Order: _Aptera_,
which is divided into two sub-orders:
1. _Thysanura_ (Bristle-tails): with ten abdominal segments; number of
abdominal appendages variable. Cerci prominent. Developed tracheal
system.
2. _Collembola_ (Spring-tails): with six abdominal segments;
appendages of the first forming an adherent ventral tube, those of the
third a minute "catch," those of the fourth (fused basally) a
"spring." Tracheal system reduced or absent.
Sub-class: EXOPTERYGOTA.
Hexapoda mostly with wings, the wingless forms clearly degraded.
Maxillulae rarely distinct. No locomotor abdominal appendages. The
wing-rudiments develop visibly outside the cuticle. Young like or
unlike parents.
Order: _Dermaptera_.
Biting mandibles; minute but distinct-maxillulae; second maxillae
incompletely fused. When wings are present, the fore-wings are small
firm elytra, beneath which the delicate hind-wings are complexly
folded. Many forms wingless. Genital ducts entirely mesodermal. Cerci
always present; usually modified into unjointed forceps. Numerous (30
or more) Malpighian tubes. Young resembling parents.
Includes two families--the _Forficulidae_ or _earwigs_ (q.v.) and the
_Hemimeridae_.
Order: _Orthoptera_.
Biting mandibles; vestigial maxillulae; second maxillae incompletely
fused. Wings usually well developed, net-veined; the fore-wings of
firmer texture than the hind-wings, whose anal area folds fanwise
beneath them. Jointed cerci always present; ovipositor well developed.
Malpighian tubes numerous (100-150). Young resemble parents.
Includes stick and leaf insects, cockroaches, mantids, grasshoppers,
locusts and crickets (see ORTHOPTERA).
Order: _Plecoptera_.
Biting mandibles; second maxillae incompletely fused. Fore-wings
similar in texture to hind-wings, whose anal area folds fanwise.
Jointed, often elongate, cerci. Numerous (50-60) Malpighian tubes.
Young resembling parents, but aquatic in habit, breathing dissolved
air by thoracic tracheal gills.
Includes the single family of the _Perlidae_ (Stone-flies), formerly
grouped with the Neuroptera.
Order: _Isoptera_.
Biting mandibles; second maxillae incompletely fused. Fore-wings
similar in shape and texture to hind-wings, which do not fold. In most
species the majority of individuals are wingless. Short, jointed
cerci. Six or eight Malpighian tubes. Young resembling adults;
terrestrial throughout life.
Includes two families, formerly reckoned among the Neuroptera--the
_Embiidae_ and the _Termitidae_ or "White Ants" (see TERMITE).
Order: _Corrodentia_.
Biting mandibles; second maxillae incompletely fused; maxillulae often
distinct. Cerci absent. Four Malpighian tubes.