Some of the species of Holocompsa remind us strongly of Hemiptera of the family Capsidae; they have an arrangement of colours similar to what prevails in that group, and their tegmina and wings which, as being those of Blattids may be said to be abnormally formed, resemble in texture and the distribution of the venation those of the Hemiptera. These Insects are closely allied to Diaphana, of which genus we have figured a species (Fig. 122).
There is very little evidence on which to base an estimate of the number of species of Blattidae existing in the world at present. Probably the number extant in collections may amount to 1000 or thereabouts, and the total existing in the world may be as many as 5000. The species of Blattidae cannot tolerate cold, and are consequently only numerous in tropical regions. Europe possesses about twenty species, and in Britain there are only three that are truly native; these are all small Insects belonging to the genus Ectobia, and living out of doors, amongst leaves, under bushes, and in various other places. We have, however, several other species that have been introduced by the agency of man, and these all live under cover, where there is artificial warmth and they are protected from the inclemencies of the winter season. The commonest of these forms is Stilopyga orientalis, the "black beetle" of our kitchens and bakehouses. This Insect is said to have been brought to Europe from "Asia" about 200 years ago, but the evidence as to its introduction, and as to the country of which it is really a native, is very slight. It is indeed said[[163]] that S. orientalis has been found in peat in Schleswig-Holstein. Periplaneta americana is a larger Insect, and is common in some places; it is apparently the species that is most usually found on board ships, where it sometimes multiplies enormously, and entirely devours stores of farinaceous food to which it obtains access: it is known that sometimes a box or barrel supposed to contain biscuits, on being opened is found to have its edible contents entirely replaced by a mass of living cockroaches. Fortunately Periplaneta americana has not spread widely in this country, though it is found in great numbers in limited localities; one of the best known of which is the Zoological Gardens in the Regent's Park at London. Periplaneta australasiae is very similar to P. americana, but has a yellow mark on the shoulder of each tegmen. This has obtained a footing in some of the glass-houses in the Botanic Gardens at Cambridge and Kew; and it is said to be fairly well established in Belfast. Another of our introduced domestic cockroaches is Phyllodromia germanica, a much smaller Insect than the others we have mentioned. It has only established itself at a few places in this country, but it is extremely abundant in some parts of Northern and Eastern Europe. It has been increasing in numbers in Vienna, where, according to Brunner, it is displacing Stilopyga orientalis. In addition to these, Rhyparobia maderae and species of the genus Blabera have been met with in our docks, and are possibly always to be found there. They are Insects of much larger size than those we have mentioned. We figure the alar organs of one of these species of Blabera of the natural size: the species in this genus are extremely similar to one another. Blaberae are known in the West Indies as drummers, it being supposed that they make a noise at night,[[164]] but details in confirmation of this statement are wanting.
Fig. 132.—Alar organs of Blabera sp. A, tegmen; B, wing.
It is a remarkable fact that no satisfactory reasons can be assigned for the prevalence of one rather than another of these domestic cockroaches in particular localities. It does not seem to depend at all on size, or on the period of development, for the three species Stilopyga orientalis, Periplaneta americana, and Phyllodromia germanica, which are the most abundant, differ much in these respects, and replace one another in particular localities, so that it does not appear that any one is gaining a permanent or widespread superiority as compared with another. There are, however, no sufficient records on these points, and further investigation may reveal facts of which we are at present ignorant, and which will throw some light on this subject. We may remark that Mr. Brindley has found it more difficult to obtain hatching of the young from the egg-capsules of Periplaneta americana and Phyllodromia germanica at Cambridge, than from those of Stilopyga orientalis.
Although much work has been done on the embryology of Blattidae, the subject is still very incomplete. The recent memoirs of Cholodkovsky[[165]] on Phyllodromia germanica contain so much of general interest as to the development of the external parts of the body that we may briefly allude to them. The earliest appearance of segmentation appears to be due to the centralisation of numerous cells round certain points in the ventral plate. The segmentation of the anterior parts is first distinct, and the appearance of the appendages of the body takes place in regular order from before backwards, the antennae appearing first; the mandibles, however, become distinct only subsequent to the maxillae and thoracic appendages. There are in the course of the development appendages to each segment of the body (he counts eleven abdominal segments); the cerci develop in a similar manner to the antennae; the first pair of abdominal appendages—at first similar to the others—afterwards assume a peculiar stalked form. The abdominal appendages subsequently disappear, with the exception of the ninth pair, which form the ventral styles, and the eleventh pair, which become the cerci. The last ventral segment is said to be formed by the union of the tenth and eleventh embryonic ventral segments.
Fig. 133.—A, Tegmen(?) of Palaeoblattina douvillei; B, of Etoblattina manebachensis. (After Brauer and Scudder.)
As regards their Palaeontological forms Blattidae are amongst the most interesting of Insects, for it is certain that in the Carboniferous epoch they existed in considerable number and variety. A still earlier fossil has been found in the Silurian sandstone of Calvados; it consists of a fragment (Fig. 133, A), looking somewhat like an imperfect tegmen of a Blattid; it was described by Brongniart under the name of Palaeoblattina douvillei, and referred by him, with some doubt, to this family. Brauer has, however, expressed the opinion[[166]] that the fragment more probably belonged to an Insect like the mole-cricket, and in view of this discrepancy of authorities we may be pardoned for expressing our own opinion to the effect that the relic has no connexion with the Insecta. The figure given by Scudder[[167]] has not, however, so uninsect-like an appearance as that we have copied from Brauer. Whatever may prove to be the case with regard to Palaeoblattina, it is certain, as we have already said, that in the Palaeozoic epoch Insects similar to our existing cockroaches were abundant, their remains being found in plenty in the coal-measures both of Europe and North America. Fig. 133, B, shows a fossil tegmen of Etoblattina manebachensis from the upper Carboniferous beds of Ilmenau in Germany. It will be noticed that the disposition of the nervures is very much like that which may be seen in some of our existing Blattidae (cf. the tegmen of Blabera, Fig. 132, A), the vena dividens (a) being similarly placed, as is also the mediastinal vein on the front part of the organ. The numerous carboniferous Blattidae have been separated as a distinct Order of Insects by Scudder under the name Palaeoblattariae, but apparently rather on theoretical grounds than because of any ascertained important structural distinctions. He also divided the Palaeoblattariae into two groups, Mylacridae and Blattinariae, the former of which was supposed to be peculiar to America. Brongniart has, however, recently discovered that in the Carboniferous deposits of Commentry in France Mylacridae are as common as in America. This latter authority also states that some of the females of these fossil Blattidae are distinguished by the presence of an elongate exserted organ at the end of the body. He considers this to have been an ovipositor by which the eggs were deposited in trees or other receptacles, after a manner that is common in certain Orthoptera at the present day. If this view be correct these Carboniferous Insects must have been very different from the Blattidae of our own epoch, one of whose marked characteristics is the deposition of the eggs in a capsule formed in the body of the parent.
In the strata of the secondary epoch remains of Blattidae have also been discovered in both Europe and America, in Oolitic, Liassic, and Triassic deposits. From the Tertiary strata, on the other hand, comparatively few species have been brought to light. A few have been discovered preserved in amber.