Life Among the Butterflies by Vance Randolph - HTML preview

CHAPTER VII
 ENEMIES AND PROTECTION

Butterflies have many enemies. Even the eggs of butterflies are often discovered by tiny, four-winged parasites, which pierce the egg-shell and deposit their own microscopic eggs inside. These eggs produce little grubs, which devour the contents of the butterfly’s egg, so that the latter develops into a caterpillar.

The chief enemies of the butterfly tribe, however, are the insectivorous birds, whose summer food often consists largely of caterpillars. Caterpillars are also attacked by wasp-like parasites which deposit their eggs in the victim’s skin; when the eggs hatch the parasitic grubs feed upon the flesh of their unwilling host, who usually dies about the time the guests are ready to pupate. One often sees dead or dying caterpillars covered with the white cocoons of these hymenopterous parasites. Certain larger wasps, too, use caterpillars as food for their young. These insects paralyze their prey by stinging it, lay their eggs on the helpless body, and seal it up in a hollow reed, or in a nest of mud. When the wasp larva hatches it finds plenty of living food at hand.

Chrysalids also are eaten by birds and by various carnivorous insects, and are frequently killed by parasitic wasps and flies. Many an amateur butterfly-hunter has been puzzled to see that some of his chrysalids produce, not beautiful butterflies, but a lot of insignificant little wasps.

Adult butterflies are not much troubled by parasitic insects, but they are eaten by many birds, particularly those of the fly-catcher type, by lizards, and by the larger dragonflies. Spiders kill a few, the great gray robber-flies carry off a butterfly now and then, and frogs and toads take them whenever possible. They are not much attacked by the mammals, but I have seen a chipmunk devour a large Papilio turnus with every symptom of satisfaction.

Long ages of struggle with these enemies have developed certain protective devices—not through any supernatural intervention or any conscious activity on the part of the butterflies, but simply by the mechanical process called natural selection. There is a certain degree of variation among all animals, and some of these variations are transmitted to succeeding generations. Now, if certain butterflies happen to vary in such a way that they are protected against their enemies, they survive at the expense of their less fortunate fellows, and such of their offspring as inherit the protective variation also survive, until at last, by a gradual process of elimination, the entire species is protected.

PROTECTIVE COLORATION

One of the commonest of the protective devices is called protective coloration. It may be noted that many caterpillars are green, a color which blends well with that of the leaves upon which they feed, and so protects them in a measure from the prying eyes of their enemies. Such chrysalids as are attached to green leaves or twigs are often green also, but most caterpillars leave the foliage to pupate, and the chrysalids are neutral gray or brown so as to be inconspicuous against a background of bark or dead wood. Many butterflies pass the winter in the chrysalis state, and these cold weather chrysalids are never green, but usually some dull color which harmonizes with the winter landscape.

Another thing to be noted, particularly in caterpillars, is the operation of the so-called law of counter-shading, which means simply that the part of the body which gets the most light is usually darkest in color. Some caterpillars habitually feed with their feet downward, and their backs are darker than their bellies; others are accustomed to feed in the opposite position, and the shading tones are reversed. One has only to look at a few living specimens in their native haunts to see the value of this arrangement.

In adult butterflies it may be observed that it is the upper side of the wings which shows the bright colors, while the lower side is much less conspicuous. The flying butterfly is not in much danger anyway, and a display of color can do no great damage, but it might be fatal to the same insect at rest. At rest, however, the wings are usually brought together vertically, so that the highly colored upper side is quite concealed, and only the dull under surface exposed to view. In some butterflies—Vanessa antiopa, for example—the lower surface so nearly resembles the bark upon which the insect is accustomed to rest that it can hardly be distinguished, even when one knows exactly where to look for it.

Some of these protected butterflies illustrate a minor protective phenomenon known as dazzling or eclipsing coloration. Many observers believe that the sudden change from the bright colors of the flying butterfly to the neutral tints of the same specimen at rest is more confusing to a pursuer than the total absence of brilliant colors. This dazzling and eclipsing effect is particularly noticeable in various species of Grapta and Vanessa.

Another thing to be remarked is the fact that many butterflies protectively colored, such as those mentioned above, usually alight on some object similar in color to the lower side of their own wings. These butterflies are accustomed to rest upon the trunks of trees, and almost invariably select one with dark-colored bark, avoiding green or light-colored trees such as birches and sycamores. It is not claimed the individual butterfly, after examining the colors of its wings, casts about for a perch to match, but it is quite conceivable that those which were attracted to light-colored bark have been gradually weeded out of the species.

OFFENSIVE ODORS AND TASTES

Besides the method of protective coloration, some caterpillars are protected against birds by sharp spines or hairs; others by peculiar markings and attitudes said to approximate the appearance of serpents or other dangerous objects. Many butterflies, it has been observed, are protected by still other methods. The common Monarch (Anosia plexippus), the Zebra butterfly (Heliconius charitonius), and the Red Silverwing (Dione vanillae) are very conspicuous butterflies, yet they flutter leisurely about unmolested by birds and other enemies. This protection is due to what Alfred Russell Wallace called “a strong, pungent, semi-aromatic odor, which seems to pervade all the juices of their system.”

It has long been known that certain butterflies produced disagreeable odors, but Fritz Müller, working in Brazil as early as 1876, was the first to give the matter any very serious attention. Since Müller’s time extensive studies have been made by Colonel Longstaff and Dr. Dixey, two English entomologists, but very little has been done in North America. It is certainly true, however, that the three butterflies mentioned above (and doubtless many others) are possessed of some taste or odor offensive to their enemies.

Dione vanillae is more abundant in South America than in the United States, and was one of the species investigated by Müller, who writes: “The males ... when seized, open wide the anal valvulae, from the inner side of which there appear two glands yielding a strong and nauseous smell. The females, on the contrary, emit a similar smell from a yellow gland extruded on the dorsum between the last and penultimate segments.” Longstaff, who studied this butterfly in Jamaica, describes it as “a beautiful but ill-smelling Fritillary” and says it smells like cow-dung.

WARNING COLORATION

It has been noticed that many dangerous and distasteful insects are rendered conspicuous by their brilliant colors, and examples of this so-called warning coloration are not lacking among the butterflies. The Swallowtail caterpillars, which produce a very disagreeable odor, are usually marked by two great staring eye-spots on the back of the thorax. Some of the protected butterflies, such as the ill-smelling Zebra and Red Silverwing, are extraordinarily conspicuous by reason of striking color-combinations. Many entomologists believe that these peculiar color-schemes have been developed by protected butterflies as an advertisement of their inedible character. This view is not as popular as it used to be, but there may be something in it; it certainly seems to explain the structure and habits of some of the higher animals—the rattlesnake for example—better than any other hypothesis yet advanced.

PROTECTIVE MIMICRY

There seems to be a tendency among certain insects which are edible and unprotected, to bear a superficial resemblance to inedible or distasteful species. Thus certain harmless flies have developed a remarkable likeness to wasps and bumblebees, although in structure and habits they are really very different. Several diurnal moths, too, have transparent wings, and yellow bands about the body which give them the appearance of gigantic and singularly ferocious hornets. It is certainly a great advantage for a harmless insect to resemble some dangerous and inedible species, and the whole phenomenon of resemblance has been called protective mimicry.

The best example of mimicry in American butterflies is the case of the Monarch (Anosia plexippus) and the Viceroy (Basilarchia disippus). The Monarch belongs to the subfamily Euploeinae, all the members of which are provided with secretions which render them distasteful to birds and predaceous insects. The Monarch advertises its inedibility by its bright brown and black wings, and its leisurely manner of flight. The Viceroy belongs to an altogether different group, the members of which are readily eaten by birds, and which do not resemble the Monarch either in form, coloration, or manner of flight. The Viceroy, however, has gradually developed so remarkable a resemblance to the Monarch that it is difficult to distinguish them at a little distance, although the Viceroy is much the smaller of the two, and has a traverse black band on the hind wing that is lacking in the Monarch. There is no doubt that the Viceroy benefits by this resemblance, as birds (doubtless mistaking it for the inedible Monarch) appear to give it a wide berth.

This novel situation is supposed to have come about as follows: In the remote past the Viceroy was a blue-and-black butterfly like its relatives, but because of the abiding principle of variation the individuals of the species were not exactly alike—some specimens were lighter than others. For some reason or other these lighter-colored butterflies had a slight advantage in the struggle for existence, and so in time the entire species was of this type, the darker specimens having been exterminated. Finally, some individuals chanced to bear a slight resemblance to distasteful butterflies of the Monarch type, so that birds avoided them, but continued to feed upon their less fortunate relatives. Thus, through a long process of natural selection, the Viceroy has come to resemble the Monarch.

HELIOTROPISM AND LIST

It was long ago observed that plants respond definitely and mechanically to the direction of rays of light; the leaves and flowers of many plants always turn toward the sun and even follow its daily course, so that the flower which turned to the East in the morning faces due West at sunset. Similar phenomena are now known to occur in animals. Among butterflies a good example is that of the Mourning Cloak (Vanessa antiopa). This butterfly, when it alights in the sunlight, almost invariably turns about until its body lies parallel with the rays of light and its head points directly away from the sun. This phenomenon is known as negative heliotropism, and a number of plausible explanations of it have been advanced. Probably it is merely another method of blending with the background, akin to protective coloration. When the Mourning Cloak alights upon the ground, or upon a log, it closes its wings and becomes well-nigh indistinguishable from its surroundings, because the under side of the wings is protectively colored. Now, if the body lies at right angles to the sun’s rays, the wings cast a large shadow, much more conspicuous than the butterfly itself, and hence attracting unfriendly eyes to the butterfly. If however the body is in line with the rays of light, and the wings brought together vertically, the shadow is insignificant. One has only to pin a few dead butterflies on a smooth neutral background in full sunlight to see the force of this theory. It is very probable, then, that negative heliotropism is simply a method of reducing the too-conspicuous shadow to its lowest terms.

Other butterflies reduce the shadow by what is known as list, leaning far over to one side like a sail-boat in a storm—hence the name. The best American example of list is found in the behavior of the Wood-nymph butterfly (Satyrus alope), which is often seen to topple over to one side, presenting the entire wing surface to the source of light, lowering the top-line and thus reducing the shadow.

FEIGNING DEATH

Many insects, particularly beetles, frequently escape their enemies by feigning death. Whether this behavior is an instinctive ruse or a genuine paralysis induced by something akin to fright we do not know, but it is doubtless of considerable value to the insect. Some butterflies have been known to play possum when in a tight place. Vanessa cardui, known as the Painted Lady, sometimes closes its wings, folds its legs close to the body, and falls motionless to the ground. Usually it is lost in the weeds or grasses, but even if found and picked up it often allows itself to be handled without any sign of life. Similar behavior has been reported in Satyrus alope, the Wood-nymph butterfly.