Butterflies and Moths (British). Furneaux William Samuel
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4. – Section of the Eye of an Insect.
A glance at the section of a compound eye will show you that all the little cones radiate from a common centre. And, as each little compartment is surrounded by opaque colouring matter, it is clear that perpendicular rays only are capable of penetrating to its base and exciting the nerve fibre that lies there. Thus each little division of a compound eye forms its own image of the object that happens to be exactly opposite its facet. But how many facets do we find in a single eye? Sometimes only a few hundreds, but sometimes as many as seventeen or eighteen thousand! We must not, however, conclude that the nature of the vision of butterflies and moths is necessarily very different from our own. We have two eyes, but the images formed by them are both blended, so that we do not see double. We can understand, therefore, that the thousands of images formed in a single eye may be blended together so as to form one continuous picture. Still there remains this difference: while in our own case the two images formed by the two eyes are practically the same, in the case of insects every one of the little conical tubes of a compound eye forms an image of an object that cannot possibly be formed by any one of the others. Thus, if the lepidopterous insect sees a continuous picture of its surroundings, such a picture is produced by the overlapping and blending, at their edges, of hundreds or thousands of distinct parts.
There is yet another interesting difference between the vision of these insects and that of ourselves. As already stated, our two eyes are both turned toward the same point at the same time. But look at the butterfly's eyes. Here are no movable eyeballs, and the two eyes, placed as they are at the sides of the head, are always turned in opposite directions. The corneæ, too, are very convex; and consequently the range of vision is vastly wider than ours. A boy is often easily surprised by a playmate who approaches him stealthily from behind, but did you ever try the same game with a butterfly? I have, many a time. After getting cautiously so near to a butterfly at rest as to be able to distinguish between its head and its hinder extremity, I have quietly circled round it so as to approach it from behind, being at the time under the impression that it wouldn't see me under those circumstances. But not the slightest advantage did I derive from this stratagem, for the position and construction of its eyes enabled it to see almost all ways at once.
In addition to the two compound eyes, the Lepidoptera, or at least most of them, are provided with two small simple eyes; but these are generally so hidden among the closely set hair that covers the head, that it is doubtful whether they are of much service as organs of vision.
Fig. 5. – Antennæ of Butterflies.
Fig. 6. – Antennæ of Moths.
The antennæ proceed from two points close to the upper borders of the eyes. They are jointed organs, and are of very different forms in the various species of Lepidoptera. They are generally long, slender, and clubbed at the extremity in butterflies, but exhibit several minor points of difference which we shall have to note later on. In moths the antennæ are sometimes long, slender, and pointed. Some are thick, and more or less prismatic in form; while others are slightly or deeply pectinated or comb-like. The antennæ of butterflies are always straight, or only slightly curved; and, although the insects can sway them bodily, they have no power to bend them, or to stow them away in any place of shelter. Moths, on the other hand, when at rest, are almost invariably found to have their antennæ snugly tucked under the wings, and brought so closely against the side of the head for this purpose that even the uncovered portion is often difficult to find.
There are two other prominent appendages belonging to the heads of the Lepidoptera. These are the labial palpi or feelers of the lips. They are generally easily seen, projecting forward on the under side of the head, sometimes so long and conspicuous as to give one the idea of a snout or long nose. The palpi are jointed – usually in three parts – are covered with scales, and often furnished with hairs or bristles.
Fig. 7. – Section of the Proboscis of a Butterfly.
If you watch a moth or butterfly when it is feeding on the sweet juices of a flower, or on some kind of artificial sweet with which you have provided it, you will observe its long trunk or proboscis, by which food is sucked up. This instrument is so long and slender that it seems almost impossible that it can be a tube through which a liquid freely passes. But a careful examination will show that this is the case. It is composed of two separate pieces – two half tubes, which, when closely applied to each other, form a very thin and flexible pipe, perfectly air-tight and adapted for suction. Sometimes you can see a butterfly or moth manipulating with its proboscis as if it required readjustment in some way or other. It has split the tube throughout its length, so that it now looks like two exceedingly fine hairs. Then, after a short time, the two halves are put together again, and immediately, as if by magic, become a single tube in which no kind of seam is to be observed without a powerful magnifier.
In order to observe the nature of such a wonderful arrangement we must have recourse to the aid of a good microscope. Thus assisted, we can see at once how the junction of the two sides of the proboscis is brought about so quickly and so perfectly. The inner edges of each half are very regularly fringed with lines of closely set hairs – so regular, in fact, are they, that they give one the idea of long yet minute beautifully formed combs. When the two parts are brought together, the hairs of two opposite edges interlock, those on one side exactly filling the spaces between those of the other.
The microscope also reveals another interesting fact, viz. that the proboscis is not a single tube, but, although so remarkably thin, is really a set of three distinct pipes, one lying on each side of the central one. It is said that the central tube only is used for sucking up the liquid food, and there seems to be some doubt as to the uses of the other two. Some naturalists are of opinion that the latter are air tubes, and are connected with the respiration of the insect; while others say that through these the insects eject a thin watery fluid with which to dissolve or dilute those sweetmeats that are not sufficiently liquid to be readily sucked up. But possibly both these opinions are correct, the proboscis serving all three of the purposes here named. The only observation of my own bearing on the subject is this. While a moth was feeding on a drop of syrup in a strong light, a powerful lens revealed drops, of liquid, mingled with bubbles of air, passing alternately up and down the two lateral tubes of the proboscis. At the same time the upward current of syrup in the central tube was by no means steady and continuous.
When this organ is not in use, it is beautifully coiled into a close spiral which lies between the labial palpi. The length varies considerably in different insects, and consequently the number of turns in the spiral must differ also. Sometimes there are less than two turns, while some of the longer ones form spirals of from six to ten turns.
In concluding our brief account of the head of lepidopterous insects it is, I suppose, hardly necessary to add that there is no kind of chewing apparatus to be described; all the members of this order, at least in the perfect state, deriving the whole of the little nourishment they require entirely by suction through the proboscis or 'trunk.'
The second division of the body is the thorax. This is much larger than the head, and consists of three ring-like segments, joined one behind the other so intimately that the lines of junction are hardly visible, even after the thick clothing of fine hair has been brushed off. Behind the thorax is the abdomen, which is composed of several segments, the junctions between the rings often being most distinct.
Fig. 8. – Diagram of the Wings of a Butterfly.
I. Fore wing.– 1-5, subcostal nervules; 6, 7, discoidal nervules; 8-10, median nervules; 11, submedian nervure; 12, internal nervure; 13-15, disco-cellular nervules; 16, interno-median nervule; 17, median nervure; 18, subcostal nervure; a, costal nervure; b, costa or anterior margin; c, apex or anterior angle; d, posterior or hind margin; e, posterior or anal angle; f, interior or inner margin; g, base; h, discoidal cell.
II. Hind wing.– 1, 2, subcostal nervules; 3, discoidal nervule; 4-6,