_Number of Species._--It is now considered that 2,000,000 is a moderate
estimate of the species of insects actually existing. Some authorities
consider this total to be too small, and extend the number to
10,000,000. Upwards of 300,000 species have been collected and
described, and at present the number of named forms increases at the
rate of about 8000 species per annum. The greater part by far of the
insects existing in the world is still quite unknown to science. Many of
the species are in process of extinction, owing to the extensive changes
that are taking place in the natural conditions of the world by the
extension of human population and of cultivation, and by the destruction
of forests; hence it is probable that a considerable proportion of the
species at present existing will disappear from the face of the earth
before we have discovered or preserved any specimens of them.
Nevertheless, the constant increase of our knowledge of insect forms
renders classification increasingly difficult, for gaps in the series
become filled, and while the number of genera and families increases,
the distinctions between these groups become dependent on characters
that must seem trivial to the naturalist who is not a specialist.
_Orders of Hexapoda._--In the present article it is only possible to
treat of the division of the Hexapoda into orders and sub-orders and of
the relations of these orders to each other. For further classificatory
details, reference must be made to the special articles on the various
orders. As regards the vast majority of insects, the orders proposed by
Linnaeus are acknowledged by modern zoologists. His classification was
founded mainly on the nature of the wings, and five of his orders--the
Hymenoptera (bees, ants, wasps, &c.), Coleoptera (beetles), Diptera
(two-winged flies), Lepidoptera (moths and butterflies), and Hemiptera
(bugs, cicads, &c.)--are recognized to-day with nearly the same limits
as he laid down. His order of wingless insects (Aptera) included
Crustacea, spiders, centipedes and other creatures that now form classes
of the Arthropoda distinct from the Hexapoda; it also included Hexapoda
of parasitic and evidently degraded structure, that are now regarded as
allied more or less closely to various winged insects. Consequently the
modern order Aptera comprises only a very small proportion of Linnaeus's
"Aptera"--the spring-tails and bristle-tails, wingless Hexapoda that
stand evidently at a lower grade of development than the bulk of the
class. The earwigs, cockroaches and locusts, which Linnaeus included
among the Coleoptera, were early grouped into a distinct order, the
Orthoptera. The great advance in modern zoology as regards the
classification of the Hexapoda lies in the treatment of a heterogeneous
assembly which formed Linnaeus's order Neuroptera. The characters of the
wings are doubtless important as indications of relationship, but the
nature of the jaws and the course of the life-history must be considered
of greater value. Linnaeus's Neuroptera exhibit great diversity in these
respects, and the insects included in it are now therefore distributed
into a number of distinct orders. The many different arrangements that
have been proposed can hardly be referred to in this article. Of special
importance in the history of systematic entomology was the scheme of F.
Brauer (1885), who separated the spring tails and bristle-tails as a
sub-class Apterygogenea from all the other Hexapoda, these forming the
sub-class Pterygogenea distributed into sixteen orders. Brauer in his
arrangement of these orders laid special stress on the nature of the
metamorphosis, and was the first to draw attention to the number of
Malpighian tubes as of importance in classification. Subsequent writers
have, for the most part, increased the number of recognized orders; and
during the last few years several schemes of classification have been
published, in the most revolutionary of which--that of A. Handlirsch
(1903-1904)--the Hexapoda are divided into four classes and thirty-four
orders! Such excessive multiplication of the larger taxonomic divisions
shows an imperfect sense of proportion, for if the term "class" be
allowed its usual zoological value, no student can fail to recognize
that the Hexapoda form a single well-defined class, from which few
entomologists would wish to exclude even the Apterygogenea. In several
recent attempts to group the orders into sub-classes, stress has been
laid upon a few characters in the imago. C. Borner (1904), for example,
considers the presence or absence of cerci of great importance, while F.
Klapalek (1904) lays stress on a supposed distinction between
appendicular and non-appendicular genital processes. A natural system
must take into account the nature of the larva and of the metamorphosis
in conjunction with the general characters of the imago. Hence the
grouping of the orders of winged Hexapoda into the divisions
Exopterygota and Endopterygota, as suggested by D. Sharp, is unlikely to
be superseded by the result of any researches into minute imaginal
structure. Sharp's proposed association of the parasitic wingless
insects in a group Anapterygota cannot, however, be defended as natural;
and recent researches into the structure of these forms enables us to
associate them confidently with related winged orders. The
classification here adopted is based on Sharp's scheme, with the
addition of suggestions from some of the most recent authors--especially
Borner and Enderlein.
Class: HEXAPODA.
Sub-class: APTERYGOTA.
Primitively (?) wingless Hexapods with cumacean mandibles, distinct
maxillulae, and locomotor abdominal appendages. Without ectodermal
genital ducts. Young closely resemble adults.