COMPARATIVE AGE OF FOSSIL BUTTERFLIES.
All the well determined fossil butterflies come from one of three localities, Aix, Rott and Radoboj, all belonging to the tertiaries of Europe. Others are reported, as will be seen further on, to have been found in Prussian amber; and it is not in the least improbable that they have been or may be. These would be of about the same age as the oldest of the others, those of Aix. Of the Aix fossils, which belong to the upper Eocene, or to speak more definitely, the Ligurian, Neorinopis sepulta, Lethites Reynesii, Thaites Ruminiana and Pamphilites abdita (the first described by Boisduval, the rest by myself) come from the calcareous marls of the gypsum quarries, the only bed in which insects had been found when visited by Messrs. Murchison and Lyell in 1829. Coliates Proserpina, however, described here for the first time, was taken from strata beneath these, and therefore, at least until we have more precise knowledge concerning the remains of butterfly larvæ in amber, may be considered the oldest known butterfly. Count de Saporta writes me concerning this fossil, the discovery of which is due to him, as follows:—“Cette empreinte ne provient pas des platrières même, c’est à dire des galeries qui servent à l’exploitation du Gypse; mais d’une assise ou groupe de couches immédiatement inférieure. Vous verrez cette provenance indiquée pour un grand nombre de mes espèces; dans ce cas, elles ne proviennent par des ouvriers mais je les ai recueillies moi même en suivant les lits sur les points où ils affleurent au dehors.”
The next in order, approaching recent times, are the lignite beds of Rott in the basin of the Rhine, which belong to the Aquitanian or the upper part of the lower Miocene. Thanatites vetula (described by Hayden) is the only butterfly known from this division of the Tertiaries.
The most recent beds containing fossil butterflies are the lacustrine deposits of Radoboj in Croatia, Austria. These belong to the Mayencian or lower portion of the middle Miocene, and have furnished Eugonia atava, Mylothrites Pluto, another fragment possibly referable to Mylothrites, and Pontia Freyeri, all described by Heer. Two of the genera of these more recent beds contain representatives now living in the same region; but none of the older beds have yet furnished butterflies referable to modern genera.
It is rather extraordinary that the upper Miocene beds of Œningen, Bavaria, which, if we except the amber, have furnished almost more insects than all the other beds of fossil insects of the world together, and which are more recent than any of those in which butterflies have been found, have yielded scarcely any remains of Lepidoptera (one species) and none whatever of butterflies.
PROBABLE FOOD-PLANTS OF TERTIARY CATERPILLARS.
Of the five butterflies from Aix, two belong to the Oreades (Neorinopis sepulta and Lethites Reynesii) the food of whose caterpillars at the present epoch has invariably been found to be either Gramineæ or, occasionally, Cyperaceæ. Both of these groups are present in the deposits of Aix, the former being represented by ten species of Poacites, and the latter by a Cyperites;[AG] and it is in the highest degree probable that these formed the sustenance of the Oreades of that epoch. A third species (Pamphilites abdita) belongs to the Astyci, a group whose principal food is the same family of plants, Gramineæ, although some species have been found also upon Althea, Malva and Lavatera (Malvaceæ), Trifolium, Coronilla and ?Lespedeza (Leguminosæ), Plantago (Plantaginaceæ), and Maranta (Scitamineæ). Of these families the Leguminosæ only are found at Aix, and in abundance, even including a plant doubtfully referred to Trifolium. It is, however, far more probable that Pamphilites lived upon grasses; and it is not a little strange that the Gramineæ, the probable food-plants of three of the five butterflies known from that fauna, were among the rarest of the plants; that is, their proportion to the whole phanerogamic flora was about the same as now obtains in New Guinea or New Grenada, countries the least favored in this respect.[AH] The proportion of the Gramineæ and Cyperaceæ to the whole of the Phanerogamia in Europe of to-day is, probably, about the same as in the United States (more than seventeen per cent.) and much greater than in the East Indies. The limited number of known fossil butterflies does not give great weight to any general considerations based upon them, but it may at least be worth while to remark that Aix, in Eocene times, had, in the point referred to, an assemblage of plants much better comparable with the East Indian flora of the present day than with the modern European flora, the proportion of known Gramineæ, etc., to the Phanerogamia being five per cent., while the proportion of its grass-feeding butterflies to the other rhopalocerous Lepidoptera is sixty per cent. To judge simply by the catalogue of the East India Museum, the only authority upon East Indian butterflies extant, the present proportion of gramnivorous to non-gramnivorous butterflies is as 1: 5·2, while in Europe it is as 1: 3. Eocene Aix, then, had a European proportion of Satyrids, composed, as will be seen, of species of an Indian aspect, feeding upon plants essentially temperate, but, as in tropical countries, numerically unimportant.
The Danai, to which the fourth species from Aix (Coliates Proserpina) belongs, feed almost exclusively upon Leguminosæ, and these have recently been found in great abundance at Aix. Count de Saporta enumerates one species each of ?Trifolium, Caragana, Ervites, Sophora, Micropodium, Cercis and Gleditschia, two of Phaseolites and six of Cæsalpinites, belonging to the Papilionaceæ, besides nine Acacias and a Mimosa of the Mimoseæ, and four species of uncertain relations; making a series larger than he has found in any other family.[AI]
Of these, two species of Phaseolites, one of Sophora, eight of Acacia and two of Leguminosites are specified as coming from the lower beds, where Coliates itself is found. But Coliates is most closely allied, as we have said, to a group of Indian forms, and the food plants of their caterpillars is almost wholly unknown. A species of Delias, however, to which genus Coliates has been specially compared, is stated to feed, not upon a leguminous plant, but upon Dioscorea, one of the Yam family; and the presence in Aix of a species of a closely allied group, Smilax rotundiloba Sap., is announced by Count de Saporta. It is not improbable, therefore, that Smilax rotundiloba was the food-plant of the larva of Coliates Proserpina.[AJ]