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NOTES ON BUTTERFLIES.
BY THE Rev. C. HOPE-ROBERTSON, F.R.M.S.
MHESE lovely creatures give as much pleasure to the student
I of science as to the child at play, who runs, cap in band, to chase them. Their colours, forms, and habits, all furnish charming subjects for observation. We propose to collect some notes taken at intervals on these points, and show the methods available for easily exhibiting their interesting peculiarities.
We begin with the distinction which serves, in general, to divide butterflies from moths, which is the existence of a clubshaped end to the antennæ of the butterfly; while the moth has sharp-pointed or plumed antenne. From this club-shaped end comes the generic word Rhopalocera, compounded of the two Greek words for a club and horn. Observation has found this distinction characteristic of the butterfly tribe; we wish further to see what there is in the habits of the creatures to explain the reason why they have such horns.
If we look at one on a bright sunny day we will see its horns erect, stretched out, delighting in the sunshine. While, if we look at a moth, which flies chiefly at dusky hours, when it is brought to the light its horns are folded back, and doubled as far as possible under shade of its body or wings, as if unable to bear the glare. They are evidently most sensitive to light.
Now this is a key to one purpose which the horns serve. They are the thermometers of these little weather-watchers. The degree of warmth needed for them is made known by the effect of light on the delicate tips of their horns or feelers. A club-shaped feeler gives a larger surface for the light to act upon; and the nervous energy needed for the butterfly to enable its muscles to work, is collected by the club-shaped points, and its general influence stored up to sustain flight against the effect of a chill. Cold would paralyse them. Now if we take the microscope to examine the structure of the clubpoints, these are found to be full of cells, or depressions, where the heat and electric energy, accompanying light, may be
r the glare under shats horns are elig at dus
me off like of these diversified stalk, to the me
collected and transmitted to the system for use. Some of these are figured at A, Plate LXIX.
The butterfly tribe is one of the order Lepidoptera, or scalewinged insects, so called from the multitude of little scales with which the wings are covered.
These are of various forms, colours, and textures, sufficiently distinct to mark the species, with a wonderful variety on the different parts of the wing. The shapes are clear and compact on all the body of the wing; while at the edges there are longer shapes, like plumes, or the wing feathers of birds. They are attached, each by a separate stalk, to the membrane of the wing. Some of these diversified forms are figured at k. They come off like dust when slightly rubbed. The lovely colours of these scales seem to depend, not always on an actual sheet of local colour, but to be due sometimes to the dispersion of light by means of fine lines or striæ drawn on each scale. The relation of the size of lines, closely ruled, to the length of a wave of light, is, by the undulatory theory, easily shown to account for the dispersion of colours by a ruled surface, in itself colourless. Buttons have been made, as curiosities, thus artificially ruled with very fine lines, giving a beautiful play of changing colour. The colours of mother-of-pearl, and other substances, seem due also to the dispersion of light on a white ground. Now, in the case of the purple Emperor a changing play of colour is seen, between brown and purple, as it catches the light at different angles. This appearance can be calculated as due to the relative breadths of the lines covering the scales (which vary from 10,000 to 30,000 in the inch), when compared with the difference between the lengths of waves of light.
These lines on the scales run always from end to end, Dever across. The reason for this seems to be that by this arrangement the drops of rain, if falling on the wing, are rolled off easily. The wing is nearly waterproof ; single drops running off very cleanly, leaving no trace, if the scales are uninjured.
The arrangement in position on the membrane of the wing is a curious point about the scales, and explains the wonderful expansion of the wing, during the first half-hour after the butterfly comes out of its chrysalis. When first emerged the comparative size of the wing to the body is seen at fig. G. In about half-an-hour it will be seen to have expanded to about the size figured at 1.
To find the rationale of this development, we must take a chrysalis when nearly full grown, and gently lift off the skin covering the future wing, whose outline is easily traced on the side of the chrysalis, as at E. Applying a microscope, we can see the scales very closely packed together, slipped under each
other both lengthways and sideways, as at fig. D. When the full development of the wing is attained, the scales are then to be seen in a much more expanded state, spread out from each other in all directions, as at fig. F. The membrane of the wing, to which the roots of the scales are fixed, is elastic when soft. In this state it emerges from the chrysalis, and, air being breathed by the air-vessels, is sent to the extreme edges of the wing, by many air-tubes which pervade the skin; the air stretches these tubes, which in turn expand the skin, and this pulls out from under each other the different rows of scales, until they assume a position in order, like the slates of a housetop. By this time the skin gets hard, and when once dried is stiff enough to remain in the flat shape required for future flight. But till this is done the insect is helpless, and many a one is then picked up as a choice morsel by quick-eyed birds.
The colours of the wing in its moist, fresh, dustless purity are surpassingly lovely. Nothing can be more attractive as a sight than the flight of a dozen or so of new-fledged Peacock butterflies about one's breakfast-table. They may easily be kept thus to come out in succession by securing some dozens of the black-looking caterpillars, off a bed of nettles, where they feed. A large box, covered with wire gauze, to keep out wasps and ichneumon flies, may be their home. Fresh nettles must be daily given them. And then, by degrees, they all go into the trance-like chrysalis stage, hanging to the sides or top of the box. Each morning after they begin to come out a fresh batch will be found, and the room may be lit up with their rainbow hues for two or three hours, till they are strong enough to brave the open air. It is sad to see the disappointing result if an ichneumon fly has pierced the caterpillar previous to its capture, and laid its murderous eggs inside. Instead of the dazzling, graceful wing of the butterfly, comes out a swarm of dingy little flies, horrible to behold, in their victim's stead. Therefore let these caterpillars be early taken and kept well covered.
We take next a very interesting point, which we believe has not been much noticed. It is the powers of a magnifying glass seen in the eyes of some butterflies. Their eyes may be divided into two classes-hairy and smooth. The general appearance of the eye is shown at fig. M, where 1, 2, 3 represent the surface lines, as seen with successively increasing powers of the microscope. It is seen that ultimately the surface is found to be covered with a number of hexagonal little lenses, each being, in fact, a complete little eye. About sixty rows of these cover an ordinary eye, such as that of the Vanessa tribe. They are about both of an inch at the largest.
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