Page:Popular Science Monthly Volume 68.djvu/248

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O insects are more abundant as fossils, and none so widely distributed through the various formations, as are the cockroaches. Their delight in moist places, often near the banks of streams, their firmly chitinized wings and body render them of insects among the best adapted for preservation. Especially are they to be expected in association with ferns and other plants among which during life they found shelter, and together with which they were transported to their resting place in the rocks. Hardly ever will persistent search among fossil leaves of land origin, when imbedded in rocks of sufficiently fine texture, fail to bring to light at least detached wings and perhaps bodies. Even when the bodies of adults have not been preserved, not infrequently will be found the cast-off integument of the young. It is this approximately complete geological record that lends an especial interest to the cockroach family.

The cockroaches have proved themselves a remarkably conservative group having retained throughout their long existence, as compared with other insects, a relatively generalized structure. The development is direct, the young resembling the adults. The mouth parts are of the biting kind common to primitive insects. The segmentation of the abdomen and thorax is distinct. The foot is five-jointed. The venation of the wing is much less complicated than that of many of the more advanced types. Not all the organs, however, have retained this primitive simplicity. In this, as indeed in every group, some organs have outrun others in degree of specialization, so that the group early became a characteristic and well delimited one. The body is flattened, the head small and turned downwards. The covering of the first thoracic segment, the pronotum, is enlarged, rounded, more or less shield-shaped. The front wing is firmly chitinized and lies flat on the back, or slightly arched to conform to the shape of the thorax. An inner area near the base of the wing is marked off by a deep curved line, the anal furrow. The hind wing is less resistant and broader, the greater width being obtained by a greater expansion of the inner border. Hardly ever will a doubt arise as to the reference to this family of even a fragment of a specimen.

The structural characters so far mentioned made their appearance early. The rounded pronotum is as characteristic of Paleozoic as of