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.

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.