Peacock Butterfly.
We have dwelt as long as our limits will permit on this wonderful apparatus of vision in insects: let us now turn our thoughts for a little space to a pair of organs which are, perhaps, of all others the most characteristic of the insect tribes: these are the Antennæ. We need scarcely explain what these are, or point them out to the reader, since they are so familiar to most persons, but under the erroneous, or, at any rate, the questionable expression—"the feelers." We have an excellent specimen of antennæ in those of the butterflies, in which insects they are very long and elegant organs. They are attached to the head by means of a beautiful joint, called a ball and socket, the same as is often adapted to garden watering engine-pipes, so as to enable them to be moved about in all directions. They are never found absent in the perfect insect, and are manifestly organs of considerable importance in the insect economy. As to their structure,—when examined under a microscope, antennæ are found to be composed of a variable number of small round pieces, covered with a horny or leathery skin, but within being softer and hollow, so as to form a series of tubes placed end to end, all connected together in such a manner as to admit of free movement between each joint, so that the insect can bend them into any shape it may find expedient for its purposes. But it must not be supposed that all antennæ are similar in form, or there would be a risk of their being continually mistaken by the reader for some other organ. The cut on the next page will, at a glance, exhibit the astonishing variety of form assumed by these organs. The common cockchafer is possessed of antennæ as strikingly different in form from those of the butterfly, as if they were really distinct organs intended and adapted to serve different purposes. Some are long and thread-like, some resemble a necklace of pearls, some are notched like a saw, some have a resemblance to a fan, some are like a club, some resemble a fork, and some a feather; in short, their variety of form is almost endless: and they differ as much in length and in thickness; for while the antennæ of some insects are as long as, or even longer than, their bodies, those of others are very short, and little prominent; and while some are as thin and fine as the finest hair, others are nearly as thick as the true body of the insect. Let us now ask what is the use of these singular organs of the insect?
Various Antennæ.
From a hint which has before been dropped, it is probable that the answer to this question will be readily anticipated. Various facts appear to show that they may be the insect's ears! "Can it be that such is their function?" some may exclaim, calling to mind that ears in other creatures are so different in appearance and structure from these organs. There have been many opinions on the subject, and some have maintained views completely opposed to this idea. But the greater number of entomologists seem to consider this as their most probable function. If they are not ears, we are not able to point to any other organs in the head which are. From a number of facts it is certain, that insects can hear; from which it is equally certain, that they must possess special organs. We may, therefore, until the contrary is shown with respect to them, fairly be allowed to consider the antennæ as these organs. The opinions of Messrs. Kirby and Spence on this point are conveyed in the following words:—"They conceive that antennæ by a peculiar structure may collect notices from the atmosphere, receive touches or vibrations, and communicate them to the sensorium (or brain), which, though not precisely to be called hearing, may answer the same purpose." The late celebrated Dr. W. F. Erichson of Berlin, by means of the microscope, has discovered in all insects, and especially in those most remarkable for their powers of smelling, that these organs are furnished with a number of fine pores, clothed with membrane, which he regards as organs of smell.[U]
Every one has heard of M. Pelisson, the prisoner at the Bastile, and his spider companions of the cell. This gentleman had, by patient training, engaged the attention of a spider, which used every day to come and listen to a solo on his violin, after which it would make its retreat again to its home. Now it is clear, that this little insect not only heard, but even enjoyed, the notes of his instrument; which would have been impossible, had it possessed no ears. Then again, every one must have noticed the start which a fly will make if some sharp noise be made near it, and how actively it will move its legs, and seem to be put all on the qui vive. A little every-day observation will furnish us with many similar instances. "A little moth," writes one of the last-mentioned authors, "was reposing on my window; I made a quiet, not loud, but distinct noise; the antenna nearest to me immediately moved towards me. I repeated the noise at least a dozen times, and it was followed every time by the same motion of that organ; till at length the insect, being alarmed, became more agitated and violent in its motions. In this instance it could not be touch; since the antenna was not applied to a surface, but directed towards the quarter from which the sound came, as if to listen." Can we question that the merry grasshopper, chirping all day in the field, is heard by its mate? or that the cricket on the hearth sings for its fellows, or only to please our ears? or does the tap of the death-tick beetle, formerly commemorated, draw forth no answering tap from its companion on the other side of the post? All these instances, and countless more that might be mentioned, indicate, beyond a question, that insects have the faculty of hearing, and render it extremely probable that the antennæ are the organs by which this function is performed.
But it is probable that they serve another and not less important purpose. The country folk, who are generally very weather-wise, and whose sayings always deserve a great deal of attention, because they are very commonly founded both upon observation of facts, and experience, will always assure us that the high flight of swallows in their search after insects is a certain token of fair weather; their low flight of approaching wet. Why is this? Can insects foretell the state of the weather? and if so, by what means? That insects have a meteorometrical power of foretelling the state of the weather, appears beyond a doubt. To select a few instances. Bees, our exemplars in so many points, might be consulted with advantage, and would often give us better information as to whether an over-coat or an umbrella would be probably required, than the not always trustworthy barometer. When engaged in their daily labours, if a change of weather to rain is at hand, although the sun may yet shine and the sky remain clear and calm, they foresee it, and return suddenly to their hives. Hence in weather of an unsettled description they never undertake journeys far from home, but make short excursions to and from their hives, being never absent long at a time; and if we find a bee wandering far from its hive, it may be taken, on the other hand, as a pretty sure prognostic that top-coat and umbrella may be left at home for that day at least. Ants, as we have already noticed, possess the same faculty of weather-wisdom, never allowing their larvæ to be caught in a shower. The abundance of insects in our houses in wet weather, is a familiar observation; we are assured that in hot countries their excessive numbers, during the rainy season, make these little creatures almost as terrible as a plague to all who are within—climbing up the walls, scrambling into the dishes, drowning themselves in the soup-tureens, or putting out the lights with their bodies. In hot and sultry weather, when a black cloud or two in the far horizon give token of a coming storm, insects abound in the air; but as the heavens grow black, they disappear; and before a drop of rain has actually fallen, we may look in vain for them.
That the antennæ are the organs by which this knowledge of coming events with regard to the weather is obtained by the insects, appears probable. They may, perhaps, perceive by their delicate surfaces, changes in the electrical condition of the air, which are quite insensible to ourselves: and as all change of weather is preceded and accompanied by changes of electrical balance of one kind or another in the air, they may be thus forewarned of the coming danger. The following is the manœuvre of a large number of insects of the beetle tribe. When they are about to move from any station where they have been at rest, the first thing they usually do before they move a step, is to bring forward and expand their antennæ, which have previously been folded up out of the way. They open them as widely as possible, so as to expose them to all the influences of the air, and then, if they be satisfied, they unfold their wings and take their flight. We might compare this proceeding with that of one of ourselves looking up at the direction of the wind, or tapping the weather-glass in the hall, previous to setting out for the day! Too much stress, however, it must be added, is not to be laid upon this point; for while it is unquestionable that insects have knowledge of the weather, it is of course very difficult to ascertain with certainty whether it is by means of their antennæ, or by the general feeling of their bodies; analogous, perhaps, to what rheumatic persons feel when the wind goes round to the north-east.
There is a still more singular use of the antennæ, which deserves to be mentioned. These organs appear to be the principal instruments of speech, if we may use such a term, or, at any rate, of the communication of intelligence. When the soldier ants go out upon their expeditions, and have left the nest, previously to setting off, they touch each other with their antennæ and forehead on the trunk; and this is their signal for marching, for as soon as ever a soldier feels this tap, he immediately puts himself in motion. When, also, they have any discovery to communicate, they strike with them those that they meet, in a particularly impressive manner; and if a hungry ant wants to be fed, it touches with its two antennæ, moving them very rapidly, those of the individual from which it expects its meal. They appear to salute one another by gently tapping each other's antennæ, as we should shake hands on meeting an old friend!