AESTIVATION OR SUMMER LETHARGY

In some species of b.u.t.terflies there is a special adaptation to pa.s.sing through the hottest part of the summer season in a state of lethargy which is suggestive of the torpor of the hibernating period.

This phase of b.u.t.terfly existence has not been extensively studied and there are indications that it exists more generally than has been commonly supposed. It has been noticed even in northern New England that some of the Angle-wings seek shelter and become lethargic during August. Apparently this is an adaptation to single broodedness, helping to carry the species through the year without the exhaustion incident to the continued activity of the b.u.t.terfly.

In more southern regions, especially in the hot, dry climates where vegetation withers in midsummer, it is well known that some caterpillars become lethargic, remaining inactive until the fall rains start vegetation into growth. The Orange-sulphur b.u.t.terfly is a good example of this.

This summer lethargy offers excellent opportunity for careful study.

Any observer who finds a b.u.t.terfly hidden away in summer under boards, the bark of a tree, or in a stone pile should look carefully to see what species it is and how the b.u.t.terfly behaves. Such observations should be sent to the entomological journals in order that our knowledge of the subject may be increased.

FEIGNING DEATH

The fact has long been noticed that various b.u.t.terflies have the habit at times of feigning death and dropping to the ground where they may lie motionless for a considerable period. This habit is most easily observed in some of the Angle-wings, especially those which hibernate as adults. Those species have the under surfaces of their wings colored in various bark-picturing patterns and apparently live through the winter to some extent, resting beneath the bark of large branches or upon the trunks of trees. Many of them also secrete themselves in hollow trees or beneath loose bark or in board piles or stone walls. It is probable, however, that during the long ages when these insects were adapting themselves to their life conditions, before man interfered with the natural order and furnished various more or less artificial places for hibernation, these b.u.t.terflies rested more generally upon the under side of branches than they do now.

Even in warm weather when one of these b.u.t.terflies is suddenly disturbed it is likely to fold its legs upon its body and drop to the ground, allowing itself to be handled without showing any signs of life. This habit is doubtless of value, especially during hibernation or possibly during the summer lethargy or aestivation, the latter a habit which may be more general among these b.u.t.terflies than is now supposed. As the insect lies motionless upon the ground it is very likely to blend so thoroughly with its surroundings that it becomes concealed, and any bird which had startled it from the branch above would have difficulty in finding it.

Some very interesting observations have been made upon the death-feigning instincts of various other insects, especially the beetles. But no one so far as I know has yet made an extended study of the subject in relation to our American b.u.t.terflies. It is an excellent field for investigation and offers unusual opportunities for photographic records. One of the pictures opposite page 32 shows a photograph which I took of a Mourning Cloak as it was thus playing 'possum. This species exhibits the instinct to a marked degree.

COLORATION

The caterpillars of b.u.t.terflies and moths form a large part of the food of insect-eating birds. These caterpillars are especially adapted for such a purpose and in the economy of nature they play a very important part in keeping alive the feathered tribes. During the long ages through which both birds and insects have been developing side by side, there have been many remarkable inter-relations established which tend on the one hand to prevent the birds from exterminating the insects and on the other to prevent the insects from causing the birds to starve. The most important of these, so far as the caterpillars are concerned, are the various devices by which these insects protect themselves from attack, by hiding away where birds are not likely to find them, by clothing their bodies with spiny hairs, by other methods of rendering themselves distasteful, or by various phases of concealing coloration. On the whole, the examples of the latter are not so numerous or so easily found in the case of the larvae of b.u.t.terflies as in those of moths.

Perhaps the basal principle of concealing coloration is the law of counter-shading, first partially announced by Prof. E. B. Poulton, and later much more elaborately worked out by Mr. Abbott H. Thayer, and discussed at length by Mr. Gerald H. Thayer in his remarkable volume, "Concealing Coloration in the Animal Kingdom." The law of counter-shading is tersely stated in these words: "Animals are painted by nature darkest on those parts which tend to be most lighted by the sky's light and _vice versa_." As this law works out on most animals that live on or near the ground, the upper part of the body exposed to the direct light from above is dark; and the under part, shut off from the upper light and receiving only the small reflection from below, is enough lighter to make the appearance of the creature in its natural environment of a uniform tone from back to breast.

Nearly all caterpillars ill.u.s.trate this law of counter-shading. If they are in the habit of feeding or resting with their feet downward the back will be darker and the under side lighter, but if they are in the habit of feeding or resting in the opposite position these color tones will be reversed. One can find examples of such conditions almost any summer's day by a little searching of trees or shrubs.

This law of counter-shading, however, is really only the basis for the coloration of caterpillars or other animals. It tends, chiefly, to make the creature appear as a flat plane when seen from the side, and may be said in a way to prepare the canvas upon which Nature paints her more distinctive pictures. A great many examples of color markings that tend to conceal the caterpillar amid its natural surroundings may be found by any one who will study the subject and it offers one of the most interesting fields for investigation. The chapter on caterpillars in the above-mentioned book by Mr. Thayer should serve as a starting point for any one taking up the subject.

b.u.t.terflies differ from caterpillars and from most other animals in the fact that their coloring is chiefly shown upon the flat surfaces of the wings. Consequently, there is less opportunity for the various phases of counter-shading which is so commonly shown in the larger caterpillars. The bodies of nearly all b.u.t.terflies do exhibit this phenomenon, but these bodies are relatively so small that counter-shading plays but a little part in the general display.

Upon the outstretched membranes of the b.u.t.terflies' wings Nature through the long ages of development has painted a great variety of pictures. Those which tend to protect the insect by concealment amid its surroundings are most commonly spread on the under surface of the wings. Especially is this true in the case of those species which pa.s.s the winter as adults or which have the habit of resting upon the bark of trees, the sides of rocks, or the surface of the ground. We here find some of the most interesting examples of obliterative coloring that occur in nature. Some b.u.t.terflies have taken on the look of tree bark, others the sombre appearance of weathered rocks, while still others are painted with the images of flowerets and their stems.

_Dazzling and Eclipsing Colors_

Many of the b.u.t.terflies, especially the Angle-wings, which are marked on the under surface in various protective colors, are admirable examples of that phase of animal coloring which is spoken of as _dazzling coloration_. This is apparently one of the most important protective devices to be found in Nature and the validity of it is now generally conceded by naturalists. One phase of it, which may be called eclipsing coloration, seems to have been first definitely formulated by the late Lord Walsingham, a famous English entomologist who enunciated it in an address as president of the Entomological Society of London. The most significant paragraphs in that address were these:

"My attention was lately drawn to a pa.s.sage in Herbert Spencer's 'Essay on the Morals of Trade.' He writes: 'As when tasting different foods or wines the palate is disabled by something strongly flavored from appreciating the more delicate flavor of another thing afterward taken, so with the other organs of sense a temporary disability follows an excessive stimulation. This holds not only with the eyes in judging of colors, but also with the fingers in judging of texture.'

"Here, I think, we have an explanation of the principle on which protection is undoubtedly afforded to certain insects by the possession of bright coloring on such parts of their wings or bodies as can be instantly covered and concealed at will. It is an undoubted fact, and one which must have been observed by nearly all collectors of insects abroad, and perhaps also in our own country, that it is more easy to follow with the eye the rapid movements of a more conspicuous insect soberly and uniformly colored than those of an insect capable of changing in an instant the appearance it presents.

The eye, having once fixed itself upon an object of a certain form and color, conveys to the mind a corresponding impression, and, if that impression is suddenly found to be unreliable, the instruction which the mind conveys to the eye becomes also unreliable, and the rapidity with which the impression and consequent instruction can be changed cannot always compete successfully with the rapid transformation effected by the insect in its effort to escape."

Lord Walsingham then goes on to suggest that this intermittent play of bright colors probably has as confusing effect upon birds and other predaceous vertebrates as upon man; and that on this hypothesis such colors can be accounted for more satisfactorily than upon any other yet suggested. Since then the significance of this theory has been repeatedly pointed out by Professor Poulton, Mr. Abbott H. Thayer, and various other authorities upon animal coloring. The terms _dazzling_ and _eclipsing_ have been applied to the phenomenon.

Shortly after Lord Walsingham propounded this theory I called attention [A] to its fitness in explaining some of the most interesting color phases shown by American insects, notably the moths and locusts which have brilliantly colored under wings and protectively colored upper wings.

[A] _Popular Science Monthly_, 1898, "A Game of Hide and Seek."

Reprinted in the _Insect World_, 1899.

The animals of the north show numberless color phases of interest. One of the most curious of these is exhibited by several families of insects in which the outer wings are protectively colored in dull hues and the under wings brightly colored. For example, there are many species of moths belonging to the genus Catocala found throughout the United States. These are insects of good size, the larger ones measuring three inches in expanse of wings, and the majority of them being at least two thirds that size. Most of them live during the day on the bark of trees, with their front wings folded together over the back. The colors and markings of these wings, as well as of the rest of the exposed portions of the body, are such as to a.s.similate closely with the bark of the tree upon which the insect rests. In such a situation it requires a sharp eye to detect the presence of the moth, which, unless disturbed, flies only at night, remaining all day exposed to the attacks of many enemies. Probably the most important of these are the birds, especially species like the woodp.e.c.k.e.rs, which are constantly exploring all portions of the trunks of trees.

The chief beauty of these Catocalas, as they are seen spread out in the museum cabinet, lies in the fact that the hind wings, which, when the moth is at rest in life, are concealed by the front ones, are brightly colored in contrasting hues of black, red, and white in various brilliant combinations. These colors, in connection with the soft and blended tones of the front wings, make a very handsome insect.

It is easy to see that when one of these Underwing Moths is driven to flight by a woodp.e.c.k.e.r or other bark-searching bird it would show during its rapid, irregular flight the bright colors of the under wings which would be instantly hidden upon alighting and the very different coloring of the upper wings blending with the bark would be subst.i.tuted. Consequently, the bird would be very likely to be baffled in its pursuit.

_Coloration of Locusts_

On the rocky hills and sandy plains of New England there are several species of gra.s.shoppers or locusts that also ill.u.s.trate these principles. If you walk along a strip of sandy land in summer, you start to flight certain locusts which soon alight, and when searched for will be found closely to a.s.similate in color the sand upon which they rest. On a neighboring granite-ribbed hill you will find few if any of this species of locust, but instead there occur two or three quite different species, which when at rest closely resemble the lichen-covered rocks. This resemblance is very striking, and is found in all stages of the insect's existence. If now you go to a lowland meadow, still another color phase will be found to prevail--the green gra.s.s is swarming with the so-called "long-horned" gra.s.shoppers, which are green throughout with linear bodies, and long, slender legs and antennae.

Each of these three groups of insects is adapted to its particular habitat. All are constantly persecuted by birds, and have been so persecuted for unnumbered ages in the past. In every generation the individuals have varied, some toward closer resemblance to environment, others in an opposite direction. The more conspicuous insects have been constantly taken, and the least conspicuous as constantly left to reproduce. Were the three groups to change places to-day, the green gra.s.shoppers from the meadows going to sandy surfaces, the sand-colored locusts going to rocky hills, and the "mossbacks" from the hills to the lowland meadows, each would become conspicuous, and the birds would have such a feast as is seldom spread before them.

The species living on sand and rocks are often "flushed" by birds.

Those which flew but a few feet would be likely to be captured by the pursuing bird; those which flew farther would stand a better chance of escaping. Similarly, those which flew slowly and in a straight line would be more likely to be caught than those which flew rapidly and took a zigzag course. As a consequence of the selection thus brought about through the elimination of those which flew slowly along the straight and narrow way that led to death, you will find that most locusts living in exposed situations when startled fly some distance in a rapid, zigzag manner.

But still another element of safety has been introduced by some species of these locusts through the adoption of the color tactics of the Catocala moths. The under wings of the common Carolina locust--the species most abundant along the highway--are black, bordered with yellowish white. The base of the hind wings of a related species living on the Western plains is bluish, while in the large coral-winged locust of the Eastern states the hind wings are red, bordered with black. In nearly all of the species of these locusts frequenting open localities where they are liable to disturbance by birds or other animals, the hind wings exhibit contrasting colors in flight. Most of them also fly in a zigzag line, and alight in a most erratic manner. Many times I have had difficulty in determining the exact landfall of one of these peculiar creatures, and I believe Lord Walsingham's suggestion is well exemplified in them. (_See page 33._)

The most famous example of a combination of this dazzling coloring of the upper wing surface with a definite protective coloring of the under wing surface is the Kallima b.u.t.terfly which is ill.u.s.trated in almost every book dealing with animal coloration. The under wing surface bears a striking resemblance to a leaf and the hind wings project to form a tail which looks like the petiole of the leaf, and there is also a mark running across the wings which mimics the midrib.

When the b.u.t.terfly is flying the brilliant colors of the upper surface are visible, but when it alights these are instantly replaced by the sombre tone of the under surface, so that apparently the insect completely disappears and in its place there is only a leaf attached to a branch in a most natural position. In Dr. Longstaff's book there is an ill.u.s.tration of another tropical b.u.t.terfly, _Eronia cleodora_, which resembles on its under surface a yellow disease-stricken leaf but on its upper surface gives a brilliant combination of black and white. This insect alights upon the leaves which it resembles and is a striking example of both dazzling and mimicking coloration.

Many of our own b.u.t.terflies, notably the Angle-wings, are excellent examples of a similar combination. In flight they reveal conspicuous colors which are instantly hidden upon alighting and then one only sees the bark-like or dead leaf-like under surface as may be seen in the plate opposite pages 160-161. The iridescence upon the upper wing surface of many b.u.t.terflies, whose under wing surface is colored in concealing tones, is doubtless also of great use to the insect in a similar way. There is a splendid opportunity here for some observer to study this phase of b.u.t.terfly activity and to get photographs of the insects amid their natural surroundings.

In their book upon "Concealing Coloration" the Messrs. Thayer have called attention to many interesting phases of dazzling coloration.

They show that bright marks like the eye-spots or ocelli, which form so prominent a feature on the wing surfaces of many b.u.t.terflies, really helped to conceal the insect amid its natural surroundings, by drawing the eye away from the outlines of wings and body so that the latter tend to disappear. Their discussion of this subject opens up another vast field for outdoor observations of absorbing interest, in which there is great need for many active workers.

[Ill.u.s.tration: _See page 23_

b.u.t.tERFLY FEIGNING DEATH, HANGING TO BARK BY ONE FOOT]

[Ill.u.s.tration: _See page 17_

b.u.t.tERFLY IN HIBERNATING POSITION]

[Ill.u.s.tration: Caterpillar feeding upon leaf of milkweed Caterpillar hung up for the change to the chrysalis The transition stage The Chrysalis