us, to the stout chitinous jaws of the Mallophaga, or bird
lice, is a sudden transition, but on comparing the rest of the head and
body it will be seen that the distinction only amounts to a family one,
though Burmeister placed the Mallophaga among the Orthoptera
(grasshoppers and crickets) on account of the mandibles being adapted
for biting. It has been a common source of error to depend too much upon
one or a single set of organs. Insects have been classified on
characters drawn from the wings, or the number of the joints of the
tarsi, or the form of the mouth parts. We must take into account in
endeavoring to ascertain the limits of natural groups, as the internal
anatomy and the embryology and metamorphosis of insects, before we can
hope to obtain a natural classification.

The family of bird lice is a very extensive one, embracing many genera,
and several hundred species. One or more species infest the skin of all
our domestic and wild mammals and birds, some birds sheltering beneath
their feathers four or five species of lice. Before giving a hasty
account of some of our more common species; we will give a sketch of the
embryological history of the lice, with special reference to the
structure of the mouth parts.

[Illustration: 114. Embryo of the Louse.]

[Illustration: 115. Mouth Parts of the Louse.]

The eggs (Fig. 114, egg of the head louse) are long, oval, somewhat
pear-shaped, with the hinder end somewhat pointed, while the anterior
end is flattened, and bears little conical micropyles (_m_, minute
orifices for the passage of the spermatozoa into the egg), which vary in
form in the different species and genera; the opposite end of the egg is
provided with a few bristles. The female attaches her eggs to the hairs
or feathers of her host.

[Illustration: 116. Mouth Parts of the Louse.]

[Illustration: 118. Mouth Parts of Louse.]

[Illustration: 117. Mouth Parts of Louse.]

After the egg has been fertilized by the male, the blastoderm, or
primitive skin, forms, and subsequently two layers, or embryonal
membranes, appear; the outer is called the amnion (Fig. 114, _am_),
while the inner visceral membrane (_db_) partially wraps the rude form
of the embryo in its folds. The head (_vk_) of the embryo is now
directed towards the end of the egg on which the hairs are situated;
afterwards the embryo revolves on its axis and the head lies next to the
opposite end of the egg. Eight tubercles bud out from the un