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.