characterized by a big head marked off
from the thorax by converging grooves. Their natural home is in the
grass, where their flat, closely woven sheets of silk, almost invisible
by reason of their transparency, but brought into plain view when
coated with dew or dust, are spread everywhere. They also are fond of
getting into cellars and old buildings, and constructing webs across
corners, bracketwise. Somewhere the web sinks like a narrow funnel into
a short tube in which the owner hides, watching hungrily until a fly
alights on his silken platform.
"The Therididæ," says Emerton, "are the builders of the loose and
apparently irregular webs in the upper corners of rooms, in fences
and among rocks, and between the leaves and branches of low trees and
bushes. They are generally small, soft and light-colored spiders, with
the abdomen large and round and the legs slender and usually without
spines.... Most of the Therididæ live always in their webs, hanging by
their feet, back downward. The webs have in some part a more closely
woven space under which the spider stands." These spiders are quick
to avail themselves of any chance to spin their shapeless meshes of
almost invisible silk, which few regard as real "webs," in closets,
cellars, and all over the house or barn. Many of them are adorned with
gay colors or striking patterns, and some are much feared, especially
_Latrodectus mactans_, about half an inch long, which is black with
scarlet spots. It is common from Canada to Chile, and everywhere is
considered fatally poisonous--why, it is difficult to say.
Last of our list, and highest in rank, are the Epeiridæ, the "orb
weavers," as they are often called, who make those regular spiral nets
which are in our mind's eye when we think of cobwebs. Most of the
moderately large and handsome house and garden spiders are of this
family, and everyone can easily examine their work, although it is less
easy to watch them at it, as the webs are built and repaired at night.
Among the obscurer and foreign species the abdomen often shows humps,
points and long forward-reaching horns that make them exceedingly
grotesque, and doubtless difficult to handle by birds and other
creatures that seize them as food.
One of the round webs of the Epeiridæ consists of several radiating
lines, varying in different species from a dozen to seventy, crossed
by two spirals--an inner spiral that begins in the center and winds
outward, and an outer spiral that begins at the edge of the web and
winds inward. The inner spiral is made of smooth thread, like that of
the rays, to which dust will not cling; the outer spiral is made of
more elastic thread which, when fresh, is covered with fine drops of
sticky liquid.
"In beginning a web, after the radiating threads are finished, the
spider fastens them more firmly at the center and corrects the
distances between them by [inserting] several short, irregular
threads, and then begins the inner spiral, with the turns at first
close together and then widening ... until they are as far apart as
the spider can reach with the spinnerets [resting] on one and the
front feet on the next, and so goes on nearly to the outside of the
web, where it stops abruptly. The spider usually rests a moment, and
then begins, sometimes at another part of the web, the outer sticky
spiral.... As soon as the inner spiral is found in the way a part of it
is cut out, and by the time the outer spiral is finished the inner is
reduced to the small and close portion near the center.... The whole
making of the web seems to be done entirely by feeling, and is done as
well in the dark as in daylight. When the spider is active and the food
supply good, a fresh web is made every day, the old one being torn down
and thrown away."
[Illustration: AMERICAN GARDEN SPIDER
(_Epeira vulgaris_)]
As a rule these orb weavers do not stay in the web in the daytime, but
hide away in their nests made in some near-by but concealed place; and
their egg cocoons are hidden in all sorts of places.
All of the spiders that have been considered so far belong to the
division of the class that has but a single pair of lungs. A second
division has been made for those having two pairs of lungs, composed
of a single family, the Mygalidæ, consisting of the so-called
"bird-catching" spiders and the trapdoor spiders. The great mygale
of Guiana has a body sometimes two inches long, and its legs will
span eight or nine inches of space. It is hairy all over, intensely
black, and a terror to all small creatures, even catching small birds,
according to tradition; but proof of this is wanting.
The trapdoor spiders are those of the genera Cteniza and Atypus which
dig and inhabit vertical holes in the soil, lined with silk and closed
at the top by a hinged stopper or "trapdoor." Several species occur in
southern Europe, one of which has a second door hanging by a silken
hinge half way down the shaft; and in case of trouble the spider goes
below it and pushes it above its head, so that the intruder is deceived
into thinking it has opened an empty nest. _Cteniza californica_ is the
common species of our Southwest. The cover of the hole is made of dirt
fastened together with threads, and is lined, like the tube, with silk,
and fastened by a thick hinge of silk. The spider holds the door shut
from inside. These underground homes are safe retreats for the spiders
during the day, and nesting places in which their eggs are deposited
and young reared; at night the spiders go forth in search of prey.
MITES AND TICKS
Mites and ticks are classified with the spiders as degenerate relatives
of arachnoid stock. Ticks are large enough to be seen without a
magnifying glass, and some become half an inch long. Ticks are wholly
parasitic. The female lays several thousand eggs at one time on the
ground or just beneath the surface. "The young 'seed ticks' that hatch
from these in a few days soon crawl up on some near-by blade of grass,
or on a bush or shrub, and wait quietly until some animal comes along.
If the animal comes close enough they leave the grass or other support
and cling to their new-found host." These parasites are the agents of
the spread of several infectious diseases of cattle, the worst of which
is the destructive Texas fever, and of mankind, as spotted fever and
other ills resulting from the presence of blood parasites.