the Poduras did
not have an independent origin and do not, perhaps, represent a distinct
branch of the genealogical tree of articulates. While the Poduras may be
said to form a specialized type, the Bristle-tails (Lepisma, Machilis,
Nicoletia and Campodea) are, as we have seen, much more highly
organized, and form a generalized or comprehensive type. They resemble
in their general form the larva of Ephemerids, and perhaps more closely
the immature Perla, and also the wingless cockroaches.

[Illustration: 186. Embryo of Diplax.]

[Illustration: 187. Embryo of Louse.]

Now such forms as these Thysanura, together with the mites and the
singular Pauropus, we cannot avoid suspecting to have been among the
earliest to appear upon the earth, and putting together the facts,
first, of their low organization; secondly, of their comprehensive
structure, resembling the larvae of other insects; and thirdly, of their
probable great antiquity, we naturally look to them as being related in
form to what we may conceive to have been the ancestor of the class of
insects. Not that the animals mentioned above were the actual ancestors,
but that certain insects bearing a greater resemblance to them than any
others with which we are acquainted, and belonging possibly to families
and orders now extinct, were the prototypes and progenitors of the
insects now known.

[Illustration: 188. Embryo of Spider.]

[Illustration: 189. Embryo of Podura.]

Though the study of the embryology of insects is as yet in its infancy,
still with the facts now in our possession we can state with tolerable
certainty that at first the embryos of all insects are remarkably alike,
and the process of development is much the same in all, as seen in the
figure of Diplax (Fig. 186), the louse (Fig. 187), the spider (Fig. 188)
and the Podura (Fig. 189), and we could give others bearing the same
likeness. We notice that at a certain period in the life of the embryo
all agree in having the head large, and bearing from two to four pairs
of mouth organs, resembling the legs; the thorax is merged in with the
abdomen, and the general form of the embryo is ovate. Now this general
embryonic form characterizes the larva of the mites, of the myriopods
and of the true insects. To such a generalized embryonic form to which
the insects may be referred as the descendants, we would give the name
of _Leptus_, as among Crustacea the ancestral form is referred to
Nauplius, a larval fo