“If the insect is cleared from the mass of froth it will crawl quite rapidly along the stem of the plant, stopping, at times, to pierce the stem for the purpose of sucking the juices within, and finally settling down in earnest, clutching the stem with its legs. After sucking for some time, a clear fluid is seen to exude from the end of the abdomen, flowing over the body first, and gradually filling up the spaces between the legs and the lower part of the body and the stem upon which it rests. During all this time not a trace of an air-bubble appears; simply a clear, slightly viscid fluid is exuded, and this is the only matter that escapes from the insect. This state of partial immersion continues for an hour or more. During this time, and even when walking, the posterior segments of the insect’s abdomen are extended at intervals, the abdomen turning upwards at the same time. It is a kind of reaching-up movement, but whether this action accompanies a discharge of fluid, or is an attempt at reaching for air, I have not ascertained. Suddenly the insect begins to make bubbles by turning its tail out of the fluid, opening the posterior segment, and grasping, as with a pair of claspers, a moiety of air, then turning the tail down into the fluid, again, and instantly allowing the enclosed air to escape. These movements go on at the rate of seventy or eighty times a minute. The tail is moved alternately to the right and left in perfect rhythm, so that the bubbles are distributed on both sides of the body, and these are crowded towards the head, till the entire fluid is filled with bubbles, and the froth thus made runs over the back and around the stem. In half a minute some thirty or forty bubbles are made in this way—a bulk of air two or three times exceeding that of the body—without the slightest diminution in the size of the body.”
It seems clear, therefore, that the air which is put to this purpose is abstracted directly from the atmosphere, and that neither it nor the bubbles manufactured through it have ever been within the body of the insect. Moreover, if the little bubble-maker be thoroughly dried—which, according to Professor Morse, is a matter of difficulty—it will continue to secrete such spare fluid as it still has, but not the tiniest bubble is seen to issue with this. If set in a drop of water it struggles to the surface, and then goes through the same process of blowing bubbles as it has done when immersed in fluid of its own distilling. The result, however, is not the same, for the water will not hold the bubbles, which constantly disappear. Such, then, is the manner in which the frothy pool is made. What purposes does it answer? That of a pond, apparently, for it would appear that in their larval state these little frog-, or tree-hoppers, are to some extent aquatic insects. If kept dry and not allowed to renew their supply of fluid, their body shrivels, and before long they die. This is not through suffocation, since they can breathe air, by means of spiracles, in the ordinary way. If, however, they are examined closely, certain leaf-like appendages may be detected upon each side of the seventh and eighth segments of the abdomen, and Professor Morse suggests that these may be of the nature of branchiæ, or gills, enabling the insect to breathe, also, in water or fluid, by abstracting the air from it, after the manner of a fish, as some other aquatic larvæ do. “As many of these,” he says, “respire in two ways, either inhaling air through the spiracles, or by means of branchial leaflets, so Aphrophora (for that is the classic name of our insect) may likewise utilise its branchial tufts for the same purpose. Thus we may see the reason for this bubble-blowing, since each fresh bubble added to the mass may aerate the fluid, so to speak, and thus secure at intervals a fresh supply of oxygen.”[[36]]
In early spring, if one examines the leaf-buds of rose trees, which now begin to swell, one may often see tiny little black specks, like grains of gunpowder, scattered over their surface, especially within any fold or crevice which it presents. These are the eggs of the Aphides, insects which, if not cicadas, are not so very far removed from them, and which, looked at from various points of view, are extremely interesting little creatures. One of these points of view, which we may conveniently start from, is their extraordinary rate of increase, which exceeds even that of the Chinese. “A single insect,” says Mr. Buckton, “hatched from one of these shining black ova may be the mother of many billions of young, even during her lifetime. Réaumur calculated that one Aphis may be the mother of the enormous number of 5,904,900,000 individuals during the month or six weeks of her existence. But neither Tongard nor Morren is satisfied with this estimate, both declaring that quintillions are within the capabilities of a single mother’s efforts. Professor Huxley (who, by the way, was not interested in the alleged phenomena of spiritualism, even if true) makes a curious calculation which, at any rate, affords some approximate idea of what a quintillion of Aphides might mean. Assuming that an Aphis weighs as little as one-thousandth of a grain (which is less than I should ever have thought), and that it requires a man to be very stout to weigh more than two million grains, he shows that the tenth brood of Aphides alone, without adding the product of all the generations which precede the tenth, if all the members survive the perils to which they are exposed, contains more ponderable substance than 500,000,000 of stout men: that is, more than the whole population of China.”[[37]] This, it appears, is an under-estimate, which is rather annoying, for one would like to call it a gross exaggeration. But facts are facts—in whatever degree they may interest one—and it is impossible not to feel respect for an insect like this, especially in these days, when the diminished returns of the census are beginning to cause alarm as to the future destinies even of our own once proudly fecund race. It is a wonderful record for a single individual—to have weighed down China—and when Mr. Buckton remarks that facts like these regarding the prolific nature of Aphides “afford sufficient explanation of the occurrence of the extraordinary swarms so often noticed by authors,”[[37]] nobody is likely to disagree with him. With billions a certainty, and quintillions in the air, swarms seem amply accounted for.
One of the authors here alluded to is our homely immortal, White of Selborne. “I shall here mention,” he says, “an emigration of small Aphides, which was observed in the village of Selborne no longer ago than August 1st, 1785. At about three o’clock in the afternoon of that day, which was very hot, the people of this village were surprised by a shower of Aphides, or smother-flies, which fell in these parts. Those that were walking in the streets at that juncture found themselves covered with these insects, which settled, also, on the hedges and gardens, blackening all the vegetables where they alighted. My annuals were discoloured with them, and the stalks of a bed of onions were quite coated over for six days after. These armies were then, no doubt, in a state of emigration and shifting their quarters; and might have come, as far as we know, from the great hop-plantations of Kent or Sussex, the wind being, all that day, in the easterly quarter. They were observed, at the same time, in great clouds about Farnham, and all along the vale from Farnham to Alton.”[[38]]
Other great migrations of Aphides have at various times been observed. In the autumn of 1834 the city of Gand was invaded, and, one may almost say, taken by a vast army of them, and at Bruges and Antwerp the same swarm is said to have darkened the sun,[[39]] a result of such gatherings more noticeable elsewhere than in England, since our sun usually is darkened. Insects, though their movements are not so regular, nor, as a rule, so noticeable as those of birds, yet often migrate—how often or how regularly it is difficult to say. Locusts are, of course, the stock example as well as the most terrific one, but perhaps dragon-flies, were they as destructive, would have been as much noted in this connexion. Their migrations seem to be tolerably frequent, and a record of them between 1494 and 1868 has been published by Koppen, a German entomologist. In 1881 a great flight of them took place in Illinois. “The air,” we are told, “for miles around seemed literally alive with these dragon-flies, from a foot above ground to as far as eye could reach, all flying in the same direction, a south-westerly course, and the few that would occasionally cross the track of the majority could all the more easily be noticed from the very regular and swift course they generally pursued; but even these few stray ones would soon fall in with the rest again. Very few were seen alighting and all carefully avoided any movable obstacles.”[[40]] This migration took place during a very dry season, and may have been caused by it owing to the drying up of swamps, ponds, etc., in which the insects would otherwise have laid their eggs, obliging them to seek other suitable places.
In the spring of 1900 a great migration of dragon-flies was observed in Belgium. “All the observers agree that the insects flew rather low, with astonishing regularity, and without resting; that they kept close to the earth, where there were no obstacles, but that they mounted to a height of 10 to 12 mètres when houses or trees were in the way. They did not go round obstacles in their line of route, but surmounted them, and descended on the other side. According to some observers, their flight was very slow, others again asserting that it was very swift. When the velocity could be estimated, however, it was found to be at 5 mètres per second or 18 kilometres (11¼ miles) per hour (so that the slows have it). In general they went in groups, more or less isolated, and more or less dense.” The writer of the above account—a Belgian—concludes thus: “All the facts point to the following conclusions: The dragon-flies of the 5th came from regions situated to the east of the country, which they entered in several columns, flying at a great altitude: between 7 and 8 a.m. they descended towards the earth, continuing their route towards the west. But we remain in ignorance of their point of departure. The swarm probably quitted its usual habitation early in the morning, and immediately flew to a great height. It was only on arriving near the earth that they flew against the wind”[[41]] (which, however, they then continued to do).
What Mr. Hudson calls “dragon-fly storms” are a special phenomenon of the Pampas. In this case the cause of the migration—for such movements seem to come under this heading—is a special wind called the pampero, that blows south-west from the interior of the Pampas. It is very violent, cold, and dry, and the dragon-flies evidently fear it. The “storm” is thus described by Mr. Hudson: “It is in summer and autumn that the large dragon-flies appear; not with the wind, but—and this is the most curious part of the matter—in advance of it; and inasmuch as these insects are not seen in the country at other times, and frequently appear in seasons of prolonged drought, when all the marshes and watercourses for many hundreds of miles are dry, they must, of course, traverse immense distances, flying before the wind at a speed of 70 or 80 miles an hour. On some occasions they appear almost simultaneously with the wind, going by like a flash, and instantly disappearing from sight. You have scarcely time to see them before the wind strikes you. As a rule, however, they make their appearance from 5 to 15 minutes before the wind strikes; and when they are in great numbers, the air, to a height of 10 or 12 feet above the surface of the ground, is all at once seen to be full of them, rushing past with extraordinary velocity in a north-easterly direction. In very oppressive weather, and when the swiftly advancing pampero brings no moving mountains of mingled cloud and dust, and is, consequently, not expected, the sudden apparition of the dragon-fly is a most welcome one, for then an immediate burst of cold wind is confidently looked for. In the expressive vernacular of the gauchos the large dragon-fly is called ‘hijo del pampero,’ son of the south-west wind.”[[42]]