1911 Encyclopædia Britannica/Spiders

From Wikisource
Jump to navigation Jump to search
4078621911 Encyclopædia Britannica, Volume 25 — SpidersReginald Innes Pocock

SPIDERS, the common English name of Arachnida (q.v.) of the order Araneae, resembling the Pedipalpi in many structural points, but differing from them as well as from all other Arachnida in retaining short abdominal appendages known from their silk-manipulating function as spinnerets or spinning mamillae, with which are associated silk glands. It is probably owing to the possession of such glands and the varied purposes for which the silk is used that spiders as a group far surpass the other orders of Arachnida, with the possible exception of the Acari (mites and ticks), in diversity of form and of size, in numbers of genera and species, in extent of geographical distribution, and in adaptation to varied habitats. Except in the extreme north and south, and on the tops of the highest mountains, where there is no insect life as food supply, spiders are found all over the world, even in isolated oceanic islands. They occur up mountain slopes as far as vegetation extends, in tropical valleys and forests, in open grassy plains, in sandy deserts, and even in fresh-water ponds and between tide-marks on the seashore. Some are nocturnal, some diurnal; some catch their prey by speed of foot, some by cunningly lying hid, some by means of silken nets. The phenomena known as “protective resemblance,” or similarity to inanimate objects or vegetation, and the kindred phenomenon of “mimicry,” or beneficial likeness to certain protected species of animals, are common in the group. In these particulars, considered in their entirety, spiders show a marked contrast to other Arachnida, such as the scorpions, pedipalps, book-scorpions and so-called harvest spiders, which by comparison are remarkably uniform, within the limits of the orders, in structure, habits and other respects. Spiders, in short, must be regarded as the most highly organized and the most successful members of the class Arachnida.

Their success in the struggle for existence, as already indicated, must be assigned in a great measure to the possession of silk glands and to their power of manipulating the silk for a variety of purposes. Several facts point to the conclusion that the primary use of this secretion was the formation of egg-cases or cocoons by the female, for this is the only constant use for which the silk is employed, without exception, by all species. The second step in the evolution of spinning instincts was probably the making of a silken chamber for the reception of the cocoon itself and for the protection of the mother while guarding it and her newly-hatched young. If an aperture for ingress and egress, for purposes of feeding, were left in the wall of such a chamber, there would arise in a rudimentary form what is known as the tubular nest or web; and the next important step was possibly the adoption of such a nest as a permanent abode for the spider. Some spiders, like the Drassidae and Salticidae, have not advanced beyond this stage in architectural industry; but next to the cocoon this simple tubular retreat—whether spun in a crevice or burrow or simply attached to the lower side of a stone—is the most constant feature to be observed in the spinning habits of spiders. From this starting-point the evolution of web-making seems to have proceeded along two main divergent lines. Along one line there was a gradual elaboration of the tube until it culminated, so far as structural complexity is concerned, in the so-called trap-door nests or burrows of various families; along the other line the tubular retreat either retains its primitive simplicity in association with a new structure, the snare or net, or is entirely superseded by the latter.

Trap-door nests are made by spiders belonging to two widely different groups, namely the Lycosidae or wolf-spiders, to which the true tarantula (q.v.) belongs, and the Mygalomorphae, containing the species which construct the best-known types of this style of burrow. Although there is no direct genetic affinity between the spiders of these two groups, an interesting parallelism in their habits may be traced. In both there are species which form no nest or burrow, others which construct a simple silk-lined tunnel in the soil, and others which close the aperture of the burrow with a hinged door; while both share the habit of lining the burrow with silk to prevent the infall of loose sand or mould; and the species which make an open burrow close the aperture with a sheet of silk in the winter during hibernation and open it again in the spring. Possibly from this habit was developed the instinct to build a door with a movable hinge. In the trap-door species of Lycosidae, like, for instance, Lycosa opifex of the Russian steppes, the hinge is weak and the lid of the burrow is kept normally shut by being very much thicker and heavier at its free margin opposite the hinge so that it readily falls by its own weight. In the burrows made by the Mygalomorphae, on the contrary, the hinge is strong and highly elastic, its component silken threads being laid on in such a way that the door shuts with a snap when the occupant has passed in or out. The lid is sometimes thin and wafer-like as in the burrow of the species of Nemesia, sometimes thick and cord-like as in that of the species of Cteniza or Pachylomerus. Its upper side is always covered by the spider with pieces of the vegetation growing hard by, so that, when the door is closed, the position of the burrow is completely concealed. If an attempt be made by any enemy to lift the lid, the spider seizes its inner side with his fangs and striking his claws into the walls of the burrow offers the greatest possible resistance to the efforts of the intruder. When on the watch for prey the spider slightly raises the lid and, peeping through the chink, darts like a flash upon any beetle or fly that unwittingly passes within reach. Quite commonly the burrow has a second passage running obliquely upwards from the main passage to the surface of the soil, and this subsidiary track may itself be shut off from the main branch by an inner door, so that when an enemy has forced an entrance through the main door, the spider retreats behind the second, leaving the intruder to explore the seemingly empty burrow.

There is no doubt that the primary influence that has guided the evolution of the architecture of the burrowing spiders has been that great necessity for the preservation of life, avoidance of enemies and protection from adverse physical conditions like rain, cold or drought. And when we turn to the other line along which the web-building instinct has been developed we find that the primary guiding influence has been that second great vital necessity, namely the necessity of getting food. Reference has already been made to the silken tube or tent, of simple structure, with an orifice at one or both ends, as the possible origin of all snares, however complex they may be. Perhaps the most rudimentary form of snare arose from the spinning of threads round the mouth of the tube to hold it in place. Be that as it may, the snare in many instances, as in that of the Agalenidae (Tegenaria, Agalena), a family closely allied to the Lycosidae, is a horizontal sheet of webbing, upon which the spider runs, continuous with the lower half of the aperture of the tube, of which it is simply an extension. A very similar sheet is spun by a species of Linyphia, one of the Argyopidae, but in this case there is no tube connected with the web and the spider hangs suspended beneath the horizontal netting. Snares of another type consisting of a tangled mass of threads amongst which the spiders pick their way with ease, but which are impassable to insects, are spun by members of the Theridiidae and Pholcidae; but by common consent the so-called orbicular web, so characteristic of the Argyopidae but by no means confined to them, is regarded as manifesting the greatest perfection of instinct in snare-spinning. These webs, which are typically subcircular in form, consist of a system of threads radiating from a common centre and crossed at intervals, and approximately at right angles, by a series of concentric lines, the whole being suspended in a triangular, quadrangular or polygonal framework formed of so-called foundation lines, attached to the branches or leaves of trees or other firm objects in the neighbourhood. Passing back from the centre of the web to the underside of an adjoining leaf or some other sheltered spot runs a single thread, the trap line affording passage to the spider to and from the sheltered spot and the snare itself. At whatever spot an insect becomes entangled in the frame, the vibration set up by its struggles is transmitted along the nearest radiating thread to the centre and thence up the trap line to the shelter where the occupant lurks awaiting the signal. No sooner is the vibration perceived than the spider descends with all speed to the centre, and by feeling the ends of the radiating lines learns which is ashake and rapidly, without the possibility of mistake, makes its way to the entangled insect. The probable reason for the wall-lines being concentric is that lines passing over the radii as nearly as possible at right angles are the shortest that can be laid on; they therefore use up a smaller quantity of silk and take a shorter time to spin than threads crossing the radii in any other direction; and at the same time they afford them the greatest possible support compatible with delicacy and strength of construction. On account of its delicacy no web is more difficult to see than one of the orbicular type above described. Its whereabouts is thus, to a great extent, concealed both from enemies searching for spiders and from insects suitable for food; and its open meshwork of strong threads makes it much less liable to be beaten down by rain or torn to shreds by winds than if it were a flat sheet of closely woven silk. In constructing, therefore, a snare of radiating and concentric lines, it seems that a spider economizes both time and silk and in addition renders the web as strong and as serviceable and yet as delicate and invisible as possible.

Perfect orbicular webs are made by many genera of Argyopidae (Zilla, Meta, Gasteracantha) , the best-known example being that of the common garden spider of England, Aranea or Epeira diademata; but these webs are not associated with any tubular retreat except such as are made under an adjoining leaf or in some nook hard by. Some tropical members of the family belonging to the genus Nephela, however, spin a web which is intermediate in structure between that of Aranea and the complete sheet-like web of Agalena. It covers an area of about one-third of a circle and its radiating threads diverge from the mouth of a funnel-shaped tube resembling in every respect the tube of the last-mentioned genus. Again some species of Dictyna, belonging to the Amaurobiidae, also have a tubular retreat opening on to the surface of a snare in which a crude attempt at a radial and concentric arrangement of the threads is perceptible. The interest of these two types of web lies in the fact that they bridge over the structural gap between the simple sheet-web of Agalena and the perfected orb-web of Aranea.

Dictyna may be cited as an example of a group of spiders, sometimes called the Cribellata, which have certain spinning glands and appliances not possessed by others. These glands are represented externally by a special plate, the cribellum, which lies in front of the ordinary spinning mamillae, and by a comb of short bristles, the calamistrum, placed in the penultimate segment of the left of the last pair. By means of the calamistrum the silk secreted by the cribellum is teased into a fine thread which is twisted round the main threads of the web, giving it a very characteristic woolly or flocculent appearance.

There are many other uses to which silk is put, besides those mentioned above. By trailing a thread behind them spiders are able to drop from any height to the ground and to retrace their steps with certainty to a particular spot. The possession of silk-glands has also profoundly influenced the geographical distribution of spiders and has enabled them to cross arms of the sea and establish themselves on isolated oceanic islands which most of the orders of Arachnida are unable to reach. This is effected by the so-called habit of “ballooning” practised by very young spiders, which float through the air, often at great altitudes, in the direction of the prevalent winds. It was formerly supposed that this custom was peculiar to a single species, which was called the “gossamer” spider from the fact that the floating webs, when brought to the earth by rain or intercepted by bushes and trees, coat the foliage or grass with a sheeting of gossamer-like silk; but the habit is now known to be practised by the newly-hatched young of a great variety of species belonging to several distinct families.

As a commercial product spider-silk has been found to be equal, if not superior, to the best silk spun by lepidopterous larvae; but the cannibalistic propensities of spiders, making it impossible to keep more than one in a single receptacle, coupled with the difficulty of getting them to spin freely in a confined space, have hitherto prevented the silk being used on any

extensive scale for textile fabrics.

The methods of catching prey adopted by spiders are extremely varied. The nets or snares are highly efficient for this purpose. Amongst the threads, which entangle the wings and legs of intercepted prey, the spiders are perfectly at home and can pounce on the struggling victim at once if it be small and harmless or keep at a respectful distance, checking all efforts at escape, if it be poisonous or strong. If in the latter case the spider be afraid to come to close quarters, various devices for securing it are resorted to. The Theridiidae eject on to the insect from their spinning mamillae drops of liquid adhesive silk; the Argyopidae, steadying it with the tips of their long front legs, sweep additional strands of silk over it with the legs of the hinder pair; the Agolenidae, attaching a long thread to a point hard by, run round and round the victim in circles, gradually winding it up beyond all hope of breaking loose. Two genera of Argyopidae (Hyptiotes and Theridiosoma) construct spring-nets out of their incomplete webs of the orbicular type. To the web is attached a trap-line which when drawn taut holds the snare stretched and tight, and when relaxed loosens the whole structure so that the threads fall together. When an insect strikes the web the spider loosens his hold of the trap-line, thus enveloping the victim in a tangle of threads which would otherwise not come into contact with it. Spiders which spin no snare are dependent for capturing prey for the most part upon their quickness or powers of lying concealed. Many Thomisidae lurk amongst the stamens and petals of flowers, which they closely match in colour, waiting to seize the insects which visit the blossoms for nectar. Examples of Selenops (Clubionidae) lie flat and absolutely still on the bark of trees, to which their coloration assimilates, and spring like a flash of light upon any insect that touches their legs; the Lycosidae dart swiftly upon their prey; and the Salticidae, which compared with other spiders have keen powers of vision, stealthily stalk it to within leaping distance, then, gathering their legs together, cover the intervening space with a spring and with unerring aim seize it and bury their fangs in its body. One genus of Thomisidae (Phognorachne), which inhabits the Oriental region, adopts the clever device of spinning on the surface of a leaf a sheet of web resembling the fluid portions of a splash of bird's dung, the more solid central portions being represented by the spider itself, which waits in the middle of the patch to seize the butterflies or other insects that habitually feed on birds' excrement and are attracted to the patch mistaking it for their natural food.

The sexes of spiders are distinct. Except in the case of the water-spider (Argyroneta) the males are smaller, sometimes very much smaller, than the females, but have proportionately longer legs and smaller bodies. When adult the males may always be distinguished from the females by the presence of a pair of horny intromittent organs, one of which is lodged in the terminal segment of each palpus or appendage of the second pair. In its simplest form this is a hollow flask-shaped horny piece, consisting of a dilated basal portion and a terminal spiniform portion with an orifice at the apex; but its structure is frequently complicated by accessory processes and outgrowths which aid copulation and serve to protect the delicate point from injury. In the breeding season the male deposits drops of sperm on a sheet of webbing, picks it up in these flasks by means of capillary attraction and carries it about until he falls in with a female. During pairing he thrusts the tip of these organs into the seminal vesicles of the female and the eggs are fertilized as they pass out of the oviduct. Cases of parthenogenetic reproduction, or reproduction without the intervention of the male, have been recorded in the case of two genera (Filistata and Tegenaria), and may be commoner than is usually supposed. All spiders are oviparous. The number of eggs produced at a time varies enormously according to the species, from about half a dozen, more or less, in some ant-mimicking Attidae or jumping spiders to many hundreds in the larger orbicular-webbed spiders of the family Argyopidae. The first act of the female after oviposition is to wrap her eggs in a casing of silk commonly called the cocoon. The cocoon varies greatly in size, shape and consistency according to the nature of the spider that makes it. Sometimes, as in Pholcus, it is merely a thin network of silk just sufficient to hold the eggs together. More often it consists of a thick felting of silk, either spun in one continuous piece into a globular form, as in the Aviculariidae, or composed of two plate-like pieces, an upper and a lower, united at the edges and lenticular in shape, as in some of the Lycosidae. Sometimes it is woolly and flocculent, sometimes smooth like parchment, and its shape depends in a large measure upon the habits of the female towards her offspring. As a rule terrestrial spiders guard the cocoon in the permanent burrow, as in the trap-door spiders, or in the silken retreat which acts as a temporary nursery, as in the Salticidae. Other species of wandering habits carry the cocoon about with them, sometimes attached to the spinnerets, as in the Lycosidae, sometimes tucked under the thorax, as in the large tropical house-spider, Heteropoda regia, one of the Clubionidae. The females of some snare-spinning species, like the Pholcidae, carry it in their jaws; but in the case of the Argyopidae the females usually leave the cocoon to its fate as soon as it is constructed, sometimes rolling it in a leaf, sometimes attaching it by a stalk to a branch. It is in this and related families that the greatest diversity in the colour and form of the cocoon is found. In these spiders, too, the newly-hatched young shift for themselves as soon as they emerge from the cocoon; in others that guard the cocoon the young stay for a longer or shorter time under their mother's protection, those of the wandering Lycosidae climbing on her back to be carried about with her wherever she goes. There is no metamorphosis during growth such as occurs in some insects, the young being hatched with its full complement of appendages and only differing from its parents in characters of comparatively minor importance. Growth is accompanied by a succession of moults, the spider emerging from its old skins by means of a fracture which extends along the front and sides of the cephalo-thorax just beneath the edge of the carapace. It is only at the final moult that the sexual organs are mature, the two sexes being alike in the earlier stages of growth. Until maturity is reached the spider has the power to repair lost or damaged limbs. If a limb be lost at an early stage it may be re-grown in perfection; but at later stages it is only imperfectly reproduced and is shorter and thinner than the other limbs. Rapidity of growth and longevity vary greatly according to circumstances and to the species. In northern and temperate latitudes where insects disappear in the winter, species of Argyopidae like Aranea diademata, live only for a single season. The young emerge from the cocoon in the early spring, grow through the summer, and reach maturity in the early autumn. The sexes then pair and perish soon after the female has constructed her cocoon. Species of other families (Lycosidae, Clubionidae) may live for a few seasons, hibernating in the soil or amongst dead leaves; and examples of the larger spiders (Aviculariidae) have been kept alive in captivity for several years.

Owing to the smaller size of the male and the greater voracity of the female, the male makes his advances to his mate at the risk of his life and is not infrequently killed and eaten by her either before or after pairing has been effected. Fully aware of the danger, he pays his addresses with extreme caution, frequently waiting for hours in her vicinity before venturing to come to close quarters. Males of the Argyopidae hang on the outskirts of the webs of the females and signal their presence to her by jerking the radial threads in a peculiar manner. Other web-spinning spiders (Tegenaria) have somewhat similar habits; and the male of the park-web spider (Atypus), one of the Mygalomorphae, taps the walls of the tubular web of the female before daring to bite a hole in it and descend into her burrow. Most curious of all is the courtship of the males of some species of Salticidae, or jumping spiders, which are decorated with plumes or coloured stripes or iridescent patches. These they display before her, posing and performing extraordinary antics in her presence exactly as cock birds behave towards their hens. Lastly, the males of some species of spiders differ from the females in possessing stridulating organs consisting of horny ridges and spikes and lodged either between the mandible and palpus as in some species allied to Linyphia, one of the Argyopidae, or between the cephalo-thorax and abdomen as in Steatoda, one of the Theridiidae and Cambridgea, one of the Agalenidae. It is believed that the males of these species signal to their females by means of the sound these organs emit. The greatest disparity in size between the sexes is met with in the tropical genus Nephila, the females of which are gigantic representatives of the Argyopidae. The male, however, is a veritable pigmy beside the female, and during copulation presents the appearance of a parasite attached to her abdomen. It has been suggested that the diminutive size of the male is of great advantage to him during courtship, because he is enabled to move easily thereby to escape from her clutches should she turn upon him with hostile intent.

All spiders possess a pair of poison-glands, one in each of the chelicerae or mandibles and opening by means of a duct at the tip of the fang. The primary function of this poison is to kill the prey upon which they feed, its action being very rapid upon insects. In a great majority of cases, however, it is comparatively innocuous to human beings, despite legends to the contrary that have arisen in connexion with certain species like the tarantula. The bite, however, of any spider, strong enough to pierce the skin, may give rise to a certain amount of local inflammation and pain depending principally upon the amount of poison injected. The bite, for example, of large species of the family Aviculariidae, sometimes called Mygales, and sometimes, but erroneously, known as tarantulas, species which have fangs half an inch long and as sharp as needles and a considerable quantity of poison, may be very painful, though seldom serious provided the health of the patient be good. There is one possible exception, however, to the innocuous nature of the poison and this is supplied by the species of the genus Lathrodectus, one of the Theridiidae. There is no actual proof that this spider is more poisonous than others, but it is a significant fact that its species, inhabiting countries as widely separated as Chile, Madagascar, Australia, New Zealand and South Europe are held in great fear by the indigenous population, and many stories are current of serious or fatal results following their bites. Many of the species of these spiders, moreover, are very conspicuously coloured, being either wholly black or black relieved by fiery red spots, forcibly suggesting that they are warningly coloured. Some of the species of Aviculariidae also appear to be warningly coloured with black or black and red, and their coloration is associated with the urticating nature of their bristles, which makes them highly unpalatable to vertebrate foes. So far as is known, however, only the large spiders belonging to this group possess this special means of defence, and in many other species this is accompanied by highly-developed stridulating organs resembling those of rattlesnakes and scorpions in function. Others again, like Gasteracantha and Acrosoma, belonging to the Argyopidae, are armed with sharp and strong abdominal spines, and these spiders are hard-shelled like beetles and are spotted with black on a reddish or yellow ground, their spines shining with steel-blue lustre. The majority of spiders, however, are soft-skinned and succulent, and are tasty morsels for insectivorous reptiles, birds and mammals. Hence as a very general rule the coloration makes for concealment under natural conditions of existence, and the instincts which lead to concealment are very highly developed. As instances of procryptic or celative coloration may be mentioned that of the species of the genus Dolomedes, one of the Lycosidae, which lives amongst reeds and is marked with a pair of longitudinal yellow lines which harmonize with the upright stalks of the vegetation, and Lycosa picta, which lives on the sand, can scarcely be seen on account of its mottled pattern: Sparassus smargdulus and the species of Pecucetia, which are found amongst grass or low green herbage, are mostly green in colour, and Salticus scenicus is banded with white and black to match the grey tint of the rocks and stone walls on which it hunts its prey. Similar instances of protective coloration could be cited without end. Sometimes the shape of the spider combines with the colour to produce the same effect, as in the species of Uloborus, which as they hang in thin shabby-looking webs exactly resemble fragments of wind-blown rubbish. The success of procryptic coloration depends, however, very largely upon stillness, and the instinct to keep stationary without moving a limb is a marked characteristic of all spiders unless engaged in hunting or fleeing from imminent danger. The instinct reaches its highest development in the phenomenon miscalled “death feigning.” Spiders of various families will, when alarmed, lie absolutely still with legs tucked up and allow themselves to be pushed and rolled, and handled in various ways without betraying that they are alive by the slightest movement. But it would be absurd to suppose that they are in reality pretending to be dead, because there is no reason to think they can have any knowledge of death. They are merely practising the inherited instinct to lie motionless, movement being the only indication of the presence of living prey known to many insectivorous animals. When concealment is no longer possible terrestrial species, like the Lycosidae, dart swiftly to the nearest shelter afforded by crevices in the soil, stones, fallen leaves or logs of wood, while those that live in bushes, like the Argyopidae, drop straight to the ground and lie hidden in the earth or in the fallen vegetation beneath.

The extent to which procryptic coloration and instincts favouring concealment are developed indicates that generation after generation spiders have been subjected to persecution from enemies. No doubt large numbers are devoured by insectivorous birds, mammals and reptiles, but the mortality due to them and other foes sinks into insignificance beside that caused by the persecution of hymenopterous insects of the families Ichneumonidae and Pompilidae, especially of the latter, many species of which systematically ransack the country for spiders wherewith to feed their young in the breeding season. It is no exaggeration to say that countless thousands of spiders of all families are annually destroyed by these insects, and there is no reason to doubt that destruction on at least as great a scale has been going on for centuries, too many even to guess at. Hence it is probable that no factor has had a greater influence than these wasps in moulding the protective instincts and habits of spiders. One interesting phenomenon in spider-life seems to be directly and certainly traceable to this influence, and that is mimicry of ants. In several families of spiders, but principally in those like the Clubionidae and Salticidae, which are terrestrial in habits, there are species which not only live amongst ants, but so closely resemble them in their shape, size, colour and actions that it requires a practised eye to distinguish the Arachnid from the insect. Now the Pompilidae or mason wasps provision their cells with insects of many different kinds, as well as with spiders; but, of the hundreds of species of these wasps that have been described from different parts of the world, only one is known to use ants for this purpose; and this species is not one that preys upon spiders. On the other hand it has been specially recorded of two of the species of spider-destroyers that they have great dislike and apparent fear of these little poisonous Hymenoptera. So, too, does it appear that ants are entirely immune to the attacks of Ichneumonidae, which destroy hosts of other insects and of spiders by laying their eggs upon their bodies. But since ants are not persecuted by these two families of Hymenoptera, the greatest enemies spiders have to contend with, it is evident that mimicry of ants is of supreme advantage to spiders. Ants, however, are not the only animals mimicked by spiders. Some members of the Argyopidae (Cyclosa) are exactly like small snails; others (Cyrtarachne) resemble Coccinellidae in shape and colour. Now, Coccinellidae (ladybirds) are known to be highly distasteful to most insectivorous mammals and birds, and snails would be quite unfit food for the Pompilid or Ichneumonid larvae, so that the reason for the mimicry in these cases is also perfectly clear. The exact extent, however, to which each particular class of enemy has affected the protective habits and attributes of spiders is by no means always evident; and it is impossible to discuss the question in detail within the limits of a short article. But two instances of extreme deviation from the ordinary mode of life due, apparently, like ant-mimicry, solely, if not wholly, to the persecution of Hymenoptera, may be cited as illustrations of the profound effect upon habit brought about by long-continued persecution from enemies of this kind. This deviation is the adoption of an aquatic mode of life by the European fresh-water spider (Argyroneta) and by the marine spider Desis, which is found on the shores of the Indian and Pacific Oceans from Cape Colony to eastern Australia. Desis lives invariably between tide-marks upon the rocks and coral reefs, and may be found at low tide either crawling about upon them or swimming in tidal pools and feeding upon small fish or crustaceans. As the tide rises the spiders take refuge in crevices and spin over their retreat a sheet of silk, impervious to water, beneath which they lie in safety with a supply of air until the ebb exposes the site again to the sun. The fresh-water spider (Argyroneta) lives amongst the weeds of lakes and ponds and, like Desis, is quite at home beneath the water either swimming from spot to spot or crawling amongst the stems of aquatic plants. As a permanent home the spider makes beneath the surface a thimble-shaped web, with inverted mouth, anchoring it to the weeds. He then ascends to the surface, carries down a bubble of air and releases it inside the mouth of the silk-thimble, thus replacing a certain amount of water. This action is repeated until the domicile is filled with air, when the spider takes possession of it. The spider owes its name Argyroneta or the silver swimmer to its silvery appearance as it swims about under water enveloped in air, and its power to retain an envelope of air on its sternum and abdomen depends upon the circumstance that these areas are beset with hairs which prevent the water reaching the integument; but the air retained by these hairs can be released when the spider wishes to fill its subaqueous home with that element. Argyroneta feeds principally upon flies or gnats, which it seizes from below as they light upon the surface of the water. In the breeding season the male spins a bell or thimble near that of the female and joins the two by means of a silken passage. The female attaches her eggs to the inner wall of her own home, and the young when large enough to shift for themselves have the bell-making instinct fully developed. Since the adoption of an aquatic mode of life by Desis and Argyroneta involves no increased facilities in getting food, and merely substitutes for ordinary terrestrial enemies fishes and crustaceans in the former case, and fishes, amphibians, and insectivorous water-insects in the latter, the supposition is justified that the change in environment is due to the unremitting persecution of Pompilidae and Ichneumonidae, which would not venture to pursue their prey beneath the water's surface. The habits of certain other spiders suggest the origin of the perfect adaptation to aauatic conditions exhibited by Desis and Argyroneta. The nature of the integument and its hairy clothing in all spiders enables them to be plunged under water and withdrawn perfectly dry, and many species, even as large as the common English house-spider (Tegenaria), are so lightly built that they can run with speed over the surface of standing water, and this faculty has been perfected in genera like Pirata, Dolomedes and Triclaria, which are always found in the vicinity of lakes or on the edges of rivers and streams, readily taking to the water or running down the stems of water plants beneath its surface when pursued. Some species of Dolomedes, indeed, habitually construct a raft by spinning dead leaves together and float over the water upon it watching for an opportunity to dash upon any insect that alights upon its surface.

Geologically, spiders date from the Carboniferous Period, Arthrolycosa and others from the coal beds of Europe and North America being closely allied to the existing genus Liphistius. Remains of spiders from the Baltic amber beds of Oligocene age and from nearly coeval fluviatile or lacustrine deposits of North America belong to forms identical with or closely related to existing genera, thus proving the great antiquity of our present spider fauna.