Imágenes de páginas
PDF
EPUB

found in large geometrical webs, two, three, or four feet in diameter, stretched between shrubs or trees, and often high up among the pines, so as to be out of reach. The webs are strong and of a yellow color; and, as with most species of geometrical spiders, the concentric circles are elastic and studded with numerous viscid globules, while the radii and other parts of the framework are composed of dry and inelastic silk; but with this species the distinction between these two portions of the web consists not only in the viscidity of the former, but also in the color; for while most of the concentric circles are of a bright yellow or golden hue, the radii and stay-lines, and also every eighth or tenth circle (the number varies in different individuals), are white or silver-colored. The circles are very near together in proportion to the size of the insect, being only one third or one fourth of an inch apart.

As might be inferred from these facts, but which, so far as I know, has never before been observed, this spider not only has the power of regulating the size of its thread, - according as one or two, or three or four of its spinnerets are pressed upon the surface from which the line is to extend, or as a greater or less number of the spinnerules in any one spinneret are employed, — but can also use in the construction of its web either the white or the yellow silk at will; for of its two principal pairs of spinnerets, one, the anterior, yields the yellow, while the other or posterior pair yields the white silk. Of this I satisfied myself by carrying the thread from the anterior pair of spinners upon one part of a spindle, and that from the posterior pair upon another part, guiding them with pins while the spindle was in motion ; the result being the formation of two circles of silk, one of a golden, the other of a silver color, as in one of the specimens exhibited ; moreover, if while both threads are being drawn out, they are slackened, the lower silver thread will wrinkle and fly up, being inelastic; while the other will contract and, within certain limits, preserve its direction.

There is a corresponding difference in the color of the glands which secrete the gum of which the silk is formed; one set, the more numerous, being yellow, and the other white.

The manner in which the spider deposits the globules of gum on the circles which she wishes to be viscid is not yet explained ; at any rate this same yellow silk, when either reeled from the body of the spider, or when employed in the formation of its cocoon, is dry and much less elastic than that of which the concentric circles are composed.

The evolution of the silk from the spider is almost wholly a mechan

ical process, and, beyond a certain expansion of the parts, by separatmg the spinners from each other, the only control exercised by the insect is by means of its hinder legs, the tips of which serve to guide the thread, and by grasping it to control the evolution. I have never been able to reel out over three hundred yards at once from a single spider ; but on opening the abdomen, the glands are found still to conLain m ore or less gum. Upon three successive days I obtained equal qua ntities of silk; so that if, as now seems probable, the emission of the silk is purely mechanical, then a certain degree of preparation is necessary, after it is secreted, before it is ready for use.

The diameter of the silk as spun by the insect, or as reeled from it, varies from one six-thousandth to one thousandth of an inch ;* it is. exceedingly strong, more so in proportion to its bulk than that of the

-W orm; as is natural, since the spider's thread is made up of huneds and even thousands of minute fibrils, while the common silk 18 single. The largest threads are those composing the outer layer

the cocoons, but these are evidently compound, and each of the two,

ee, or four strands is apparently such as proceeds from the single spinners; the minute fibrils of which have united at once on leaving the spin Derules, so as to form the ordinary silken fibre which generally appears simple under the microscope.

The habits of this spider are very interesting. It seems to obey ree principal instincts : first, to ascend ; second, to seek the light, bether natural or artificial; and, third, to maintain a position with the ad downward. It has eight eye-spots, but, so far as I have observed,

can only distinguish light from darkness, and is not able to see P e cts. There is not here space to give in detail an account of all that

Observed in case of several which made their webs in my room in puth Carolina; but all seems to indicate that these spiders do not sees as the term is generally understood ; the touch is, however, very cute, and is exercised by the palpi and by the tips of the legs, specally the anterior pair. Unlike some other geometrical spiders, it seizes 3. p rey at once in its jaws, and never envelopes it in a silken net till has expired. The sense of hearing is evidently very acute.

It is very quiet in its disposition, and never leaves its web unless molested. The female builds the web, and even carries the male on wer back or belly when moving about; she never attempts to bite

* The micrometer measurements were made by Mr. R. C. Greenleaf.

unless provoked, and may le salire to run over one's per:un with impunity.

Perhaps the most remarkable fact in connection with this spider is, that it can be fed and watered by hand ; a live fly held to its jaws is seized as soon as a buzz makes its presence known; so also a bit of chicken-liver, if touched to the jaws; and if a drop of water be presented on a camel's-hair pencil, it will be readily taken and gradually swallowed. It is evident that the spiders drink the drops of water which are left in the web from the rain or dew; and they thrive best in a moist atmosphere.

The female lays four or five hundred eggs, half as large as a pin's head, and slightly agglutinated together in a rounded mass, which is secured on the lower side of a leaf by a strong silken cocoon of loose texture, and varying in color. Many of the eggs which were laid by my spiders in September were hatched in about thirty days. The young differ much from the adult in form and color; and the changes which they pass through in growth will prove a most interesting branch of the subject. The young do not leave the cocoon for some time; and even after they have, are more or less gregarious, — always keeping in companies, and preserving good order while moving. They need water, and, if not supplied with food, are prone to eat one another. If properly attended to, they grow quite rapidly ; * and although at first they make only an irregular web in common, yet after they have attained a length of half an inch, they will, if separated, construct regular geometrical webs.

In a state of nature, not many over one per cent of the spiders which are hatched live to maturity; so that the question of a practical value of this silk depends upon the success of the attempts to prevent this destruction, which is apparently due to their own voracity, to the elements, and to other insects.

Much more might be related concerning the habits of the insect, of the manner of keeping and feeding the young, of the means of securing the spider while its silk is obtained, and of the various apparatus employed ; but I am so impressed with the peculiarities thus far observed in themselves, and with the beauty and strength of the silk, that if time and means permit I shall continue the inquiry as far as possible. And having now, as I hope, established my claim to the

* Feb. 23d, 1866. Some of these young are now more than an inch in length.

discovery of this new method of obtaining a silken material, (namely, by a reeling or circular motion applied to the insect itself,) I will defer to a future occasion a more complete account of the spider, of its habits, anatomy, and embryology, and of the various qualities of its silk, with whatever conclusion can be reached concerning the practicability of rearing the young; and also how far it is possible to apply the same process to the silk-worm, and other silk-producing larvæ.

Fivo hundred and arty-ninth Mooting.

December 12, 1865. — MONTHLY MEETING. The PRESIDENT in the chair.

A letter was read from Mr. Samuel Eliot in acknowledgment of his election into the Academy; also letters relative to exchanges.

The President called the attention of the Academy to the recent decease of Dr. John Lindley of London, of the Foreign Honorary Members.

Five hundred and sixtioth Mooting.

January 9, 1866. — MONTHLY MEETING. The PRESIDENT in the chair.

The President called the attention of the Academy to the recent decease of Colonel James Duncan Graham of the Resident Fellows, formerly an Associate Fellow.

A memoir by Professor Child was presented by title, namely, “ Remarks on the Language of Gower's Confessio Amantis : a Sequel to Observations on the Language of Chaucer, printed in Vol. VIII. of the Memoirs of the Academy.”

Professor Cooke made the following communication :

On the Aqueous Lines of the Solar Spectrum. By JOSIAH

P. COOKE, JR. A CAREFUL examination of the solar spectrum, continued during several months with the spectroscope described in a recent article of the

VOL. VII.

American Journal of Science, has led me to the conclusion that a very large number of the more faint lines of the solar spectrum, hitherto known simply as air lines, are due solely to the aqueous vapor of our air, and hence that the absorption of the luminous solar rays by the atmosphere is at least chiefly owing to the aqueous vapor which it contains. .

The appearance of the Fraunhofer's line D, seen under precisely the same conditions, but with increasing quantities of aqueous vapor in the atmosphere, is shown in Figures 1, 2, 3, and 4. The D line is selected, because, being a favorite test object for the spectroscope, its general appearance is well known to all observers. But even more marked changes than those here illustrated have been noticed in other, although chiefly in contiguous, portions of the solar spectrum.

These changes attracted my attention from my earliest observations with the spectroscope ; but with my first instrument, and the bisulphide of carbon prisms then employed, it was almost impossible to eliminate the effects which might be caused by the variations in the condition of the instrument itself; and as these were known to be very great, it was possible that they might account for all the variations observed. With the improved instrument, however, just referred to, absolute constancy of action is obtained, and all merely instrumental variations avoided.

A peculiar condition of the atmosphere gave the first clew as to the cause of the changes under consideration. The weather on the 17th of November, 1865, at Cambridge, Massachusetts, was very unusual even for that peculiar season known in New England as the Indian Summer. At noon the temperature on the east side of my laboratory was 70° F., while the wet-bulb thermometer indicated 66°, showing an amount of moisture in the atmosphere equal to 6.57 grains per cubic foot. At the same time the atmosphere was beautifully clear, and the sun shone with its full splendor. I have never seen the aqueous lines of the spectrum more strongly defined than they were on this day; and the total number of lines visible in the yellow portion of the spectrum was at least ten times as great as are ordinarily seen. The appearance of the D line on that day is shown in Fig. 4. Between the two familiar broad lines D, and D, there were eight sharply defined lines of unequal intensity, which is only very imperfectly represented by the woodcut. In addition to these, on the more refrangible side of the space

* American Journal of Science and Arts, Vol. XI., November, 1865.

« AnteriorContinuar »