Here we may inquire—Why is the ground in these serene days covered so thickly by these webs, and what becomes of them? What occasions the spiders to mount into the air, and do the same species form both the terrestrial and aërial gossamer?—And what causes the webs at last to fall to the earth? I fear I cannot to all these queries return a fully satisfactory answer; but I will do the best I can. At first one would conclude from analogy, that the object of the gossamer which early in the morning is spread over stubbles and fallows—and sometimes so thickly as to make them appear as if covered with a carpet, or rather overflown by a sea, of gauze, presenting, when studded with dew-drops, as I have often witnessed, a most enchanting spectacle—is to entrap the flies and other insects as they rise into the air from their nocturnal station of repose, to take their diurnal flights. But Dr. Strack's observations render this very doubtful; for he kept many of the spiders that produce these webs in a large glass upon turf, where they spun as when at liberty, and he could never observe them attempt to catch or eat—even when entangled in their webs—the flies and gnats with which he supplied them; though they greedily sucked water when sprinkled upon the turf, and remained lively for two months without other food[536]. As the single threads shot by other spiders are usually their bridges, this perhaps may be the object of the webs in question: and thus the animals may be conveyed from furrow to furrow or straw to straw less circuitously, and with less labour, than if they had travelled over the ground. As these creatures seem so thirsty, may we not conjecture that the drops of dew, with which they are always as it were strung, are a secondary object with them? So prodigious are their numbers, that sometimes every stalk of straw in the stubbles, and every clod and stone in the fallows, swarms with them. Dr. Strack assures us that twenty or thirty often sit upon a single straw, and that he collected about 2000 in half an hour, and could have easily doubled the number had he wished it: he remarks, that the cause of their escaping the notice of other observers, is their falling to the ground upon the least alarm.
As to what becomes of this immense carpeting of web there are different opinions. Mr. White conjectures that these threads, when first shot, might be entangled in the rising dew, and so drawn up, spiders and all, by a brisk evaporation, into the region where the clouds are formed[537]. But this seems almost as inadmissible as that of Hooke, before related. An ingenious and observant friend, thinking the numbers of the flying spiders not sufficient to produce the whole of the phenomenon in question, is of opinion that an equinoctial gale, sweeping along the fallows and stubbles coated with the gossamer, must bring many single threads into contact, which, adhering together, may gradually collect into flakes; and that being at length detached by the violence of the wind, they are carried along with it: and as it is known that such winds often convey even sand and earth to great heights, he deems it highly probable that so light a substance may be transported to so great an elevation, as not to fall to the earth for some days after, when the weather has become serene, or to descend upon ships at sea, as has sometimes happened. This, which is in part adopted from the German authors, is certainly a much more reasonable supposition than the other; but some facts seem to militate against it: for, in the first place, though gossamer often occurs upon the ground when there is none in the air, yet the reverse of this has never been observed; for gossamer in the air, as in the instance recorded by Mr. White, is always preceded by gossamer on the ground. Now, since the weather is constantly calm and serene when these showers appear, it cannot be the wind that carries the web from the ground into the air. Again, it is stated that these showers take place after several calm days[538]: now, if the web was raised by the wind into the air, it would begin to fall as soon as the wind ceased. Whence I am inclined to think that the cause assigned by Dr. Lister is the real source of the whole phenomenon. Though ordinary observers have overlooked them, he noticed these spiders in the air in such prodigious numbers, that he deemed them sufficient to produce the effect. I shall not, however, decide positively; but, having stated the different opinions, leave you to your own judgement.
The next query is, What occasions the spiders to mount their chariots and seek the clouds? Is it in pursuit of their food? Insects, in the fine warm days in which this phenomenon occurs, probably take higher flights than usual, and seek the upper regions of the atmosphere; and that the spiders catch them there, appears by the exuviæ of gnats and flies, which are often found in the falling webs[539]. Yet one would suppose that insects would fly high at all times in the summer in serene warm weather. Perhaps the flight of some particular species constituting a favourite food of our little charioteers—the gnats, for instance, which we have seen sometimes rise in clouds into the air[540]—may at these times take place; or the species of spiders that are most given to these excursions, may not abound in their young state—when only they can fly—at other seasons of the year.
Whether the same species that cover the earth with their webs produce those that fill the air, is to be our next inquiry. Did the appearance of the one always succeed that of the other, this might be reasonably concluded:—but the former, as I lately observed to you, often occurs without being followed by the latter. Yet, since it should seem that the aërial gossamer, though it does not always follow it, is always preceded by the terrestrial, this warrants a conjecture that they may be synonymous. Two German authors, Bechstein[541] and Strack[542], have described the spider that produces gossamer in Germany under the name of Aranea obtextrix[542]. But it is not clear, unless they have described it at different ages, when spiders often greatly change their appearance, that they mean the same species. The former describes his as of the size of a small pin's head, with its eight eyes disposed in a circle, having a black-brown body and light-yellow legs: while Dr. Strack represents his A. obtextrix as more than two lines in length; eyes four in a square, and two on each side touching each other; thorax deep brown with paler streaks; abdomen below dull white, above dark copper brown, with a dentated white spot running longitudinally down the middle. The first of these, if distinct, as I suspect they are, agrees very well with the young of one which Lister observed as remarkable for taking aërial flights[543]; and which I have most usually seen so engaged. The other may possibly be that before noticed, which he found in such infinite numbers in Cambridgeshire[544]. If this conjecture be correct, it will prove that the same species first produce the gossamer that covers the ground, and then, shooting other threads, mount upon them into the air.
My last query was, What causes these webs ultimately to fall to the earth? Mr. White's observation will I think furnish the best answer. "If the spiders have the power of coiling up their webs in the air, as Dr. Lister affirms, then when they become heavier than the air they will fall[545]." The more expanded the web, the lighter and more buoyant, and the more condensed, the heavier it must be.
I trust you will allow from this mass of evidence, that the English Arachnologists—may I coin this term?—were correct in their account of this singular phenomenon; and think, with me, that Swammerdam (who however admits that spiders sail on their webs), and after him De Geer, were rather hasty when they stigmatized the discovery that these animals shoot their webs into the air, and so take flight, as a strange and unfounded opinion[546]. The fact, though so well authenticated, is indeed strange and wonderful, and affords another proof of the extraordinary powers, unparalleled in the higher orders of animals, with which the Creator has gifted the insect world. Were indeed man and the larger animals, with their present propensities, similarly endowed, the whole creation would soon go to ruin. But these almost miraculous powers in the hands of these little beings only tend to keep it in order and beauty. Adorable is that Wisdom, Power, and Goodness, that has distinguished these next to nothings by such peculiar endowments for our preservation as if given to the strong and mighty would work our destruction.
After the foregoing marvellous detail of the aërial excursions of our insect air-balloonists, I fear you will think the motions of those which fly by means of wings less interesting. You will find, however, that they are not altogether barren of amusement. Though the wings are the principal instruments of the flight of insects, yet there are others subsidiary to them, which I shall here enumerate, considering them more at large under the orders to which they severally belong. These are wing-cases (Elytra, Tegmina, and Hemelytra); winglets (Alulæ); poisers (Halteres); tailets (Caudulæ); hooklets (Hamuli); base-covers (Tegulæ), &c. Besides, their tails, legs, and even antennæ, assist them in some instances, in this motion.
As wings are common to almost the whole class, I shall consider their structure here. Every wing consists of two membranes, more or less transparent, applied to each other: the upper membrane being very strongly attached to the nervures (Neuræ), and the lower adhering more loosely, so as to be separable from them. The nervures[547] are a kind of hollow tube,—above elastic, horny, and convex; and flat and nearly membranaceous below,—which take their origin in the trunk, and keep diminishing gradually, the marginal ones excepted, to their termination. The vessels contained in the nervures consist of a spiral thread, whence they appear to be air-vessels communicating with the tracheæ in the trunk.—The expansion of the wing at the will of the insect, is a problem that can only be solved by supposing that a subtile fluid is introduced into these vessels, which seem perfectly analogous to those in the wings of birds; and that thus an impulse is communicated to every part of the organ, sufficient to keep it in proper tension. We see by this, that a wing is supported in its flight like a sail by its cordage[548]. It is remarkable that those insects which keep the longest on the wing, the dragon-flies (Libellulina) for instance, have their wings most covered with nervures. The wings of insects in flying, like those of other flying animals, you are to observe, move vertically, or up and down.
In considering the flight of insects, I shall treat of that of each order separately, beginning with the Coleoptera or beetles. Their subsidiary instruments of flight are their wing-cases (Elytra), and in one instance, winglets (Alulæ). The former[549], which in some are of a hard horny substance, and in others are softer and more like leather, though they are kept immoveable in flight, are probably, by their resistance to the air, not without their use on this occasion. The winglets are small concavo-convex scales, of a stiff membranaceous substance, generally fringed at their extremity[550]. I know at present of only one coleopterous insect that has them (Dytiscus marginalis). They are placed under the elytra at their base. Their use is unknown; but it may probably be connected with their flight. The wings of beetles[551] are usually very ample, often of a substance between parchment and membrane. The nervures that traverse and extend them, though not numerous, are stronger and larger than those in the wings of insects of the other orders, and are so dispersed as to give perfect tension to the organ. When at rest—except in Molorchus, Atractocerus, Necydalis, and some other genera—they are folded transversely under the elytra, generally near the middle, with a lateral longitudinal fold, but occasionally near the extremity[552]. When they prepare for flight, their antennæ being set out, the elytra are opened so as to form an angle with the body and admit the free play of the wings, and they then fly off, striking the air by the vertical motion of these organs, the elytra all the while remaining immoveable. During their flight the bodies of insects of this order, as far as I have observed them, are always in a position nearly vertical, which gives to the larger sorts, the stag-beetle for instance, a very singular appearance. Olivier, probably having some of the larger and heavier beetles in his eye, affirms that the wings of insects of this order are not usually proportioned to the weight of their bodies, and that the muscular apparatus that moves them is deficient in force. In consequence of which, he observes, they take flight with difficulty, and fly very badly. The strokes of their wings being frequent, and their flight short, uncertain, heavy, and laborious, they can use their wings only in very calm weather, the least wind beating them down. Yet he allows that others, whose body is lighter, rise into the air and fly with a little more ease; especially when the weather is warm and dry, their flights however being short, though frequent. He asserts also, that no coleopterous insect can fly against the wind[553]. These observations may hold perhaps with respect to many species; but they will by no means apply generally. The cockchafer (Melolontha vulgaris), if thrown into the air in the evening, its time of flight, will take wing before it falls to the ground. The common dung-chafer (Geotrupes stercorarius)—wheeling from side to side like the humble-bee—flies with great rapidity and force, and, with all its dung-devouring confederates, directs its flight with the utmost certainty, and probably often against the wind, to its food. The root-devourers or tree-chafers (Melolontha, Hoplia, &c.) support themselves, like swarming bees, in the air and over the trees, flying round in all directions. The Brachyptera and Donaciæ, in warm weather, fly off from their station with the utmost ease;—their wings are unfolded, and they are in the air in an instant, especially the latter, as I have often found when I have attempted to take them. None are more remarkable for this than the Cicindelæ, which, however, taking very short flights, are as easily marked down as a partridge, and affords as much amusement to the entomologist, as the latter to the sportsman.—It is to be observed that many insects in this order have no wings, and the female glow-worms neither wings nor elytra.
Many persons are not aware that the insects of the next order, the Dermaptera, can fly: but earwigs (Forficula), their size considered, are furnished with very ample and curious wings, the principal nervures of which are so many radii, diverging from a common point near the anterior margin. Between these are others which, proceeding from the opposite margin, terminate in the middle of the wing[554]. These organs, when at rest, are more than once folded both transversely and longitudinally.