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.