Above: a side-view of an adult Aleochara bilineata. Indicated are the planes of various sections shown
below. Aleochara bilineata is a rove beetle or Staphylinid. Rove beetles belong to the class of beetles
(Coleoptera). Rove beetles have short elytra (wing coverings, singular of elytra is elytron) covering the
second thoracic segment (or mesothorax) and the third thoracic segment (metathorax). The elytra are
modified fore-wings (the first pair of wings) and are characteristic of beetles. These short elytra free the
abdomen to move about and the abdomen of rove beetles is very agile and usually contains defensive
glands. However, to make use of the short elytra, the hind-wings (second pair of wings) have to be able to
elaborately and precisely fold away beneath the elytra when the wings are not in use. The elytra protect the
wings when beetles burrow or charge around in the undergrowth.
Click the images below to enlarge them.
The face of Aleochara!
The various exoskeletal armour plates that make up the head covering are shown above. The labrum is the
top lip. Beneath the labrum are the mandibles ('jaws' that work horizontally to cut and crush prey). The
maxillary palps handle and taste the food to ensure its suitability. Beneath the mandibles (and so not visible
from this angle) is a pair of maxillae which guide the food to the mouth and the smaller labial palps attached
to the labium or lower lip. The lips (labrum and labium) are hard, chitinous plates. The scape is teh first of the
11 antennal segments and articulates with the head. Aleochara bilineata adults are predators and eat
primarily the maggots of flies, such as onion and cabbage root flies.
The muscles are coloured red in these sections.
The insect head and brain:
Section B-B is a section through the head and shows the muscles operating the mandibles and part of the
brain. The brain comprises the sub-oesophageal ganglion, is below the oesophagus and is made of 3 pairs of
segmental ganglia fused together - the mandibular, maxillary and labial segments, and controls and receives
sensory data from the mandibles, maxillae, maxillary palps, labia and labial palps (i.e. the mouthparts). The
circumoesophageal connectives are stout nerve trunks connecting the sub-oesophageal ganglion to the
supra-oesophageal ganglion. As their name suggests, these connective pass either side of the oesophagus
(part of the fore-gut) and so surround it. The supra-oesophageal ganglion is above the oesophagus and
consists of the tritocerebrum, the deuterocerebrum (or deutocerebrum) and the protocerebrum. The
tritocerebrum is formed from the ganglia of the third segment of the head. The deutocerebrum consists of the
fused ganglia of the antennary segment of the head and contains the antennal lobes that control and receive
sensory data from the antennae. The protocerebrum is the fused ganglia of the optic segment and innervates
the compound eyes and ocelli. Thus, we see that the insect head is comprised of at least 6 fused segments.
The protocerebrum makes up the bulk of the supra-oesophageal ganglion. Sometimes the term 'brain' is
used to refer only to the supra-oesophageal ganglion, with the sub-oesophageal ganglion regarded as a
separate structure, however, this is an anthropomorphism that makes no sense (the human brain is entirely
above the oesophagus and buccal cavity). Here the whole structure is regarded as the brain. The
protocerebrum contains the higher centres associated with intelligence and learning. The central body and
the two mushroom bodies (corpora pedunculata) on either side are especially well-developed in social
insects like ants and bees. Aleochara bilineata is often regarded as a solitary insect, but can form communal
burrows or aggregates in which limited social interactions can be observed between the individuals. If food is
scarce, however, then the adults will eat their own eggs and even each other, but never seem to do so in
culture in the lab where food is abundant. Thus, we should regard this insect species as subsocial (half-way
between solitary and truly social).
The insect thorax
The thorax of winged insects is packed with the powerful flight muscles. The thoracic cuticle is thickened and
hardened to form a skeleton upon which these muscles operate. The thorax comprises three segments, each
with its own pair of legs and with its own nervous centre, called a thoracic ganglion. The ganglia are
essentially swellings of the ventral nerve cord (the insect equivalent of the dorsal spinal cord in vertebrates).
The nerve cord is a paired structure and runs from the brain to near the end of the abdomen, along the
ventral surface. The pair of cords lie so close together that they essentially fuse into a single nerve cord,
though this is more properly called the double ventral nerve cord. The foregut leads into to the midgut, which
occupies the abdomen and the rear part of the thorax. This gives off numerous hollow tubes, called caecae
or diverticula. These midgut caecae are digestive glands and their lumens are continuous with the lumen of
the midgut. Insect flight muscles are amongst the most powerful of all animal muscles (per unit mass) and
have a power output similar or greater than that of hummingbird flight muscle.
[This calls for a cautionary note. Estimates of muscle power output vary enormously, probably in large part due to the
methodology used - power is the rate at which a muscle does work and depends upon the activity that the muscle is asked to
perform. For example, a human sprinter has very powerful leg muscles over short intervals of time, but their power output
drops below that of marathon runners in the long run. Power is not the same as speed or strength! The speed of a muscle is
proportional to its length, the strength to its cross-sectional area and its power is proportional to its volume. However, longer
muscles, though faster, tend to be less strong than shorter muscles, and yet if both short and long muscles have the same
volume, then their powers may be equal! Even current biomechanical research articles in mainstream journals often fail to
appreciate these important subtleties! It is actually quite meaningless to compare the power outputs of two muscles without
specifying the task. Furthermore, muscle A may outperform muscle B in task 1 whilst muscle B might outperform A in task 2,
so which muscle is the more powerful?]
In dragonflies and cockroaches, muscles operate the wings directly - each contraction raising or lowering the
wing. However, in many insects, including beetles, the flight mechanism is indirect with power for the
downstroke of the wings being provided by contractions of the powerful longitudinal muscles. These
muscles shorten the thorax slightly. This strain energy is converted into the downward movement of the wings
by an ingenious mechanism (called a click mechanism). Contraction of the dorsoventral muscles (muscles
that run almost vertically in columns, actually at slight angles, connecting the dorsal and ventral surfaces of
the thorax) flattens the thorax slightly and this strain energy is converted into the lifting up of the wings. This
indirect flight mechanism relies on the elasticity of the thoracic cuticle. This allows the cuticle to store
elastic energy, indeed most of the energy stored from contraction of the dorsoventral muscles is stored for
the downstroke, with only a small fraction going into the upstroke. The downstroke generates most of the lift
and it is here that most power is needed. This is why, in the sections, the longitudinal muscles are so much
thicker than the dorsoventral muscles. The tips of the wings typically trace a figure-of-eight through the air
once each cycle. Insect wings typically beat several hundred times a second, with the wings of midges
reaching an astonishing 1047 beats per second! The flight mechanisms of insects are complicated and will be
dealt with elsewhere.
The insect abdomen
The abdomen contains little muscle - just enough to give it mobility. In Aleochara there are four main
longitudinal muscle blocks in the abdomen, a dorsal pair and a ventral pair. Contraction of the dorsal
longitudinal muscles arches the abdomen over the back of the animal - a classic threat or defensive display
of rove beetles, with the tip of the abdomen positioned to release its noxious chemicals on any would be
predator. Rove beetles contain large glands towards the tip of the abdomen that release chemicals with
various functions, including bad-smelling and bad-tasting chemicals to deter predators. However, apart from
the blood-filled haemocoel cavity, the abdominal space is mostly taken up by the reproductive organs,
especially toward the tip, and by the midgut (including the large ventriculus, which is the insect equivalent of
a stomach) especially in the forward section.
Readily visible in the abdominal sections are the pair of longitudinal tracheae. These are long silvery tubes,
one on each side of the insect body that run almost the whole length of the insect and give off numerous
branches that branch repeatedly, eventually carrying small tubes directly to many of the cells. These
tracheal trunks are connected by side branches to openings along each side of the insect called spiracles.
Air enters the tracheae through the spiracles and moves along the tracheal branches to the cells, delivering
oxygen. The air moves through the tubes partly by diffusion (remember that diffusion of gases in air is much
more rapid than in water) and also by mass movement as movements of the insect pump the tracheae. These
breathing movements are especially obvious in large insects such as wasps, in which the abdomen
continuously pumps, even when the insect is resting. Also occupying the abdomen of insects are one or two
horizontal membranous sheets, or diaphragms. A dorsal diaphragm was visible in the sections of the
Aleochara abdomen (these structure can be difficult to discern amongst the web-like fat-body tissue).
Present toward the rear of the abdomen are a series of Malpighian tubules - long thin tubes that extend
from the hindgut. These structures are excretory (and are the insect equivalent of the vertebrate kidneys).
Also visible is the dorsal vessel. This tube is contractile and rhythmically beats, propelling blood from the
abdomen toward the head of the insect (it is the insect equivalent of a heart). The insect circulatory system is
an open circulatory system. Vertebrates, like humans, have a much more closed blood system, in which
blood is confined to well-defined blood vessels throughout its journey around the body (except in certain
organs like the liver which have open sinuses) whereas in the insect, the blood spends most of its time
circulating in the large haemocoel cavity of the body from which it enters the side of the dorsal vessel
through a series of pores and then gets pumped to the head of the insect where it typically exits the open end
of the dorsal vessel, though it may enter a series of blood vessels that run part-way through the head before
opening into the haemocoel of the head. Remember that the blood of insects does not contain red pigments
like human blood and has very little role in delivering oxygen (this is done by the tracheal system). Instead it
serves to transport nutrients and to regulate the ionicity and acidity of the tissues. Insects evolved from
aquatic crustaceans and essentially they have carried the sea with them, inside their haemocoel, onto the
Below is a diagram of general insect anatomy to help you to understand the sections (this diagram was
redrawn and simplified from a classic diagram by Nicholas Jago).
Section E-E through the male:
Section F-F through male:
The two main longitudinal tracheal trunks have been omitted from the above diagrams for clarity. The
photo-micrographs below illustrate the structure of the insect tracheal system:
Above left: top, a spiracle of the silkworm moth,
looking in! Bottom: The spiracles lead into
tracheal tubes, like the one shown here. The
spiral rings prevent the tube from collapsing,
ensuring a steady supply of air to the insect's
tissues. The spiracles of many insects are
euipped with valves and can close when oxygen
demands are low, reducing evaporative water
loss from inside the insect and conserving water.
main tracheal branch in the antenna of
Aleochara. Notice the spiral rings give the
inside of the tracheal tube a 'sawtooth' contour
in cross-section. Above the trachea are two
muscle cells and beneath it are two bundles of
the antennal nerve - nerves and muscles are
hungry tissues for oxygen and the trachea is
targetting its supply of air to the most
oxygen-demanding tissues. Bottom, another
section through the antennal trachea (centre)
with a branch of the antennal nerve above it
and a muscle cell beneath it. All structures sit
in an apparent void, which is actually the
haemocoelic cavity of the antenna, which is
filled with blood in life.
Click on the thumbnails below for labelled
versions of these micrographs:
vessel of Aleochara. This vessel branches from
the anterior end of the dorsal vessel and enters
the antenna where it ends openly in the
haemoceol. Blood pumped through this vessel
exits the vessel's open end and circulates into
the antennal haemocoel - a large blood-filled
cavity that fills the centre of the antenna. Click
the thumbnail below for a labelled version.
Key to label abbreviations: BV, blood vessel; HE, haemolymph fluid occupying the haemocoel; M'
mitochondria; MU, muscle fibre; NE, nerve; TE, taenidium - the spiral thickening that strengthens the
trachea; TR, trachea; and Tr, a tracheole branching from the main trachea. Tracheoles, or tracheal
capillaries, are the very fine terminal branches of the tracheal system. These are very thin-walled
tubes across which oxygen diffuses into the tissues, and into which carbon dioxide diffuses from the
tissues. Tracheoles are about 2-5 micrometres in diameter and have small taenidia.
Left: a trachea of the silkmoth, Bombyx
Since, in life, the tracheae are filled with air, they appears silvery in fresh dissections and when seen
under dissecting microscopes.
Diagram redrawn on: 5/12/12
Colour version coming soon!