Since insects constitute the highest subdivision of Arthropods, they include the very highest forms of animal life except the Vertebrates. The oldest known fossil insects are from Pennsylvanian strata, more than 1,000 species having been described from rocks of that age. They were all simple or primitive types like cockroaches and dragon flies, and were remarkable for size. Giant cockroaches got to be four inches long. One form of dragon fly, with a spread of wing of over two feet, was probably the largest insect which ever lived (Plate 15). Development of insect life was especially favored during the great Coal Age because of the prolific vegetation, but more than likely insects originated somewhat earlier. Early in the Mesozoic era a great progressive change began to come over insect life and higher forms gradually evolved until by the close of the era many of the highest types like flies, ants, and bees were common. As might be expected, the highest insects did not develop until after the appearance of the true flowering plants in later Mesozoic time, butterflies apparently not having evolved until early in Cenozoic time. Many of the thousands of known species of fossil insects are from strata of Tertiary age during which time they may have been even more numerous than to-day, although there are about 400,000 species now living. An almost incredible case is a Tertiary stratum only a few feet thick in Switzerland from which nearly 1,000 species of insects have been unearthed. Another famous locality is Florissant, Colorado, where during early Tertiary time there was a small lake into which showers of fine volcanic dust fell and entombed vast numbers of insects, more than 2,000 species having been unearthed. Still another extraordinary occurrence is along the shores of the southern Baltic Sea where more than 2,000 species of insects have been found in a fossil resin called amber. The insects were caught in the still soft sticky resin while it was exuding from the trees, and thus we have the insects, fully two or three million years old, literally embalmed and marvelously preserved, often in beautifully transparent amber. CHAPTER XIX GEOLOGICAL HISTORY OF VERTEBRATE ANIMALS (INCLUDING MAN)