Читать книгу Intelligence in Plants and Animals - Thomas G. Gentry - Страница 16

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Closely allied to the bugs is a group of remarkable insects to which naturalists now apply the name of Cicada, but which are generally, though improperly, designated Locust by the common people. They are readily distinguished by their broad heads, large prominent eyes, with three eyelets triangularly placed between them, and delicately transparent, veined wing-covers and wings. The abdomen is short and pointed, and the legs are short, the anterior femora being much thickened and toothed beneath. The hinder extremity of the body of the female is conical, and the under-side has a longitudinal channel for the reception of the ovipositor, or piercer, which is furthermore protected by four short-grooved pieces which are immovably fixed to the sides of the channel. The piercer itself consists of two outer parts grooved on the inside and slightly enlarged and angular at the tips, which are externally beset with small saw-like teeth, and a central spear-pointed borer which plays between the other two, thus combining the advantages of an awl and a double-edged saw, or rather of two key-hole saws cutting opposite to each other. A hard, horny substance, called chitine, the same as exists in the stings of bees and wasps, is the material of its composition. It would be impossible to conceive of anything more exactly fitted for its required uses than is this beautiful complicated instrument.

But the most peculiar characteristic of this family, however, consists in the structure of the mechanism by which the males make the trilling sound for which they have been so long famous. In the male of the Seventeen-year Cicada the musical instrument consists of two stretched membranes, one on each side of the body, which are plainly to be seen immediately behind the wings. These membranes are gathered into numerous fine plaits, and are played upon by muscles or cords fastened to their under surfaces. When these muscles contract and relax, which they do with great rapidity, the drum-heads, which the membranes resemble, are alternately tightened and loosened, the effect of this alternate tension and relaxation being the production of a rattling sound very much like that caused by a succession of quick pressures upon a slightly complex and elastic piece of tin-plate. Certain cavities within the body of the insect, which may be seen on raising two large valves beneath the abdomen, and which are separated from each other by thin transparent partitions of the brilliancy of mica or highly polished glass, tend to increase the intensity of the sound.

In the winged state Cicada septendecim, as the subject of our sketch was named by the immortal Linnæus, is of a black color, with transparent wings and wing-covers, the thick anterior edge and veins of which being orange-red. Near the tips of the latter there is a dusky zig-zag line which resembles in shape the letter W. The eyes, when living, are also red, while the legs are a dull orange, which color is conspicuous along the edges of the rings of the body. The wings expand from two and a half to three and a quarter inches.

SEVENTEEN-YEAR CICADA.

Adult, Chrysalis-Case, Pupa, Entrances to Burrows and Egg-Nests.

About the middle of June the perfect insects make their appearance, and as they generally come in large numbers they do a great deal of damage. In some localities they congregate to such an extent upon the trees as to bend and even to break down the limbs by their weight. The din of their discordant drums resounds in the woods and orchards from morning to evening. As their life is of rather short duration, not lasting for a longer period than a month, they soon begin to pair, and it is not long afterwards that the females may be seen preparing nests for the reception of their eggs. Branches of moderate size are selected for this purpose. Their manner of perforation is curious and interesting. Clasping the branch on both sides with their legs, and bending the ovipositor at an angle of forty-five degrees, they repeatedly thrust it into the bark and wood in the direction of the fibres, at the same time setting the lateral saws at work, thereby detaching little splinters of wood at one end, which are intended to serve as a kind of fibrous cover for the nest. The hole is bored obliquely to the pith, and by a repetition of the same operation is gradually enlarged until is formed a longitudinal fissure of sufficient extent to receive from ten to twenty eggs. The side-pieces of the piercer act as a groove to convey the eggs to the nest, where they are deposited in pairs, but separated from each other by a narrow strip of wood. When two eggs have been thus placed, the piercer is withdrawn for a moment, and then inserted till two more eggs are dropped in a line with the first, and thus the operation is repeated until the fissure has been filled, when the insect removes to a little distance and commences to make another nest to contain two more rows of eggs. It takes about fifteen minutes to prepare a groove and fill it with eggs. As many as twenty grooves are sometimes made in a branch by a single insect, and when the limb has been sufficiently stocked she goes from it to another, or from tree to tree, until she has got rid of her complement of from five hundred to seven hundred eggs. So weak does she at length become, in her continued endeavor to provide for the succession of her race, as to fall, in an attempt to fly, an almost lifeless lump to the earth, where her spirit soon goes out never more to enliven its frail house of clay.

Although Cicadas abound most upon the oaks, yet there seem to be no trees or shrubs that are exempt from their attacks, unless it be the various species of pines and firs. The punctured limbs languish and die soon after the eggs are laid, and as often happens are broken off by the winds; but when this is the case the eggs never hatch, for the moisture of the living branch seems necessary for their proper development.

The eggs are one-twelfth of an inch in length, and one-sixteenth of an inch through the middle, but taper to an obtuse point at each end. They are of a pearl-white color. The shell is so thin and delicate that the form of the inclosed insect can be seen before the egg is hatched. One writer claims that fifty-two days, and others that fourteen days, constitute the period required for the hatching of the egg.

When it bursts the shell the young insect is one-sixteenth of an inch long, and is of a yellowish-white color, excepting the eyes and the claws of the fore-legs, which are reddish. It is clothed with small hairs. In form it is grub-like, larger proportionally than the parent, and provided with six legs, the first pair being very large, shaped like lobster-claws, and armed beneath with strong spines. Little prominences take the place of wings, and under the breast is a long beak for suction. Its movements, after leaving the egg, are very lively, and nearly as quick as some of the ants. But after a few moments their instincts prompt them to reach the ground. They do not attain this end by descending the body of the tree, nor by casting themselves off precipitately, but, running to the side of the limb, deliberately loosen their hold and drop to the ground, making the perilous descent with the utmost safety. This seems almost incredible, but it has been repeatedly observed by scores of honest witnesses.

NEW-BORN CICADA.

Line Below Shows Natural Size.

On reaching the ground the young insects immediately burrow their way into the soil, using their broad and strong fore-feet pretty much after the fashion of the mole. They apparently follow, in their descent, the roots of plants, fastening their beaks into the most tender and succulent, and thus imbibing their juices, which constitute their sole aliment. They do not descend very deeply into the ground, probably not more than ten or twelve inches, although accounts have been published of their discovery at a depth of ten or twelve feet, but their occurrence at such great distances from the top of the ground is doubtless the result of accident.

The only alteration to which the insects are subject during the seventeen years of their subterranean confinement, is an increase in size, and the more complete development of the four small scale-like prominences of the back, which contain their future wings.

When the time of its transformation draws near, the larva, in which stage the insect passes the greater part of its existence, works its way up towards the surface, oftentimes in a very circuitous manner, for local changes make it necessary for it to bore through hard woods and between stones well beaten down. The burrow which it thus produces is cylindrical, about five-eighths of an inch in diameter, and firmly cemented and varnished so as to be water-proof. The upper portion, to the extent of five or six inches, is empty, and serves as a habitation till the period of its exit arrives, while the lower is filled with earthy matter removed by the insect in its progress. In this cell it remains during several days, ascending to the top for the benefit of the sunshine and air when the weather is auspicious, even venturing to peep forth occasionally, but descending on the occurrence of cold or wet weather. But when the favorable moment to leave their subterranean retreats arrives, the Cicada-grubs, or more properly pupæ, for such they are now to be considered, although they still retain something of the grub-like form, issue from the ground in great numbers as evening draws on, crawl up the trunks of trees, the stems of herbaceous plants, or on to whatever is convenient, which they grasp securely with their claws. After resting awhile, their skins, which have become dry and of an amber color, are by repeated exertions rent along the back, and through the slit formed the included Cicada pushes its head and body, and withdraws its wings and legs from their separate cases, and, crawling to a short distance, leaves its empty pupa-case fastened to the tree. At first the wing-covers and wings are small and opaque, but in a few hours they acquire their natural size and shape. It is not, however, for three or four days that the muscles harden sufficiently for them to assume their characteristic flight. The males make their appearance some days in advance of the females, and also disappear sooner. During several successive nights the pupæ continue to issue from the ground, and in some places, as was the case in May of 1868, when these insects appeared in great numbers in the vicinity of Philadelphia, the whole surface of the soil was made by their operations to assume a honey-combed appearance.

DOME-LIKE HOUSE OF CICADA.

Longitudinal Section Showing Pupa in Two Positions.

In localities where the soil is low and swampy, a remarkable chamber is built up by the larva, where the pupa may be found awaiting the time of its change to the winged state. These chambers were first noticed by S. S. Rathvon, at Lancaster, Pa., and are from four to six inches above the ground, and have a diameter of one inch and a quarter. When ready to emerge the insect backs down to an opening which is left in the side of the structure on a level with the surface of the ground, issues forth and undergoes its transformation in the usual manner. This peculiar habit of nest-building, which is so unlike what is customary with the Cicadidæ, or with Hemiptera in general, points to a high degree of intelligence among these insects, showing a remarkable ability to adapt themselves to environing circumstances. Undue moisture would be prejudicial to the pupa, as the larva seemed to know, through the guidance of the same dumb and unerring instinct which teaches it to cement its underground dwelling, but would that same instinct teach it to construct so wonderful a dome-like house as the one described for the preservation of its after-life, and one so eminently fitted by its position, shape, size and entrance to secure the necessary shelter, warmth and air for its protection and development? I apprehend not. Nothing short of a reason, similar to that in man, but differing in degree, would enable it to grasp the situation in which it found itself to be placed when nearing its final change, and plan with the view of carrying out the ultimate aim of its existence.

Fortunately, these insects are appointed to return at periods so distant that vegetation has a chance to recover from the injuries which they inflict. Were they to appear at shorter intervals, our forest- and fruit-trees would be entirely destroyed by them. They are, moreover, subject to many accidents, and have many enemies, which contribute to diminish their numbers. Their eggs are eaten by birds, and the young, when they leave the egg, are preyed upon by ants, who mount the trees for that purpose, or take them upon the ground as they are about to enter upon their protracted larval career. Blackbirds eat them in the spring when turned up by the plough, and hogs, when allowed to run at large in the woods, root them up and devour large numbers, especially just before the arrival of the period of their final transformation, when they are lodged only a few inches below the surface of the soil. Many perish in the egg by the closing up of the bark and wood that constitute the walls of the perforations, thus burying the eggs before they have hatched, and others, no doubt, are killed by their perilous descent from the trees.

As its name implies, this insect generally requires seventeen years to complete its transformations, a fact that was first pointed out many years ago by the botanist Kalm. The late Prof. Riley, who had given this species a great deal of study, was the first to work out the problem of its periodical returns. He found that there are also thirteen-year broods, and that both sometimes occur in the same locality, but that in general terms the thirteen-year brood might be called the southern form, and the seventeen-year the northern form. At the limits of their respective ranges these broods overlap each other. The shorter-lived form he named provisionally Cicada tredecim. It was the existence of this brood that led entomologists to doubt the propriety of Linné’s name, because, in calculating each appearance as occurring in any locality at the end of every seventeen years, they could not make the dates of its periodical returns correct. But it was Prof. Riley that cleared up the matter. It happened in the summer of 1868 that one of the largest seventeen-year broods occurred simultaneously with one of the largest thirteen-year broods. Such an event, so far as these two particular broods are concerned, has not taken place since 1647, nor will it take place again till the year 2089. There are absolutely no specific differences between the two broods other than in the time of maturing. There is, however, a dimorphous form that appears with both these broods. It is smaller, of a much darker color, has an entirely different voice, appears a fortnight sooner, and is never known to pair with the ordinary form. Dr. J. C. Fisher, in 1851, described it as Cicada cassinii, but the specific differences are not sufficiently well defined to entitle it to rank as a species.