WHAT IS A SPIDER?
Not many years ago the group Insecta was held even by Zoologists
to include numberless small creatures—centipedes, spiders, mites,
etc.—which further study has shown to present essential differences
of structure, and in popular language any fairly minute animal is
still an insect, just as any insect is popularly a “fly”—or, in the
United States, a “bug.” Scientifically the use of the term Insect is
now much restricted, though still extensive enough in all conscience,
since it includes many more than a quarter of a million known species.
Zoologists recognise a large group of animals characterised by having
no internal skeleton but a more or less firm external coating of a
peculiar substance called _chitin_, often strengthened by calcareous
deposits, which necessitates the presence of joints in their bodies,
and especially in their limbs if they are to move freely, just as
medieval suits of armour required to be jointed. These are the
Arthropoda. One subdivision of this group consists of aquatic animals,
breathing by gills, and known as Crustacea. Crabs, lobsters, shrimps
and “water-fleas” are familiar examples, and with the exception of the
so-called land-crabs the only Crustaceans habitually found on land are
wood-lice.
The other Arthropoda are air-breathing, and since their characteristic
breathing organs are branching tubes known as _tracheae_, the term
Tracheata is sometimes used to include them all. They fall naturally
into three divisions, the Myriapoda, the Insecta and the Arachnida, and
it is in this last-named division that we shall find the spiders.
The Myriapoda are the centipedes and millipedes, and having said this
we may dismiss them, for insects and arachnids are strictly limited as
to legs; and no myriapod can ever be mistaken for a spider.
The Arachnida are so varied in structure that it is not easy to give
characteristics common to them all, and to any general statement there
are bound to be exceptions, but for practical purposes it may be said
that while an insect, when mature, has only six legs, and a pair of
feelers or antennae of quite different structure, Arachnids have
normally eight legs, and their feeling organs are not antennae but
leg-like “pedipalps.”
Most insects are distinguishable at once by the possession of wings,
which are never found among the Arachnida, and they generally undergo
a marked transformation or metamorphosis in their progress from the
egg to maturity, taking on at first the form of a caterpillar or grub
and then that of a chrysalis; but as there are many wingless insects
and many in which the metamorphosis is very slight, the test supplied
by these characteristics is only of partial application, and we shall
do better to rely on the number of legs, and the nature of the feeling
organs. If, therefore, we find a small wingless animal with eight legs
and a pair of feelers which are not thread-like but much of the same
character as the legs, though not used for locomotion, we may be sure
that we are concerned with an Arachnid.
But is it a spider?
Now some groups of the Arachnida may be put out of court at once
as having an appearance so characteristic that no confusion is
possible. Such are the Scorpions, and the minute Chernetidea or “False
Scorpions,” but this cannot be said of the Phalangidea or “harvestmen”
or of the Acarina or “Mites,” members of which groups not only may be,
but frequently are popularly taken for spiders. In fact the Phalangidea
are very commonly spoken of as “harvest spiders” and the “red spider”
is a mite. A very brief inspection, however, with a pocket lens will
settle the matter without the least difficulty.
A spider’s body consists of two parts, a cephalothorax (head + thorax)
and an abdomen. There is a waist, but no neck. The eight legs are
attached to the cephalothorax, and the abdomen is not segmented or
ringed like that of an insect, but entire, and bears at its extremity
or on its under surface a little group of spinnerets or finger-like
projections from which the spider’s silk proceeds. For the moment these
three characteristics will suffice—the “waist” behind the leg-bearing
portion of the body, the unsegmented, legless, abdomen, and the
spinnerets (fig. 1 _B_). A harvestman, for instance, lacks the waist,
and its abdomen is segmented. Mites are of very varied form and in
some the body is more or less divided into two portions, but at least
two pairs of legs will be found to be attached to the hinder portion;
and neither harvestmen nor mites possess the spinnerets which are
the most striking characteristic of the spider; some mites—like the
“red spider”—can spin, but the mechanism by which that operation is
performed is of quite a different nature.
[Illustration: Fig. 1. _A_, a Mite; _B_, a Spider; _C_, a Phalangid.]
Having, then, very readily determined our specimen to be a true spider,
we may as well use it to note some further structural points the
detailed examination of which may be deferred till we have considered
their functions. Note the jaws or _chelicerae_, consisting of a stout
basal part and a fang which, when not in use, is shut down like the
blade of a knife; note the pedipalps or feelers, exactly like small
legs, but showing by their action that their function is sensory and
not locomotor. If they are knobbed at the end, the specimen is a male,
otherwise it is a female or as yet immature. Look closely at the front
part of the cephalothorax, and several eyes will be visible—probably
eight. They are not compound—divided into innumerable facets, like
those of insects—but simple and smooth, though to make sure of this
the use of a microscope would be necessary. Finally, obtain a view of
the under surface of the abdomen, and note in front, on either side of
the middle line, two semilunar patches of a lighter colour. These are
the “lung-books,”—special breathing organs peculiar to these animals;
two is the usual number, though certain spiders possess a second pair
behind the first.
But the spinning mammillae or spinnerets are still more characteristic
and more easily seen, though, curiously enough, it is not among the
cleverest spinners that they are most conspicuous. In the family
to which most of the cellar spiders belong (Agelenidae) and in the
elongate brown or mouse-coloured spiders found lurking under stones
(Drassidae) they are visible as little finger-like projections at the
posterior end of the abdomen, but if we have taken our specimen from a
circular web (Epeiridae) we shall have to look for them more closely.
In these spiders they are beneath the abdomen near its termination,
and are not visible from above. Moreover when at rest their tips are
applied together so that they form a small rosette in surface-view, or,
in profile, a slight cone.
The best way to capture a spider for examination is to induce it to
run up into a small glass specimen tube—for spiders readily part with
their legs if handled roughly—and if we have adopted this method we
shall see the spinnerets in use as the animal crawls about the tube. It
will not move without first attaching a silken cable to the glass, and
this cable lengthens as the spider progresses, so that before long the
interior of the tube will be a network of silken threads, and its sides
will be flecked with little white specks where the threads have been
re-attached for a new departure; and by observing closely we shall be
able to note the extreme mobility of the spinnerets in action.
All spiders spin, but it is by no means all spiders that make snares
for the purpose of catching prey. The fundamental purpose of the
spinning organs seems to be to connect the spider with its point of
departure. The jumping spiders (Attidae) make no snare, but this
“drag-line” as it has been called comes in very useful when stalking
prey on the vertical surface of a wall, when a miscalculation at the
moment of pouncing upon it would entail a considerable fall were it not
for such an anchorage. It can hardly be doubted—though of course it is
incapable of proof—that all the more complicated spinning operations
originated in this universal spider habit, but all known spiders have
learnt to apply their power of making silk to other purposes. If they
do not make snares they at least spin “cocoons” for the protection of
their eggs, and if they have a definite home from which they emerge
to seek food, such a retreat is always more or less lined with silk.
It is clear that a spider cocoon is quite different from that of an
insect; it encloses the eggs and is manufactured by the mother, whereas
among the insects the larva makes the cocoon for the protection of the
pupa or chrysalis into which it is about to turn. However far from
exhaustive the foregoing study of spider structure may be it will
suffice for our purposes, at least for the present, and we may proceed
at once to an investigation of one of the most remarkable achievements
in the way of spinning—the familiar circular snare or wheel-web of the
garden spider.