A comparison of the neuration of the tegmina of mesozoic and recent Cockroaches, to determine as far as possible the immediate relations of the former to existing types, gives as yet little satisfaction. The prolific genera, Mesoblattina and Rithma, may be said to bear considerable resemblance to the Phyllodromidæ, and the peculiar neuration of Elisama is in part repeated in the Panchloridæ, as well as in some Phyllodromidæ and Epilampridæ. Scutinoblattina also reminds one in certain features of some Epilampridæ, like Phoraspis. The other genera appear to have no special relations to any existing type. As a whole, it would appear as if the Blattariæ spinosæ approached closer to the mesozoic forms than do the Blattariæ muticæ.
Fig. 125.—Pterinoblattina intermixta Scudd. × 4. Upper Lias, England.
As to the tertiary Cockroaches we know very little, exceedingly few having been preserved, even in amber—that wonderful treasury of fossil Insects. Here first we come across apterous forms, Polyzosteria having been recognised in Prussian amber,[199] together with winged species, which seem to be Phyllodromidæ; these are the only Blattariæ spinosæ known from the Tertiaries. Of the other group, we have Zetobora, one of the Panchloridæ, and Paralatindia, one of the Corydidæ, from American rocks, and Heterogamia and Homœogamia, one from Parschlug in Steiermark, the other from Florissant in Colorado, belonging to the sub-family Heterogamidæ. Others are mentioned, generally under the wide generic term Blatta, from Oeningen, Eisleben, Rott, and even from Spitzbergen and Greenland; but little more than their names are known to us. Paralatindia, from the Green River beds of Wyoming, U.S., is the only tertiary Cockroach yet referred to an extinct genus; but close attention has not yet been paid even to the few tertiary Cockroaches which we know. There is no reason to suppose that they will be found to differ more from the existing types than is generally the case with other Insects. The more we learn of cænozoic Insects, the more truly do we find that the early Tertiary period was in truth the dawn of the present, the distinction between the faunas of these remotely separated times (though not to be compared in character) being scarcely greater than is found to-day between the Insects of the temperate and torrid zones.
We began this review with the statement that no Insect was so important palæontologically as the Cockroach. This would more clearly appear had we space to pass in review the geological history of all the Insect tribes; for then it could be shown that it was only in the passage from palæozoic to mesozoic times that the great ordinal groups of Insects were differentiated, and that the Triassic period therefore becomes the expectant ground of the student of fossil Insects. Up to the present time we do not know half a dozen Insects besides Cockroaches from these rocks. Yet, notwithstanding this advantage on the part of the Cockroaches, how meagre is the history, how striking the “imperfection of the geological record” concerning them, the following tabulation of the fossil species by their genera will show.
It here appears that there are about 80 species known from the palæozoic rocks, two or three more than that from the mesozoic, and only nine from the cænozoic! When we call to mind that half the palæozoic Insects were Cockroaches, and that seven or eight hundred species exist to-day, what shall we say of the paltry dozen[200] from the rich tertiaries? Shall we claim that these figures represent their true numerical proportion to their numbers in the more distant past? Then, indeed, must the palæozoic period have been the Age of Cockroaches; for all research into the past shows that a type once losing ground continues to lose it, and does not again regain its strength. The Cockroaches of to-day are no longer, as once, a dominant group; they are but a fragment of the world’s Insect-hosts; yet even now the species are numbered by hundreds. If this be a waning type, what must its numbers have been in the far-off time, when the warm moisture which they still love was the prevailing climatic feature of the world; and how few of that vast horde have been preserved to us! The housekeeper will thank God and take courage.
GEOLOGICAL DISTRIBUTION OF FOSSIL COCKROACHES.