The classification just given has been drawn up with reference to
existing insects, but the great majority of the extinct forms that have
been discovered can be referred with some confidence to the same orders,
and in many cases to recent families. The Hexapoda, being aerial,
terrestrial and fresh-water animals, are but occasionally preserved in
stratified rocks, and our knowledge of extinct members of the class is
therefore fragmentary, while the description, as insects, of various
obscure fossils, which are perhaps not even Arthropods, has not tended
to the advancement of this branch of zoology. Nevertheless, much
progress has been made. Several Silurian fossils have been identified as
insects, including a Thysanuran from North America, but upon these
considerable doubt has been cast.
The Devonian rocks of Canada (New Brunswick) have yielded several
fossils which are undoubtedly wings of Hexapods. These have been
described by S. H. Scudder, and include gigantic forms related to the
Ephemeroptera.
In the Carboniferous strata (Coal measures) remains of Hexapods become
numerous and quite indisputable. Many European forms of this age have
been described by C. Brongniart, and American by S. H. Scudder. The
latter has established, for all the Palaeozoic insects, an order
Palaeodictyoptera, there being a closer similarity between the
fore-wings and the hind-wings than is to be seen in most living orders
of Hexapoda, while affinities are shown to several of these
orders--notably the Orthoptera, Ephemeroptera, Odonata and Hemiptera. It
is probable that many of these Carboniferous insects might be referred
to the Isoptera, while others would fall into the existing orders to
which they are allied, with some modification of our present diagnoses.
Of special interest are cockroach-like forms, with two pairs of similar
membranous wings and a long ovipositor, and gigantic insects allied to
the Odonata, that measured 2 ft. across the outspread wings. A
remarkable fossil from the Scottish Coal-measures (_Lithomantis_) had
apparently small wing-like structures on the prothorax, and in allied
genera small veined outgrowths--like tracheal gills--occurred on the
abdominal segments. To the Permian period belongs a remarkable genus
_Eugereon_, that combines hemipteroid jaws with orthopteroid
wing-neuration. With the dawn of the Mesozoic epoch we reach Hexapods
that can be unhesitatingly referred to existing orders. From the Trias
of Colorado, Scudder has described cockroaches intermediate between
their Carboniferous precursors and their present-day descendants, while
the existence of endopterygotous Hexapods is shown by the remains of
Coleoptera of several families. In the Jurassic rocks are found
Ephemeroptera and Odonata, as well as Hemiptera, referable to existing
families, some representatives of which had already appeared in the
oldest of the Jurassic ages--the Lias. To the Lias also can be traced
back the Neuroptera, the Trichoptera, the orthorrhaphous Diptera and,
according to the determination of certain obscure fossils, also the
Hymenoptera (ants). The Lithographic stone of Kimmeridgian age, at
Solenhofen in Bavaria, is especially rich in insect remains,
cyclorrhaphous Diptera appearing here for the first time. In Tertiary
times the higher Diptera, besides Lepidoptera and Hymenoptera, referable
to existing families, become fairly abundant. Numerous fossil insects
preserved in the amber of the Baltic Oligocene have been described by G.
L. Mayr and others, while Scudder has studied the rich Oligocene faunas
of Colorado (Florissant) and Wyoming (Green River). The Oeningen beds of
Baden, of Miocene age, have also yielded an extensive insect fauna,
described fifty years ago by O. Heer. Further details of the geological
history of the Hexapoda will be found in the special articles on the
various orders. Fragmentary as the records are, they show that the
Exopterygota preceded the Endopterygota in the evolution of the class,
and that among the Endopterygota those orders in which the greatest
difference exists between imago and larva--the Lepidoptera, Diptera and
Hymenoptera--were the latest to take their rise.