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Page:Popular Science Monthly Volume 70.djvu/64

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60
THE POPULAR SCIENCE MONTHLY

the Mesozoic, or the age of reptiles. Of its three chief divisions, Cretaceous, Jurassic and Triassic, the first mentioned and youngest has thus far yielded only a small number of fossil insects. During the Cretaceous, the flowering plants came into existence, and on this account it may be concluded that a multitude of new conditions were furnished for many kinds of insect forms. The bees and various other honey-eaters could thus have originated. The fact that insects immediately adapted themselves to these new plants is to be seen in the few specimens thus far obtained; that is, in the galls and eaten places on the leaves of the oak, willow and Eucalyptus, etc. Other than these, unfortunately, but little evidence of insects has been found in the Cretaceous.

On the contrary, the remains of this group preserved in Jurassic deposits are very large in number. These have been discovered in England, Spain and Russia, but nowhere in such quantities and remarkable preservation as in the Jura of Franconia in northern Bavaria, where in previous epochs a shallow sea between coral reefs became filled up with the finest calcareous silt. Many of the insects which peopled the neighboring land found their graves in this mud. By a fortunate chance, after perhaps millions of years, these forms have now come to us, for this same hardened mud is to-day used by us as lithographic stone or paving-stone.

Now what does this rich collection of Jurassic insects teach us? It shows that in that period probably an entire series of groups of living forms either then had no existence or were just in the process of evolution. As yet are found no locusts, no earwigs, termites, thrips and wood-lice. Of the Diptera, the only representatives are those which are in the minority to-day; of the Hymenoptera, the wood-wasp, saw-fly and ichneumon-fly alone appear to have been present, while bees, ants, etc., are wanting. Some primitive forms of butterflies have been discovered, but these were at first erroneously regarded as cicadas. Grasshoppers were abundantly developed and some of them, judging from the structure of their legs, may have run about on the water or wet mud quite as water-striders, a genus of aquatic insects, do at the present time. Through their changed habits of living, these water locusts thus appear to have modified the legs no longer needed for jumping, and in this way the specters, or walking-sticks, may have finally originated. Dragon-flies, May-flies, Neuroptera and Hemiptera were represented in great variety, and of the last group there were aquatic species as well as those terrestrial; also small cicadas. Beetles, too, were not wanting, although no particularly striking forms are to be distinguished.

The fact that Jurassic insects were so extremely abundant clearly indicates a warm climate, and the school children of Bavaria would