Popular Science Monthly/Volume 10/December 1876/On Variation in the Moths

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THERE are a large number of different kinds of moths, inhabiting North America and Europe, which entomologists have classified under the technical family term, Noctuæ. Of this family, 1,028 different species have been catalogued as European. There being very many students, and a sufficient time having elapsed to secure a thorough collecting throughout the territory, this number may be taken as sufficiently corresponding with the actual representation of the family in Europe. In North America nearly 1,200 species are now catalogued,[1] but, since much of our territory remains to be explored in this respect, we may expect considerable additions to the number of known Noctuæ inhabiting our continent. The greater number of the species may be easily distinguished on comparison, the American from the European. There are, however, certain American species which differ but very slightly from certain European, and hence are generally called “representative species,” or “species of replacement.” For instance, Apatela occidentalis (G. and R.) “represents” the European Apatela psi (Linn.); Agrotis Normaniana (Grote), the European Agrotis triangulum; Calocampa nupera (Lintner), the European Calocampa vetusta; Catocala relicta (Walk.), the European Catocala fraxini, etc.

Although the number of such species appears relatively small, it is probably in reality larger than as yet suspected. For even the species acknowledged to occupy this relationship to each other exhibit quite different degrees of resemblance. I say “relationship,” for we must reasonably conclude that his resemblance is not accidental or meaningless. It must indicate that both the European and American species are derived from a common stock. More than two years ago, I suggested that the faunæ had become separated through the physical action of the glacial epoch, showing that from the present distribution of species, and their degree of resemblance, no other hypothesis would explain all the facts in the case so satisfactorily. For we have to account, first, for the occurrence of nearly-related forms in localities as widely separated, geographically, as Texas and Germany; and then, again, for the occurrence, within our territory, of nearly-related species on the Alpine summits of Mount Washington and the distant arctic regions. The method of probable distribution of these latter species, through the action of the retreating ice, I have explained in the American Naturalist for March of this year.

In studying specimens of these related American and European species of Noctuæ, some new light bearing upon the question of their differences is attempted to be thrown in the present paper. I have endeavored to localize these differences upon some portion of the insect. In all the cases I have been able to investigate, these differences are expressed on the upper surface of the body and wings, and principally on the upper surface of the front pair of wings. Here it is that the first variations, which now have grown into specific differences, were probably expressed. And the reason for this seems to be, that this portion of the body is that usually exposed to the light and the action of external influences. The moths, during the daytime, rest with the front-wings dependent over the hind-wings, and nearly covering the body. While in the American and European related species the differences which lead us to call them distinct “species” are located on the front-wings chiefly, the under surfaces of both wings remain exceedingly similar in the contrasted forms. This is the portion which in the day time (the period of inaction for moths) is applied to the surface against which the insect rests, and is entirely shielded from exposure. Take, for instance, the European Catocala fraxini and the American Catocala relicta. The ground-color of the American species above and below is a bright, clear white. Now, beneath, both species show this color, but the upper surface of the fore-wings, in the European species, is obscured by an evenly-distributed admixture of blackish scales, so that the wings appear of a uniform obscure gray. The hind-wings, in the European species, are crossed by a band of bluish scales; in the American, this band is white, while I have recently detected a slight powdered edging of blue scales, difficult to perceive, and apparently hitherto unnoticed. The principal difference in ornamentation is, again, to be found on the upper surface of the fore-wings. It lies in the course and shape of the transverse posterior line—a line which is a general feature of ornamentation throughout this group of insects.

This position might be sustained by parallel facts, but the limit of this article does not allow of their being presented. It remains to see what light these observations throw upon the origin of all these different kinds of moths. They seem to me to point to a method of variation in this group, and to a reason for its display. And, if we can apply these observations to particular instances, they may lead to a better understanding of the value of these specific forms, by allowing us to appreciate with more exactness the amount of differentiation they have undergone in the lapse of time. In a wider sense, we may attempt a classification by the ornamentation and coloration in the moths based on method, and such studies cannot fail to lend fresh interest to the barren but necessary work of describing the different species or forms. It seems to me, also, that these observations vindicate the importance of studying the characters of color and pattern in the group, and are, perhaps, a criticism on the remarks of those writers who purposely allow these no higher value than the subordinate one of dividing their material into “species” in their collections. In fact, these characters may give us a clew to the genealogy of the group, and seem to be of sufficient importance to be noted in descriptions of genera.


  1. “List of the Noctuidæ of North America, 1875–’76.” Buffalo, Reinecke & Zesch.