Читать книгу Intelligence in Plants and Animals - Thomas G. Gentry - Страница 13

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Simple nests and tubes are all the majority of spiders construct for their homes. The larger and better known webs for catching insects are made by comparatively few species. He who is astir in the grass-fields on damp summer mornings, will everywhere see innumerous flat webs, from an inch or two to a foot in diameter, which weather-wise folks consider prognostic of a fair day. These webs may always be found upon the grass at the proper season, but only become visible from a distance when the dew is upon them, making the earth appear as covered by an almost continuous carpet of silk.

By far the greater number of these nests is of the form which is termed funnel-webs, which consist of a concave sheet of silk, constituted of strong threads, crossed by finer ones, which the author spins with the long hind-spinnerets, swinging them from side to side, and laying down a band of threads at each stroke, the many hundred threads extending in all directions to the supporting spears of grass. The web is so close and tight that the footsteps of the spider can be distinctly heard by the attentive, listening ear as she runs hither and thither over its scarcely bending surface. At one side of the web is a tube, leading down among the grass-stems, which serves as a hiding-place for the owner of the web. Here, at the top, and just out of sight, the spider ordinarily stands, waiting for something to light upon the web, when she eagerly rushes out, seizing the prey unluckily caught and carrying it into her tube to eat. If too formidable an insect comes upon the web, she turns herself round, beating a precipitate retreat out of the lower end of her funnel and soon is lost beneath the mesh of enveloping and interlacing grasses.

Where favorably located, these webs remain through the entire season, and are enlarged, as the spider grows, by additions on the outer edges, and are supported by threads running up into the neighboring plants. Sometimes the webs are built in close proximity to a stone partially imbedded in the earth, the bottom of the funnel opening slightly underneath the stone, which secures to the spider a convenient harbor in case of threatening danger.

Agalenidæ, as our funnel-web weavers are called, are long-legged, brown spiders, in which the head part of the cephalo-thorax is higher than the thoracic part, and distinctly separated from it by grooves or marks at the sides. The eyes are usually in two rows, but in Agalena the middle eyes of both rows are much higher than the others. The feet have three claws, and the posterior pairs of spinnerets are two-jointed and usually longer than the others. Agalena nævia, the technical name of our Common Grass Spider, abounds in all parts of the United States, but its very commonness is the principal reason why it is so little known except by the trained naturalist, its very familiarity leading the average man and woman to look upon it with contempt.

AGALENA AND HER FUNNEL-WEB.

House-Fly, Caught in the Toils, Becomes a Victim.

Persons unfamiliar with spiders find it difficult to distinguish the young from the old, and male from female. This is caused, in part, by the great differences between different ages and sexes of the same spider, on account of which they are supposed to belong to distinct species. The adult males and females, however, are easily distinguished from each other, and from the young, by the complete development of organs peculiar to each sex, the palpal organs on the ends of the palpi in the males, and the epigynum, a hard swollen place just in front of the opening of the ovaries in the females. Usually the males are smaller than their partners, and have, in proportion to their size, smaller abdomens and longer legs. They are generally darker colored, especially on the head and front part of the body, and markings which are distinct in the female coalesce and become darker in the male. In most species these differences are not very great, but in some, Argiope and Nephila for examples, where the males are about one-tenth as large as the females, one would hardly suppose, without other evidence, that the males and females had any relationship to each other. The palpal organs and the epigynum are sexual characters which do not attain their functional value until after the last moult has been effected.

Spiders are naturally very selfish creatures. Their chief concern in life seems to be the gratification of their desires for food. They are eminently unsocial, the sexes preferring to live solitary lives. It is only when actuated by amatory influences that the females will tolerate their weaker lords, and in some instances it is only by stratagem and agility that the latter are able to accomplish the fulfilment of the law of their being, the females by their ugly, vicious tempers resisting to the utmost. In the case of Agalena the male is the stronger of the two. He, at the proper time, when the reproductive cells are matured, takes the female in his powerful mandibles, lays her gently on one side, and inserts one of his palpi, whose little sacs had previously been filled with the fecundating discharge, into the epigynum underneath. After a time, necessarily brief, he rises on tiptoe, turns her around and over, so that she comfortably lies on the other side, her head being in the opposite direction, and inserts the other palpus. All through the operation the female lies as though she was dead. The ends of nature being served, the sexes separate, the male returning to the solitary life he previously led, while the female busies herself in providing for the duties of maternity.

The eggs becoming mature, the latter proceeds to make a little web and lays them in it, practising the utmost care. She now covers them over with silk, which she weaves into a cocoon, where the young remain some time after they are hatched. Seldom is the laying seen, for it generally happens in the night-time, or in retired places. Often, in confinement, the spider refuses to lay at all. An egg of a spider, like that of any other animal, is a cell which separates from the body of the female, and subsequently unites with one or more cells that have separated from the body of the male. This process of union, termed fertilization, doubtless takes place when the eggs have attained their full size and are about to be laid. After being laid and hardened it is a very easy matter to watch their development. All that is necessary to be done is to cover the egg to be examined with oil, alcohol or any liquid that will wet it, for this tends to make the shell transparent. Eggs laid in summer are ready to hatch in a fortnight, while those laid in autumn develop slowly all through the winter. A day or two are occupied in hatching. When the time has arrived the shell, or more properly the skin, cracks along the lines between the legs, and comes off in rags, and the spider slowly stretches itself and creeps about. Pale and soft it appears, and devoid of hairs or spines, but its feet are armed with small claws. In two or three days it gets rid of another skin, and begins to assume a spider-like appearance, the eyes becoming dark-colored, the thoracic marks growing more distinct, and a dark stripe appearing across the edge of each segment of the abdomen. The hairs are now long, but few in number, and arranged in rows across the abdomen and along the middle of the thorax. Before the next moult they usually forsake the cocoon, and live together for a short time in a web spun in common. Where larger broods of young spiders live together, they soon show cannibal-like qualities, and if kept in confinement one or two out of a cocoon-full may be raised without recourse to any other food.

As spiders grow larger, they must moult from time to time. This is an interesting process. The spider hangs herself by a thread from the spinnerets to the centre of the web. In a short time the skin cracks around the thorax, just over the first joints of the legs, and the top part falls forward, being held only at the front edge. The skin of the abdomen now breaks irregularly along the sides and back, and shrinks together in a bunch, leaving the spider suspended only by a short thread from the spinnerets, her legs still being trammelled by the old skin. Fifteen minutes of violent exertion releases her from the encumbrance, when she drops down, hanging by her spinnerets like a wet rag. She can do nothing in this condition, not even draw her legs away from an approaching hand. In ten or twelve minutes the legs show signs of strengthening, and she is able to draw them gradually towards her. A few up-and-down movements, and she manages to get into the web again.

That which, more than anything else, discriminates spiders from other animals is their habit of spinning webs. Some of the mites spin irregular threads upon plants, or cocoons for their eggs, and many insects cocoons in which to undergo their changes from larva to imago, but in the spiders the spinning-organs are much more complicated, and used for a greater variety of purposes, for making egg-cocoons, silk linings to their nests, and nets for catching insects. The spider’s thread differs from that of insects, in being constituted of a great number of finer threads laid together, while soft enough to coalesce into one. Each spinneret is provided with a number of little tubes, which convey the viscid liquid that forms the thread from glands in the spider’s body. In Agalena the two hinder spinnerets are long, and have spinning-tubes along the under side of the last joint.

When about to produce a thread the spider presses the spinnerets against some object and forces out from each tube enough of the secretion to adhere to it, when the spinnerets are moved away, drawing the viscid liquid out, which hardens at once into threads for each tube. A band of threads is formed when the spinnerets are kept apart, but when closed together the fine threads unite into one or more large ones. Commonly the spinning is aided by the hinder feet, which guide the thread, keeping it clear of surrounding objects, and even pulling it from the spinnerets.

Spiders are best known and hated as animals that bite. Their biting-apparatus, the mandibles, are located in front of the head. Partly in the basal joints of these organs and partly in the head, the poison-glands are seated, from which is discharged through a tube the venom, which makes spiders so much to be feared. This tube opens at the point of the claw of the mandible. When the apparatus is not in use the claws are closed up against the parts between the rows of teeth; but when the jaws are opened to bite the claws are turned outward, so that their points can be made to penetrate anything that comes between the jaws. The ordinary function of the mandibles is the killing and crushing of insects, so that the soft parts can be eaten by the spider, and in this preparation they are substantially aided by the maxillæ. Spiders will sometimes chew an insect for hours, until it becomes a mere ball of skin, only swallowing such bits as may happen to be sucked in with the blood. Let alone and unmolested, they bite nothing except insects that are useful for food. But when attacked and cornered, all species open their jaws and bite if they can, their ability to do so depending upon their size and the strength of their jaws. Notwithstanding the large number of pimples and stings ascribed to spiders, undoubted cases of their biting the human skin are exceedingly rare, and the stories of death, insanity and lameness from spider-bites are probably all untrue. Many experiments have been made to test the effect of the bites of spiders on animals. Insects succumb most readily to their bites, some sooner than others, but birds, except when bitten by the larger Mygale, recover after the lapse of a few hours. The effect upon man, even when the bite is deep enough to draw blood, is like the pricks of a needle, attended by little or no inflammation or pain. Even in cases where death among insects and birds ensues it is claimed by the authorities, men as eminent as Blackwall, Moggridge and Dufour, that the secretion from spiders’ jaws is not poisonous, but that the animals die, when bitten, from loss of blood and mechanical injury.

Such is the prejudice against the spider, that its presence, no matter where found, whether in the open field or in a corner of the house, is an inducement for its inveterate enemy, man, to sweep it to the ground or floor and crush its frail life out with one blow of the foot. Few know, or care to know, it would seem, the good it does for man. He owes to it, in a large measure, the protection of his crops, and no little of the comfort he enjoys in life. Spiders are carnivorous creatures, and destroy vast number of insects, many of which are man’s worst enemies. They merit, and deservingly, too, his kindness and protection for the benefits they confer.

Tarantulas have been supposed to produce epilepsy by their bites, which could only be relieved by music of certain kinds. Such stories, and they have been widely circulated and believed, are the veriest nonsense, for tarantula-bites produce no such effects nowadays. These spiders, which live in holes in sand, out of which they reach after passing insects, are no more savage in their habits than other spiders, for Dufour, a celebrated French naturalist, once kept one that soon learned to take flies from his fingers without manifesting the least disposition to bite. Different species quickly learn, when treated with kindness, to regard man as their friend. I have seen Agalena take food from the hand out of a pair of forceps, or water from a brush, and even to reach on tiptoe after it from the mouth of a bottle placed for her accommodation. Though naturally timid and shy, and prone to flee to her funnel on man’s approach, yet she has been known to permit the most unexpected familiarities without fear or resentment. Many a female has taken from my hand the proffered fly, and submitted to the gentle caresses of my finger down the back and abdomen with the most pleasurable satisfaction. They have come at the sound of my voice, dancing upon their sheeted web like one gone mad, so perfectly carried away with delight. An interesting experience of last summer during a brief stay in the country seems apropos at this time. While sauntering carelessly along a forest-road I came unexpectedly upon a rustic bridge, with a railing on one side, which overspanned a small water-course. Leaning for rest and support against the railing, soon my attention was arrested by a huge female spider, which I recognized as Epeira domiciliorum. She was evidently in quest of something, as I was led to suspect from her seemingly thoughtful and deliberate movements. I watched her closely and criticisingly for a long while, and in one of her contemplative moods, when she stood perfectly motionless and fixed as it were to the railing, I reached out my finger rather impulsively and began stroking her along the abdomen, a familiarity which she did not resent, and which seemed to give her the most intense delight. When the caressing had ceased, she would turn round and confront her newly-made acquaintance, but the lifting of the finger was always the signal for her to assume an attitude of the most perfect quiescence. That she enjoyed these little attentions there cannot be a shadow of doubt, or actions are no use in the interpretation of feeling. Had they been painful, she would have sought relief in flight, or in the manifestation of an untoward disposition towards her unintentional persecutor.