The extreme variability of P. Tharos was alluded to more than a century ago by Drury, who stated:—“In short, Nature forms such a variety of this species that it is difficult to set bounds, or to know all that belongs to it.” Of the different named varieties, according to Mr. Edwards, “A appears to be an offset of B, in the direction most remote from the summer form, just as in Papilio Ajax, the var. Walshii is on the further side of Telamonides, remote from the summer form Marcellus.” Var. C leads from B through D directly to the summer form, whilst A is more unlike this last variety than are several distinct species of the genus, and would not be suspected to possess any close relationship were it not for the intermediate forms. The var. B is regarded as nearest to the primitive type for the following reasons:—In the first place it is the commonest form, predominating over all the other varieties in W. Virginia, and moreover appears constantly in the butterflies from pupæ submitted to refrigeration. Its distinctive peculiarity of colour occurs in the allied species P. Phaon (Gulf States) and P. Vesta (Texas), both of which are seasonally dimorphic, and both apparently restricted in their winter broods to the form corresponding to B of Tharos. In their summer generation both these species closely resemble the summer form of Tharos, and it is remarkable that these two species, which are the only ones (with the exception of the doubtful Batesii) closely allied to Tharos, should alone be seasonally dimorphic out of the large number of species in the genus.
Mr. Edwards thus explains the case under consideration:—“When Phaon, Vesta, and Tharos were as yet only varieties of one species, the sole coloration was that now common to the three. As they gradually became permanent, or in other words, as these varieties became species, Tharos was giving rise to several sub-varieties, some of them in time to become distinct and well marked, while the other two, Phaon and Vesta, remained constant. As the climate moderated and the summer became longer, each species came to have a summer generation; and in these the resemblance of blood-relationship is still manifest. As the winter generations of each species had been much alike, so the summer generations which sprung from them were much alike. And if we consider the metropolis of the species Tharos, or perhaps of its parent species, at the time when it had but one annual generation, to have been somewhere between 37° and 40° on the Atlantic slope, and within which limits all the varieties and sub-varieties of both winter and summer forms of Tharos are now found in amazing luxuriance, we can see how it is possible, as the glacial cold receded, that only part of the varieties of the winter form might spread to the northward, and but one of them at last reach the sub-boreal regions and hold possession to this day as the sole representative of the species. And at a very early period the primary form, together with Phaon and Vesta, had made its way southward, where all three are found now.”
Experiments with Grapta Interrogationis.[64]
[Communicated by Mr. W. H. EDWARDS, November 15th, 1879.]
The experiments with this species were made in June, 1879, on pupæ from eggs laid by the summer form Umbrosa of the second brood of the year, and obtained by confining a female in a bag on a stem of hop. As the pupæ formed, and at intervals of from six to twenty-four hours after pupation (by which time all the older ones had fully hardened), they were placed in the ice-box. In making this experiment Mr. Edwards had three objects in view. 1st. To see whether it was essential that the exposure should take place immediately after pupation, in order to effect any change. 2ndly. To see how short a period would suffice to bring about any change. 3rdly. Whether exposing the summer pupæ would bring about a change in the form of the resulting butterfly. Inasmuch as breeding from the egg of Umbrosa, in June, in a former year,[64] gave both Umbrosa (11) and Fabricii (6), the butterflies from the eggs obtained, if left to nature, might be expected to be of both forms. The last or fourth brood of the year having been found up to the present time to be Fabricii, and the 1st brood of the spring, raised from eggs of Fabricii (laid in confinement), having been found to be wholly Umbrosa, the latter is probably the summer and Fabricii the winter form. The two intervening broods, i.e. the 2nd and 3rd, have yielded both forms. This species hibernates in the imago state.
After the pupæ had been in the ice-box fourteen days they were all removed but 5, which were left in six days longer. Several were dead at the end of fourteen days. The temperature most of the time was 1°-2° R.; but for a few hours each day rose as the ice melted, and was found to be 3°-6° R.
From the fourteen-day lot 7 butterflies were obtained, 3 males and 4 females. From the twenty-day lot 4 males and 1 female; every one Umbrosa. All had changed in one striking particular. In the normal Umbrosa of both sexes,[65] the fore wings have on the upper side on the costal margin next inside the hind marginal border, and separated from it by a considerable fulvous space, a dark patch which ends a little below the discoidal nervule; inside the same border at the inner angle is another dark patch lying on the submedian interspace. Between these two patches, across all the median interspaces, the ground-colour is fulvous, very slightly clouded with dark.
In all the 4 females exposed to cold for fourteen days a broad black band appeared crossing the whole wing, continuous, of uniform shade, covering the two patches, and almost confluent from end to end with the marginal border, only a streak of obscure fulvous anywhere separating the two. In the case of the females from pupæ exposed for twenty days, the band was present, but while broad, and covering the space between the patches, it was not so dark as in the other females, and included against the border a series of obscure fulvous lunules. This is just like many normal females, and this butterfly was essentially unchanged.
In all the males the patches were diffuse, that at the apex almost coalescing with the border. In the 3 from chrysalides exposed fourteen days these patches were connected by a narrow dark band (very different from the broad band of the females), occupying the same position as the clouding of the normal male, but blackened and somewhat diffused. In the 4 examples from the twenty-day pupæ, this connecting band was scarcely as deeply coloured and continuous as in the other 3. Beyond this change on the submarginal area, whereby a band is created where naturally would be only the two patches, and a slight clouding of the intervening fulvous surfaces, there was no difference of the upper surface apparent between these examples of both sexes, and a long series of natural ones placed beside them.