_Respiratory System._--As mentioned above, respiration by means of
air-tubes (tracheae) is a most characteristic feature of the Hexapoda.
An air-tube consists of an epithelium of large polygonal cells with a
thin basement-membrane externally and a chitinous layer internally,
the last-named being continuous with the outer cuticle. The chitinous
layer is usually strengthened by thread-like thickenings which, in the
region close to the outer opening of the tube, form a network
enclosing polygonal areas, but which, through most of the tracheal
system, are arranged spirally, the strengthening thread not forming a
continuous spiral, but being interrupted after a few turns around the
tube. The tracheal system in Hexapods is very complex, forming a
series of longitudinal trunks with transverse anastomosing connexions
(fig. 11), and extending by the finest sub-division and by repeated
branching into all parts of the body. In insects of active flight the
tubes swell out into numerous air-sacs, by which the breathing
capacity is much increased.
Atmospheric air gains access to the air-tubes through paired
_spiracles_ or _stigmata_, which usually occur laterally on most of
the body-segments. These spiracles have firm chitinous edges, and can
be closed by valves moved by special muscles. When the spiracles are
open and the body contracts, air is expired. The subsequent expansion
of the body causes fresh air to enter the tracheal system, and if the
spiracles be then closed and the body again contracted, this air is
driven to the finest branches of the air-tubes, where a direct
oxygenation of the tissues takes place. The physiology of respiration
has been carefully studied by F. Plateau (1884). In aquatic insects
various devices for obtaining or entangling air are found; these
modifications are described in the special articles on the various
orders of insects (COLEOPTERA, HEMIPTERA, &c.). Many insects have
aquatic larvae, some of which take in atmospheric air at intervals,
while others breathe dissolved air by means of tracheal gills. These
modifications are mentioned below in the section on metamorphosis.
[Illustration: From Miall and Denny, _The Cockroach_, Lovell Reeve &
Co.
FIG. 12.--Food Canal of Cockroach.
s, Salivary glands and reservoir.
c, Crop (the gizzard below it).
coe, Caecal tubes (below them the stomach).
k, Kidney tubes.
i, Intestine.
r, Rectum.]
_Digestive System._--A striking feature in the food-canal of the
Hexapoda, as in other Arthropods, is the great extent of the
"fore-gut" and "hind-gut," lined with a chitinous cuticle, continuous
with the exoskeleton. The fore-gut is composed of a tubular gullet, a
large sac-like crop (fig. 12, c) and a proventriculus or "gizzard,"
whose function is to strain the food-substances before they pass on
into the tubular stomach, which has no chitinous lining. This organ,
usually regarded as a "mid-gut," gives off a number of secretory
caecal tubes (fig. 12, coe). At its hinder end it is continuous with
the hind-gut, which is usually differentiated into a tubular coiled
intestine (fig. 12, i) and a swollen rectum (fig. 12, r). From the
fore-end of the hind-gut arise the slender Malpighian tubes (fig. 12,
k), which have a renal function.
On either side of the gullet are from one to ten pairs of salivary
glands (fig. 12, s) whose ducts open into the mouth. Some of these
glands may be modified for special purposes--as silk-producing glands
in caterpillars or as poison-glands in blood-sucking flies and bugs.
The food passing into the crop is there acted on by the saliva and
also by an acid gastric juice which passes forwards from the stomach
through the proventriculus. As the various portions of the food
undergo digestion, they are allowed to pass through the proventriculus
into the stomach, where the nutrient substances are absorbed.
_Excretory System._--Nitrogenous waste-matter is removed from the body
by the Malpighian tubes which open into the food-canal, usually where
the hind-gut joins the stomach. These tubes vary in number from four
to over a hundred in different orders of insects. The cells which line
them and also the cavities of the tubes contain urates, which are
excreted from the blood in the surrounding body-cavity. This cavity
contains an irregular mass of whitish tissue, the fat-body, consisting
of fat-cells which undergo degradation and become more or less filled
with urates. When the worn-out cells are broken down, the urates are
carried dissolved in the blood to the Malpighian tubes for excretion.
The fat-body is therefore the seat of important metabolic processes in
the hexapod body.
_Reproductive System._--All the Hexapoda are of separate sexes. The
ovaries (fig. 13) in the female are paired, each ovary consisting of a
variable number of tubes (one in the bristle-tail _Campodea_ and
fifteen hundred in a queen termite) in which the eggs are developed.
From each ovary an oviduct (fig. 13, od) leads, and in some of the
more primitive insects (bristle-tails, earwigs, may-flies) the two
oviducts open separately direct to the exterior. Usually they open
into a median vagina, formed by an ectodermal inpushing and lined with
chitin. The vagina usually opens in front of the eighth abdominal
sternite. Behind it is situated a spermatheca (fig. 14, sp) and the
ovipositor previously mentioned, with its three pairs of processes
(Fig. 14, G, g).
[Illustration: From Miall and Denny, _The Cockroach_, Lovell Reeve &
Co.
FIG. 13.--Ovaries of Cockroach, with Oviducts Od and Colleterial
Glands CG.]
[Illustration: From Miall and Denny, _The Cockroach_, Lovell Reeve &
Co.
FIG. 14.--Hinder Abdominal Segment and Ovipositor of Female Cockroach.
Magnified.
T^8 &c. Tergites.
S^7, 7th Sternite.
S^8, Sclerite between 7th and 8th sterna.
S^9, 8th Sclerite.
Od, Vagina.
sp, Spermatheca.
G, Anterior, and g, posterior gonapophyses.]
The paired testes of the male consist of a variable number of seminal
tubes, those of each testis opening into a _vas deferens_. In some
bristle-tails and may-flies, the two _vasa deferentia_ open
separately, but usually they lead into a sperm-reservoir, whence
issues a median ejaculatory duet. The male opening is on the ninth
abdominal segment, to which belong the processes that form the
claspers or genital armature. Accessory glands are commonly present in
connexion both with the male and the female reproductive organs. The
poison-glands of the sting in wasps and bees are well-known examples
of these.