ntipedes, the millipedes, the Arachnida
(scorpions, spiders, &c.) and the Crustacea. In modern zoology the term
has become restricted to the lowest order of the class Hexapoda or true
insects. This order includes the bristle-tails and the springtails.

[Illustration: From _Knowledge_

FIG. 1.--A typical Thysanuran (_Machilus maritima_). Female, ventral
view.

ii.-x., Appendages on 2nd to 10th abdominal segments. The eversible
sacs on the abdominal segments are shown, some protruded and some
retracted.
Ovp, Ovipositor.
Mn, Mandible, and Mxl, maxillula, dissected out of head.]

Many wingless insects--such as lice, fleas and certain earwigs and
cockroaches--are placed in various orders together with winged insects
to which they show evident relationships. In such cases the absence of
wings must be regarded as secondary--due to a parasitic or other special
manner of life. But the bristle-tails and springtails, which form the
modern order Aptera, are all without any trace of wings, and, on account
of several remarkable archaic characters which they exhibit, there is
reason for believing that they are primitively wingless--that they
represent an early offshoot which sprang from the ancestral stock of the
Hexapoda before organs of flight had been acquired by the class.

_Characters._--In addition to the complete absence of wings and of
metamorphosis, the Aptera are characterized by peculiar elongate
mandibles (figs. 1, Mn.; 2, 4), with toothed apex and sub-apical
grinding surface, like those of certain Crustacea; by the presence
between the mandibles and maxillae of a pair of appendages (superlinguae
or maxillulae), fig. 1, Mxl., which are absent or vestigial in all other
insects; and, in most genera, by the presence in the adult of abdominal
appendages used for locomotion, these latter varying in number from one
to nine pairs. Among peculiarities of the internal organs the segmental
arrangement of the ovaries in most members of the order is noteworthy.
Many Aptera are covered with flattened scales like those of moths.

_Classification._--The Aptera are divided into two divergent sub-orders,
the _Thysanura_ (q.v.) or bristle-tails, and the _Collembola_ or
springtails.

_Thysanura._--The bristle-tails have an abdomen of eleven segments, the
tenth usually carrying a pair of long many-jointed tail-feelers (cerci,
fig. 1, x.); sometimes a median, jointed tail-appendage is also present.
To these feelers the p