Fig. 121.—Mylacris anthracophilum Scudd. × 2. Carboniferous, Illinois.
This view of the relative antiquity of the two tribes of Palæoblattariæ is supported by the fact that while in both of them the internomedian branches show a tendency to repeat the general course of the anal nervules, as in the corresponding veins of the costal region, this tendency is lost in modern types; and among those ancient Blattinariæ, which are esteemed highest in the series, there is a marked tendency toward a loss of this repetition of the style of branching of the mediastinal and anal offshoots by the scapular and internomedian respectively.
A certain amount of geological evidence may also be claimed in support of this view. A survey of the species of the two groups found up to the present time in America, published and unpublished, shows that all the Mylacridæ are found below the Upper Carboniferous, while more than half the Blattinariæ are found in or above it. This results largely from a recent and as yet unpublished discovery of Blattinariæ in the Upper Coal Measures of Ohio and West Virginia, which in their general features are much nearer than previously discovered American Cockroaches to the European Blattinariæ, the latter of which come generally from Upper Carboniferous beds. The Mylacridæ have therefore been found in America in strata generally regarded as older than those which in Europe have yielded Cockroaches, and this gives a sufficient explanation why no Mylacridæ have yet been found in the Old World. In America one is mostly dealing with absolutely older forms, and they naturally give that continent a more old-fashioned look, when we regard the Carboniferous fauna as a whole. As already stated, a wing from the French Silurian (Palæoblattina Douvillei Brongn.) has been claimed as a Cockroach, but without good reason, and to see a real old Cockroach one must look to America.
Up to this point we have contrasted the palæozoic Cockroaches with the existing forms only, and finding such important distinctions between them, we naturally turn with some curiosity to the intermediate mesozoic and tertiary formations.
Now, not only are the mesozoic species as numerous (actually, but not relatively) as the palæozoic, but a recent discovery of a Triassic fauna of considerable extent, in the elevated parks of Colorado, presents us with a series of intermediate forms between those peculiar to the Coal Measures and those characteristic of the later mesozoic rocks. Excluding, however, for a moment this Triassic fauna, we may say of the later mesozoic species that they are Neoblattariæ, not Palæoblattariæ, though they still show some lingering characteristics of their ancestry. Thus the front wings are in general of a less dense texture than in modern times, but without the perfect diaphaneity of the palæozoic species; in some the anal veins fall in true palæoblattarian fashion on the inner margin, while in others which cannot be dissociated generically from them, the anal veins are disposed as in modern types. But in all there is a loss of one of the principal veins, or rather an amalgamation of two or more—a characteristic of more fundamental character. As a general rule, moreover, to which we shall again advert, the mass of the species are of small size, in very striking contrast to the older types.
Fig. 122.—Neorthroblattina Lakesii Scudd. × 5. Trias, Colorado.
To return now to the Triassic deposits of Colorado, we recognize here an assemblage of forms of a strictly intermediate character. Here are Palæoblattariæ and Neoblattariæ, side by side. The larger proportion are Palæoblattariæ, but all of them are specifically, and most of them generically, distinct from palæozoic species, and all rank high among Blattinariæ; still further, the species are all of moderate size, their general average being but little above that of mesozoic Cockroaches, and only a little more than half that of palæozoic types. The Neoblattariæ of this Triassic deposit are still smaller, being actually smaller than the average mesozoic Cockroach, and one or two of them, of the genus Neorthroblattina (see figure of N. Lakesii), have marked affinity to one of the genera of Palæoblattariæ (Poroblattina) peculiar to the same beds, differing mainly in the union or separation of the mediastinal and scapular veins; while others, as Scutinoblattina, have a Phoraspis-like aspect and density of membrane. This novel assemblage of species bridges over the distinctions between the Palæoblattariæ and Neoblattariæ. We find, first, forms in which the front wings are diaphanous, with distinct mediastinal and scapular veins, and the anal veinlets run to the border of the wing (Spiloblattina, Poroblattina); next, those having a little opacity of the front wings, with blended mediastinal and scapular, and the anal veins as before (some species of Neorthroblattina); then those with still greater opacity, with the same structural features (other species of Neorthroblattina); next, those having a coriaceous or leathery structure, blended mediastinal and scapular, and anal veins falling on the inner margin (some species of Scutinoblattina); and, finally, similarly thickened wings with blended mediastinal and scapular, and anal veins impinging on the anal furrow (other species of Scutinoblattina).
It is not alone, however, by the union of the mediastinal and scapular stems that the reduction of the veins in the wings of later Cockroaches has come about; for in many mesozoic types the externomedian vein is blended with one of its neighbours, while in others not only are the mediastinal and scapular united, but at the same time the externomedian and internomedian.