The term "insect" comes from the Latin _insectum_, and signifies "cut
into." It expresses one of the prime characteristics of this class of
animals, namely, that of segmentation. This feature of having the body
divided into rings or segments by transverse incisions is possessed by
other large groups of animals, and was considered of sufficient
importance by Cuvier to lead him, in his system of classification, to
group with Insects, under the general term Articulata, Worms, Crustacea,
Spiders, and Myriapods. Worms differ from the other four groups in
having no articulated appendages, and in having a soft body-wall or
integument instead of a dense chitinous covering, and are separated as a
special class _Vermes_. The other four groups of segmented animals
possess in common the feature of jointed appendages and a covering of
chitinous plates, and are brought together under the term _Arthropoda_.
The division of the body into a series of segments by transverse
incisions, characteristic of these animals and these only, justifies the
use of Cuvier's old name, Articulates, as this segmented feature
represents a definite relationship and a natural division--as much so as
the vertebral column in Vertebrates. The Cuvierian name should be
retained as a coordinate of Vertebrates, Molluscs, etc., and the terms
Vermes and Arthropods may be conveniently used to designate the two
natural divisions of the Articulates.
The term "insect" has been employed by authors in two different
senses--one to apply to the tracheated animals or those that breathe
through a system of air tubes (tracheae), comprising Spiders, Myriapods,
and insects proper or Hexapods,[1] and the other in its restricted sense
as applied to the Hexapods only. To avoid confusion, the latter
signification only should be used, and it will be thus used in this
article.
[1] From the Greek [Greek: exapous], having 6 feet.
We see, then, that insects share, in common with many other animals, the
jointed or articulated structure. Wherein, then, do they differ?
_Briefly, in having the body divided into thirteen joints and a
subjoint_, including the head as a joint, and in the adult having six
true, jointed legs, and usually, though not always, wings. The five
classes of Articulates differ from each other in the number of legs they
possess in the adult form, as follows: Hexapoda, 6 legs; Arachnida, 8
legs; Crustacea, 10-14 legs; Myriapoda, more than 14 legs; Vermes, none.
This system holds for the adult form only, because some mites
(Arachnida) when young have only 6 legs, and many true insects in the
larva state either have no legs at all, or have additional abdominal
legs which are not jointed, but membranous, and are lost in the perfect
or adult state. These are called false or prolegs.
It will serve to make these instructions clear if I at once explain that
the life of an insect is marked by four distinct states, viz., the egg,
the larva, the pupa, and the imago, and that the last three words will
constantly recur. We have no English equivalent for the words larva and
pupa, for while some authors have written them with the terminal _e_, so
as to get the English plural, yet "larves" and "pupes" so shock the ear
that the terms have not been (and deserve not to be) generally adopted.
We have seen that an insect in the final state has six true legs. Yet
even here many species depart from the rule, as there are many in which
the perfect insect, especially in the female sex, is apodous or without
legs, just as there are also other cases where they are without wings.
Sometimes the legs seem to be reduced in number by the partial or total
atrophy of one or the other pair, but in all these exceptional cases
there is no difficulty in realizing that we have to deal with a true
insect, because of the other characters pertaining to the class, some of
which it will be well to allude to.
Insects are further characterized by having usually three distinct
divisions of the body, viz.: head, thorax, and abdomen, and by
undergoing certain metamorphoses or transformations. Now, while a number
of other animals outside of the insect world go through similar
transformations, those in the Crustacea being equally remarkable, yet,
from the ease with which they are observed and the completeness of the
transformations in most insects, the metamorphoses of this class have,
from time immemorial, excited the greatest curiosity.