Heliotropism In Butterflies
( Originally Published 1917 )
It has long been known that the green surfaces of plants respond to the stimulus of the sun's rays in a most remark-able manner. This response has commonly been called heliotropism and it has been carefully studied by botanists all over the habitable world. More recently, the fact has been observed that many animals respond in certain definite ways to the stimulus of direct sunshine and the same term has been applied in this case. Very Iittle attention has been given to the subject of heliotropism until within a few recent years. But the observations which have been made by Parker, Longstaff, Dixey, and others open up most interesting field for further observation. An admirable summary of our present knowledge of the subject has been published by Dr. Longstaff in his book "Butterfly Hunting in Many Lands."
One of the earliest observations upon this subject was that published in my book "Nature Biographies" which appeared in June, 1901, concerning the habit in the Mourning Cloak: "On a spring-like day early in November (the 8th) I came across one of these butterflies basking in the sun-shine upon the ties of a railway track. It rested with its wings wide open. On being disturbed, it would fly a short distance and then alight, and I was interested to notice that after alighting it would always turn about until the hind end of its body pointed in the direction of the sun, so that the sun's rays struck its wings and body nearly at right angles. I repeatedly observed this habit of getting into the position in which the most benefit from the sunshine was received, and it is of interest as showing the e treme delicacy of perception toward the warmth of sunshine which these creatures possess."
A little later, some very elaborate observations were made upon this habit of the Mourning Cloak by Prof.
G. H. Parker of Harvard University. Professor Parker noticed that during the warm spells in winter the butterflies came out of their hiding places and after alighting, always placed themselves with their heads away from the direction of the sun and their bodies lying nearly at right angles to the sun's rays. By experiment, he found that they adjusted themselves to this position as soon as they were fully exposed to direct sunshine, even if at the time of alighting they were in a shadow. He found that this movement was a reflex action through the eyes, for when the eyes were blinded no such adjustment took place. He called it negative heliotropism.
Dr. Longstaff uses the term orientation for this adjustment of the butterfly to the sun's rays and he finds it is a very general habit, especially with the Angle wings, for the butterfly thus to orient itself after alighting, in such a way that the hind end of the body points toward the sun. This occurs not only with those species which keep their wings spread open when they alight but also with those in which the wings are closed together and held in a vertical position on alighting.
Various explanations of this phenomenon have been offered but apparently none of them are yet generally accepted. Were the habit confined to butterflies like the Mourning Cloak, it would seem easy to prove that a main advantage was found in the benefit derived from the heat rays of the sun. Were it confined to those species which always fold their wings on alighting, it would seem easy to believe that it was a device for reducing the shadow cast by the insect to its lowest terms. It has also been suggested that the habit is for the purpose of revealing to the fullest extent the markings of the butterfly. Evidently there is here an ample field for further investigation before definite conclusions are reached.
LIST AND SHADOW OBSERVATIONS
Another field for most interesting studies upon the habits of living butterflies has been opened up by the very interesting discussion of list and shadow in Colonel G. B. Longstaff's fascinating book, "Butterfly Hunting in Many Lands." He there summarizes his numerous observations upon butterflies in various localities which he has seen to lean over at a decided angle when they alight. He de-fines " Lūt as "an attitude resulting from the rotation of the insect about its longitudinal axis, as heliotropism results from a rotation about an imaginary vertical axis at right angles to this." The name is adapted from the sailors' term applied to a vessel leaning to one side or an-other in a storm.
Apparently this interesting habit was first called to the attention of European entomologists by an observation of Colonel C. T. Bingham made in 1878, but not published until long afterward. The observation was this:
" The Melanitis was there among dead leaves, its wings folded and Iooking for all the world a dead, dry leaf itself. With regard to Melanitis, I have not seen it recorded any where that the species of this genus when disturbed fly a little way, drop suddenly into the undergrowth with closed wings, and invariably lie a little askew and slanting, which still more increases their likeness to a dead leaf casually fallen to the ground."
Long before this was printed, however, a similar habit had been observed by Scudder in the case of our White Mountain butterfly (Denis semidea). But this species is so exceptional in its habitat that the habit seems to have been considered a special adaptation to the wind-swept mountain top. The possibility of its being at all general among the butterflies in lowlands seems to have been over-looked.
The observations recorded by Longstaff relate chiefly to various members of the Satyrid group. For example, a common Grayling, Satyrus semele, was watched many times as it settled on the ground. As a rule three motions are gone through in regular sequence: the wings are brought together over the back; the forewings are drawn between the hind wings; the whole is thrown over to right or left to the extent of thirty, forty, or even fifty degrees.
This habit, of course, is of advantage to the insect. It seems possible that the advantage might be explained in either of two ways : first, the leaning over on the ground among grasses and fallen leaves might help to render the disguising coloration of the insect more effective, the large ocelli serving to draw the eye away from the outline of body and wing; second, the listing of the butterfly to-ward the sun tends to reduce the shadow and to hide it beneath the wings. There is no doubt that when a Grayling butterfly lights upon the ground in strong sunshine the shadow it casts is more conspicuous than the insect itself and the hiding of this might be of distinct advantage in helping it to escape observation. It is significant that in England the butterflies observed appear to lean over more frequently in sunshine than in shade. An observation of Mr. E. G. Waddilove, reported by Colonel Langstaff, is insteresting in this connection:
"A Grayling settled on a patch of bare black peat earth, shut up its wings vertically, and crawled at once some two yards to the edge of the patch to where some fir-needles, a cone or two, and a few brittle twigs were lying, and then be-coming stationary threw itself over at an angle of some forty-five degrees square to the sun. It thus became quite indistinguishable from its surroundings."
Apparently, some of the Angle wings may have the same habit, for in Barrett's "Lepidoptera of the British Islands," there is a note in regard to Grapta C-album to the effect that it is fond of sunning itself in roads, on warm walls, or on the ground upon dead leaves in sheltered valleys. "Here, if the sun becomes overclouded, it will sometimes close its wings and almost he down, in such a manner that to distinguish its brown and green marbled under side from the dead leaves is almost impossible.''
"Here is a most fascinating opportunity for American observers to determine definitely the facts in regard to our numerous species of butterflies that may show this habit. An observer with a reflex type of camera might easily be able to get pictures that would be of great value in helping to determine the principal facts in regard to the subject. Our common Graylings and numerous species of Angle-wings are so abundant and easily observed that they offer splendid opportunities to any one who will undertake a serious study of the subject.