Page:Popular Science Monthly Volume 68.djvu/250

246 given off which strengthen the upper edge of the wing. The following vein, the radius, reaches as a rule to the apex, and its branches given off from the upper side fill the apex. The medius, which is often of lesser importance, terminates on the inner border not far from the apex, and gives off a few usually superior branches. The fifth vein of the wing, the cubitus, is of more importance. It reaches along the inner border of the wing one half to two thirds the distance towards the apex, giving off strong inferior branches which strengthen the lower edge of the wing. The remaining small area on the lower inner corner of the wing is marked off by a deep furrow. The veins of this area are all simple or once forked, and pass with a uniform curve from their origin to the inner border.

The next type (Fig. 2) is from later deposits—the Permian. Here two of the main veins, the radius and media, have become fused for a little distance from their origin, so that instead of four strong veins arising from the middle of the base there are here only three. Otherwise the wing is not unlike the Coal Measure form. Many of the mesozoic wings have, in addition to a partial fusion of two or more of the main veins, a further radical change in the anal area, the veins of which, instead of ending on the inner border, run to and end on the anal furrow. The front wing of one of the living cockroaches, the common 'croton bug,' is shown in figure 3. Here cross veins are numerous, and the anal veins, as in most of the Mesozoic forms, end on the anal furrow. The wings described are those which are in a general way typical of their time. Along with each of these are found wings, some of which are more advanced, while others are of more simple structure than those illustrated. Among modern cockroaches not a few genera and species have both front and hind wings so incompletely developed as to be entirely useless, and present only as functionless wing pads. All Paleozoic cockroaches, so far as known, were provided with fully developed functional wings, the modern wingless forms having descended without doubt from winged ancestors.

The hind wings of Carboniferous cockroaches are as a rule broad with rounded inner border; the veins are evenly distributed; there are no cross veins; no indication of any fan-like plaiting such as is found in the hind wings of modern forms; apparently there was no folding, the wing lying spread out across the abdomen. Neither is there any differentiation into a thicker, brownish, outer part, and a thinner, membranous folded part. The hind wing, being less firm than the front, is naturally less often preserved. Nevertheless a considerable number have now been obtained. In the structure of the hind wing the Carboniferous cockroaches present fairly uniform characters, only occasionally, and from late Carboniferous deposits, are there indications that a folding of the wing had originated.