THE "INSECT YOUTH."
THE " insect youth," as the poet Gray in his " Ode on the Spring" respectfully calls the mayflies, have been much less "eager to taste the honied spring, And float amid the liquid noon," than could be desired. The poet remarks that—
"To Contemplation's sober eye Such is the race of Man ; And they that creep, and they that fly, Shall end where they began."
But there is a chorus of complaint this year from people devoted to the Contemplative Man's Recreation that they (the mayflies) never began at all. Perhaps a spell of warm weather may yet evoke the broods ; but so far they have been rare, irregular, and capricious. There are many rivers holding the best trout in England, and rented, when not owned by the fisherman, at prices which are the best evidence of the fascination of fly-fishing, on which the rise of the mayfly gives the only chance in the year of making large baskets, and of enjoying many hours of sport on the same day. There are a few other streams, such as some of the Upper Thames tributaries, like the Windrush, where the trout scarcely ever rise except to the mayfly. The failure of these ephemerae to appear takes the backbone out of the season. It is the great disappointment of the year.
The ephemerae are such delicate creatures, so tender and fragile, that compared with them a butterfly is of cast- iron. It cannot excite wonder that matters sometimes go amiss with the hatch, or that there are seasons when " their airy dance" fails to take place because the sun has failed to issue his invitations in due form. Their life history is simple enough. The gauzy, floating ephemera lays her eggs upon the water, and then becomes a " spent gnat," in fisherman's phrase, and dies. The eggs hatch and become tiny larvae, which descend to the bottom, and in the mud go through such periods of growth as bring them to the point at which they incline to rise to the surface. Even then they assume two forms before developing into the perfect creature, whose life, after all these metamorphoses, is often not longer than a day.
Modern changes in the drainage of the land, and the dis- turbance to the natural condition of river-beds, are felt more in the South than in the Northern parts of England. Among the results of this, and of other improved methods of agricul- ture, are the floods caused by the rapid throwing off of the land water, the violent alterations of level, and the scooping out of the mud and gravel at the bottom and sides of the streams. The delicate balance of Nature is consequently upset. If the owners of valuable trout streams were to compare notes as to the disappearance of " fly," or the survival of some forms while others become scarce, they could scarcely fail to discover the particular causes of these changes. In the present season the cold of April and the abnormal floods of May were sufficient in themselves to affect the more delicate subaqueous life of the gently flowing rivers of the South. In Cumberland and Westmoreland the rise of fly, other than mayfly, which is not native to those waters, was so scarce and fitful that the season has been out of joint from first to last.
The effect of temperature in hastening or retarding the development of insects from the pupa stage to the perfect creature is more remarkable than any other results of heat or cold, even on the life of plants. Reaumur was among the first to prove this. He found that in the hothouses of certain gardens in Paris he could in some cases cause a pupa to develop into a perfect insect in ten or twelve days. By diminishing the heat the same class of pupae took three, four, five, or six weeks to develop ; yet in each case the butterflies were perfect and laid eggs. He also took pupae which would naturally have hatched in the warm days of July, and prevented their undergoing any change at all by placing them in unnaturally cold surroundings. These remained torpid through the winter and spring of the year following. Apparently this postpone- ment of development may be produced by other causes. It was stated that of thirty-six caterpillars of the spotted muslin moth, which all spun their cocoons in the normal way, one third developed into butterflies in due course. The rest were supposed to be dead, but were left undisturbed. In the second season some of these developed, and the remainder in the third.
That many insects are quite aware of the effect of temperature on the immature generation is evident from the precautions which they take to secure favourable condi- tions for the brood. Bees regulate the heat of the hive by ventilation, using their wings as air-fans. Possibly, also, the widening or blocking of the entrance with wax is done in part with the same object. Ants are as fussy and particular about the temperature to which their eggs are exposed as if they were cooking them. They wet the eggs, as a careful gamekeeper would do. They pile them in heaps, which no doubt generates warmth, and take the stacks to pieces when they deem it necessary to let the air have access to them. They also move them about from one chamber to another, probably knowing that there is a difference in the heat of these various cellars. They do this from hour to hour, as the day becomes warm or cold. If there is a burst of hot sun, they will carry many of the eggs out, and leave them on the surface to be warmed by the direct rays. If clouds appear, or a shower falls, they hurry them all down again underground. The purpose of all this attention, both in the ante and the bees, is obvious. Their young, when they do develop, are not like the
mayflies or cockchafers, mere unattached, irresponsible items. They have to form a swarm, and to take part in a communal enterprise, which includes a journey, for which each and every one must be physically fit on or about the day on which they emerge from the pupa-case. This is especially the case with the bees, where the rivalry of the queens makes instant departure imperative. We have known a swarm of bees go straight off, each and every insect keeping the same rate of speed, which was rather deliberate, for a distance of considerably more than a mile, till they came to a hollow tree, to which they, by some instinct, knew the way. The ant swarms of England under- take less considerable journeys. But sometimes the swarms of a whole neighbourhood seem to assemble together, and fill the air in crowded myriads, covering treee, roads, gates, and palings, and dusting the water with their bodies. This shows that exactly the same tactics of warming and refrigerating must have been carried on, not in one or two nests, but in a great number, scattered promiscuously and with no communica- tion, otherwise the exit of the perfect ants would not have been simultaneous.
What is more surprising than the simultaneous trans- formation of the encased motionless pupae into winged and active insects after the care and attendance which they receive in hives or anthills is the scarcely less simultaneous development of other insects which during the intermediate state have received no attention whatever, and have been as much at the mercy of chance as seeds scattered on the ground. During the few warm days which marked the month of May the tops of the elm and other trees were seen in places to be surrounded with what looked like a cap of smoke. This proved to be composed of thousands of cockchafers flying in a swarm round the upper boughs. These insects, which appeared on the same day and almost at the same hour, are said to have spent at least two years underground in various stages as grub and pupa. The swarms of beetles which appear on the heaths of Norfolk and Suffolk must be due, as in the case of the cockchafers, to practically identical conditions and surround. ings, affecting both grubs and pupae at the same time. These beetle swarms are the nearest approach to a flight of locusts ever seen in this country. Some years ago on the great warren near Thetford the air was fall of small beetles, which came down like rain. They covered the carriage, harness, and horse, and gave to the telegraph-wires the appearance of having been dipped in soot.
The enormous reproductiveness of insect life suggests the possibility of artificially multiplying species which it may be desirable to propagate as food for fish. There is very little doubt that this could be done were the conditions of their life fully understood. Those writers who, like Mr. Halford, have studied the life history of the flies on which trout feed could probably supply much of the data needful for their artificial propagation, together with facts as to experiments which have been already made. But the dominant factor of temperature would always have to be reckoned with, and a season like the spring of 1003 would probably be a failure, in spite of any precautions which human ingenuity could devise.
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