aws and palpi. Given a mouth and limbs surrounding it, and we at
once have a rude head set off from the rest of the body. And in fact
such is the history of the development of these parts in the embryo. At
first the head is indicated by the buds forming the rudiments of limbs;
the segments to which they are attached do not form a true head until
after the mouth-parts have attained their jaw-like characters, and it is
not until the insect is about to be hatched, that the head is definitely
walled in.

[Illustration: 202. Foot of Chironomus.]

[Illustration: 203. Ephydra.]

We have arrived, then, at our Leptus, with a head bearing two pairs of
jaws. The spiders and mites do not advance beyond this stage. But in the
true insects and Myriopods, we have the addition of special sense
organs, the antennae, and another pair of appendages, the labial palpi.
It is evident that in the ancestor of these two groups the first pair of
appendages became early adapted for purely sensory purposes, and were
naturally projected far in advance of the mouth, forming the antennae.

Before considering the changes from the mandibulate form of insects to
those with mouth parts adapted for piercing and sucking, we must
endeavor to learn how far it was possible for the caterpillar or maggot
to become evolved from the Leptus-like larvae of the Neuroptera,
Orthoptera, Hemiptera and most Coleoptera. I may quote from a previous
article[28] a few words in relation to two kinds of larvae most prevalent
among insects. "There are two forms of insectean larvae which are pretty
constant. One we call _leptiform_, from its general resemblance to the
larvae of the mites (Leptus). The larvae of all the Neuroptera, except
those of the Phryganeidae and Panorpidae (which are cylindrical and
resemble caterpillars), are more or less leptiform, _i. e._, have a
flattened or oval body, with large thoracic legs. Such are the larvae of
the Orthoptera and Hemiptera, and the Coleoptera (except the
Curculionidae; possibly the Cerambycidae and Buprestidae, which approach
the maggot-like form of the larvae of weevils). On the other hand, taking
the caterpillar or bee larva, with their cylindrical, fleshy bodies, in
most respects typical of larval forms of the Hymenoptera, Lepidoptera
and Diptera, as the type of the _cruciform_ larva, etc. * * * The larvae
of the earliest insects were probably leptiform, and the cruciform
condition is consequently an acquired one, as sugges