Morphological studies of existing insects prove that the first true insects were wingless (Apterygota), like the Thysanura (silverfish) and Entotrophi, combining the generalized characteristics of both of these groups.

The development of winged insects (Pterygota) was the first great evolutionary step within the insect line. The origin of wings is by no means clear; they probably were developed from lateral flaps, like those on the first thoracic segment of the Palaeodictyoptera. These primitive flying insects, termed the Palaeoptera and exemplified by the Odonata, Ephemeroptera, and several extinct orders, were unable to flex their wings over the abdomen at rest.

The second main evolutionary change was the development of an articulation that made it possible for the wings to be held over the abdomen when the insect was not in flight. All living Pterygota except the Odonata and Ephemeroptera belong in this category, which is termed the Neoptera. The acquisition of this wing-flexing mechanism was an important change, for it enabled the insects, in adult as well as the immature stages, to hide in debris or under stones or logs. The first neopterous types had a simple or direct type of postembryonic development and are usually termed the hemimetabolous Neoptera.

The third main evolutionary step was the attainment of the more complex type of metamorphosis, with larval and pupal stages, resulting in the holometabolous Neoptera.

The fossil record of the insects, though incomplete, has given us a general idea of the time of occurrence of the three events. The existence of two orders of insects (Mecoptera and Neuroptera) with complete metamorphosis in lower Permian rocks can only mean that this step was attained at least by late Upper Carboniferous time. Similarly, the presence of species with wing-flexing abilities in the early Upper Carboniferous shows that the hemimetabolous Neoptera arose in the Lower Carboniferous. Unfortunately, since no insects have been found in strata older than those of the Upper Carboniferous period, we have no actual record of the existence of Palaeoptera before these Neoptera; nor, for that matter, is there any Paleozoic record of the Apterygota. Because all evidence derived from other sources indicates the primitive nature of these two categories, however, we can infer that the Palaeoptera Preceded the Neoptera, and therefore that they existed in the early part of the Lower Carboniferous. Similarly, We can infer that the Apterygota, which must have preceded them, arose still earlier in the Lower Carboniferous or, more likely, in the Devonian. The conclusion to be drawn from the record, at any rate, is that all three of the main steps in insect evolution took place before the end of the Carboniferous period, about 250 million years ago. Nothing nearly so important has happened to the insects since then.

Another contribution that fossil insects have made to our understanding of the evolution of the group pertains to the progressive increase in the relative numbers of species having complete metamorphosis in the geological periods since the lower Permian. Starting from the beginning of the Permian, during which only about 10 percent of the known species had complete metamorphosis, there has been an increase up to 88 percent at the present time. The most rapid change (10 to 40 percent) seems to have taken place in the interval of the Permian period. Although there is a possibility that such a marked shift in the insect population did actually take place in that time, the more probable explanation is that complete metamorphosis arose further back in the geologic time than the lower Permian, and that the change was more gradual.

Those are two examples of the way in which the study of fossils has contributed to our understanding of insect evolution. There is every indication that the insects have been as numerous on earth as they are now for at least the time since the Jurassic period, about 150 million years; and also that the insect fauna of our time is but a small part of the total parade of insect life that has lived on the earth during the past 250 million years. It is not surprising, therefore, that our understanding of insect evolution depends to a large extent on a knowledge of this extinct population.

FRANK M. CARPENTER is professor of entomology, Alexander Agassiz professor of zoology, and curator of fossil insects in the Museum of Comparative Zoology at Harvard University. He joined the staff of Harvard in 1932 and has done research on fossil insects, insect evolution, and the taxonomy of Mecoptera and Neuroptera.