Seven orders of insects were originally recognized by Linnaeus, namely,
Neuroptera, Diptera, Hemiptera, Lepidoptera, Coleoptera, Hymenoptera,
and Aptera. This classification was based on the organs of flight only,
and while in the main resulting in natural divisions which still furnish
the basis of more modern classifications, was faulty in several
particulars. For instance, the Aptera, which included all wingless
insects, was soon found to be a very unnatural assemblage and its
components were distributed among the other orders. The establishment of
the order Orthoptera by Olivier to include a large and well-defined
group of insects associated with the Hemiptera by Linnaeus, restored the
original seven orders, and this classification has, in the main, been
followed by entomologists up to the present time.
[Illustration: FIG. 1.--Pyramid showing the nature of the mouth, and
relative rank of the Orders, and the affinities of the Suborders of
Insects.]
All insects are, in a broad way, referable to one or the other of these
seven primary orders by the structure of the wings and the character of
the mouth-parts in the imago, and by the nature of their
transformations.
Some of these orders are connected by aberrant and osculant families or
groups, which have by other authors been variously ranked as independent
orders, but which, following Westwood substantially, I have considered,
for convenience, as suborders. (_See_ Fifth Report, Insects of Missouri,
etc., 1872.)
In the article just cited, I made use of the accompanying diagram in the
form of a pyramid (Fig. 1), which gives a graphic representation of the
distinguishing characters and the relative rank as usually accepted, of
the orders and suborders.
Full discussion of the different classifications is unnecessary in this
connection. Authors have differed in the past and will differ in the
future as to what constitutes a natural system, and it would require
many pages to give even a brief survey of the various schemes that have
been proposed. As I have elsewhere said, "We must remember that
classifications are but a means to an end--appliances to facilitate our
thought and study--and that, to use Spencer's words, 'we cannot, by any
logical dichotomies, actually express relations which in nature graduate
into each other insensibly.'"
The most philosophical, perhaps, of the more modern systems of
classification is that of Friedrich Brauer, who has carefully studied
the subject, and has given us an arrangement consisting of sixteen
orders. This has many merits and has been adopted, with slight
modifications, by Packard in his "Entomology for Beginners," and by
Hyatt and Arms in their recent and valuable text-book "Insecta."
Comstock, in his "Introduction to Entomology" strongly recommends
Brauer's classification, but for reasons of simplicity and convenience
adheres to a modification of the old classification of Westwood.
For purposes of comparison the classification by Hyatt and Arms, which
is substantially that of Brauer, may be introduced.
In linear arrangement it is as follows:
I. Thysanura (_Spring-tails_, etc.).
II. Ephemeroptera (_Ephemeridae_; May-flies). (=_Plectoptera_ Pack.)
III. Odonata (_Libellulidae_; Dragon-flies).
IV. Plecoptera (_Perlidae_; Stone-flies).
V. Platyptera (_Termites_, _Mallophaga_, etc.).
VI. Dermaptera (_Forficulidae_; Earwigs).
VII. Orthoptera (Locusts, Grasshoppers, etc.).
VIII. Thysanoptera (_Thripidae_; Fringe-wings).
IX. Hemiptera (Bugs).
X. Coleoptera (Beetles).
XI. Neuroptera (_Sialidae_, _Hemerobiidae_; Lace-wings, etc.).
XII. Mecoptera (_Panorpidae_; Scorpion-flies).
XIII. Trichoptera (_Phryganeidae_; Caddis-flies).
XIV. Lepidoptera (Butterflies and Moths).
XV. Hymenoptera (Bees, Wasps, etc.).
XVI. Diptera (Two-winged flies).
The relationship of these orders cannot be indicated in a linear
arrangement, and is admirably shown by Hyatt and Arms by means of
diagrams which I reproduce (Figs. 2, 3.)
[Illustration: FIG. 2.--Scheme illustrating origin and relationship of
Orders. (After Hyatt.)]
The relation of these sixteen orders to the older, septenary scheme is
shown by the following arrangement: