A first book in organic evolution

by D. Kerfoot Shute

Plate II.—Kallima paralekta. A butterfly of Sumatra illustrating the work of Natural Selection: C, butterfly with expanded wings (dorsal surface) which are conspicuously colored through Sexual Selection; B, same butterfly with wings closed (ventral surface) and presenting a close resemblance to a dead leaf, A, through the agency of Natural Selection. B, illustrates the Protective Resemblance of an animal to an inanimate object.

Protective Imitation of Particular Objects. Insects often exhibit a very great amount of detailed resemblance to the leaves, flowers, and twigs of plants among which they live. Those that live on grass are striped longitudinally, while those that feed on ordinary leaves have an oblique striation. There is a larva of a Georgia butterfly (Sphinx fuciformis) which feeds on a plant having small blue flowers and linear, grass-like leaves. This larva has a blue head and a green body striated like the leaves. The resemblance of the insect to its environment is very striking. There is another species that feeds on a plant with small red flowers situated in the axils of the leaves, and this larva has a row of seven red spots of unequal size, which corresponds quite closely with the size and color of the flowers. There is a caterpillar in Borneo that resembles a piece of moss with two exquisite pink-white seed-capsules. Its general hue is greenish, with two little pink spots on its upper surface, and it is covered with hair. Its movements are very slow, and when eating it withdraws its head beneath a mobile fleshy hood that it possesses, so that its motions in feeding are not noticeable. When living in its native haunts it is all but impossible to detect it, so completely does it resemble the surrounding moss. Other insects resemble green or dead leaves in all their varieties of form and color, and to show what a great protection such resemblance affords to insects in concealing them from view the following observation of a naturalist in Nicaragua may be related. In that country there are armies of foraging ants that devour every insect they can catch. Among a multitude of these ants he observed a locust that looked very much like a green leaf. The ants, many of them, were continually running over the body and legs of the insect without detecting its character. In many parts of the world there are many butterflies (Kallima, for example) the under surfaces of whose wings very closely resemble dead leaves. They frequent dry forests and are rapid flyers. They are rather large, and the upper surfaces of their wings are quite showy, having bluish and orange colors ([Plate II]). It is their habit always to settle on some twig where there are decaying or dead leaves. In doing so it folds its wings together over the back, thus concealing the gay upper surfaces and presenting the protectively colored under surfaces. The resemblance to a dead leaf is much more striking from the fact that the short tails of the hind wings just touch the branch on which the insect rests and look very much like the stalk of a leaf. From this stalk a dark curved line extends to the elongated tip of the upper wings, thus imitating the midrib of a leaf. On both sides of this midrib are oblique lines that are partly markings and partly nervures, which give the appearance of a leaf with its veining. The head and antennæ fit in such a way between the closed upper wings as not to interfere with that irregular outline which is characteristic of the withered and dry leaves. Often the closed wings are covered with small black dots gathered into circular groups that exactly resemble the minute fungi found on decaying leaves, and it is sometimes difficult to believe that the insects themselves are not attacked by a fungus. Wallace states that this wonderful imitation is most complete, and that in Sumatra he has often seen a butterfly enter a bush and then disappear as if by magic. He states that once he was so fortunate as to see the exact spot on which the insect settled, but even then lost sight of it for some time, and was able to discover it close to his eyes only after persistent and careful search.

The curious and interesting leaf-insects of Java are veined and colored in such a way that, with the leaf-like expansions from various parts of the body, not one person in a dozen can detect them when they are resting upon their food-plants right under one’s eyes. Other insects resemble pieces of stick ([Plate III]), with all the minute details of branches and knots. An eminent naturalist has stated that after being a practical entomologist for thirty years, he was deceived by one of these stick insects and took out his pruning knife to cut from a plum-tree what seemed to him to be a projecting spur. This spur proved to be the caterpillar of a geometer-moth, about two inches in length. He placed a portion of the plum-tree on a table, and showed it to several members of his family, designating a space of several inches in which the caterpillar was to be found, but none of them could detect the insect until it was pointed out to them. These protective resemblances of living creatures to inanimate objects are beautiful illustrations of adaptation to environment through Natural Selection.

Beautiful illustrations of protective resemblance to particular objects are furnished by leaf-hoppers (insects) in Central America. They resemble the thorny and prickly growths of the plants on which they presumably live. Some of them also resemble gall growths on the plants ([Plate IV]).

Plate IV.—Central American Leaf Hoppers resembling the prickly and thorny growths of plants on which they presumably live. Certain of them also represent gall growths on the plants. Protective Resemblance. [Figures collected by Dr. L. O. Howard, from various plates published in the Biologia Centrali-Americana.]

Alluring Coloration. Besides those insects which secure protection from enemies by their resemblance to the inert objects among which they live, there are others whose adaptive resemblance, and therefore concealment, is for the purpose of securing their food,—for alluring their prey. A most interesting case of alluring coloration is that furnished by a wingless insect of India, the mantis. Its color and form are such as to closely resemble such a fantastic flower as the pink orchis. The insect rests motionless among the bright green foliage, being very conspicuous on account of its pink color, and looking so much like a flower that it allures and captures the butterflies which settle upon it.

There is a species of spider (Thomisus citreus) of a creamy-white color whose abdomen completely resembles in color and contour the unopened buds of the flowers among which it rests. It has been seen to capture flies that were attracted to the flowers. There is another species of spider that looks exactly like the excreta of birds, and through this alluring resemblance captures certain butterflies. A naturalist has related how, in pursuing a butterfly through a jungle in Java, he was stopped by a dense growth of bushes. Here he observed a leaf with a bird’s dropping upon it, and sitting on this dropping was a beautiful butterfly. Surprised at such a usually dainty and pretty butterfly seeking such inappropriate food, he carefully approached to study the actions of the insect. The insect permitted him to get so close that he seized it by the wings, and to his astonishment a part of the body remained behind as if the bird’s dropping was very adhesive. He touched the dropping to see if it really was sticky, and found that his eyes had been deceived and that what he took for the excreta of a bird was a most artfully colored spider, lying on its back with its feet crossed and depressed closely to the body. The spider had been firmly holding the butterfly.

Plate III.—Caterpillar, B, of a Geometer Moth (Prochœrodes transverrata) on the stem of a plant (Ailanthus), A. Illustrating Protective Resemblance.