Above: unicellular glands on the antennae of Aleochara bilineata open to the outside world via rosette
clusters of pores. Each pore is only about 100 nanometres (one ten thousandth of a millimetre) in diameter.
Notice the circle of pore clusters around the large hair (which is a taste and touch sensor). Bottom right: a
close-up view of one of the pore clusters.

Living things need to export and import materials on a daily basis. Putting aside the ejection of waste, which is
excretion, individual cells and whole organisms need to take in important things, like nutrients, respiratory
gases such as oxygen, heat and sensory energy from the environment and to secrete useful chemicals. We
have considered the import of sensory information from the environment, under the section on
insect
antennae and insect vision. The insect cuticle posses special problems for insects - this cuticle is a tough
exoskeleton designed to be as near water-tight as possible so as to prevent the insect from dehydrating
(imagine how much water there is in an insect, then imagine how quickly it would evaporate if it was just a
naked drop of liquid). As mediaeval knights discovered, being encased in armour is excellent protection from
the kinetic energy of various heavy weapons, but creates problems of sensing the environment (it was difficult
to see and hear with those helms on!) and of overheating. In short, thick armour hampers the exchange of
energy and materials between the organism and its environment, which can be both a good and a bad thing.
Insects face exactly the same problems, but have found ingenious solutions.

We have already seen, how the exoskeleton is equipped with thousands of sensors that are specially
designed to conduct stimulus energy (such as light or mechanical contact) across the cuticle, whilst
transforming it into electrochemical energy. Here we look at export, and how insects have a multitude of
glands for exporting chemicals to the outside world.

Glands that secrete/excrete materials across the body surface (insect cuticle in this case) are called exocrine
glands. Insects possess two fundamental types of cuticular exocrine glands: type 1 glands are unicellular, that
is they are small glands made up of single (epidermal) cells, and type 2 glands which are larger multicellular
organs. Both types of gland secrete chemicals to the outside world via cuticular ducts - one or more channels
that carry the secretion across the cuticle to the outside world, opening via pores on the surface of the cuticle.

The type pictured above is a type 1 gland that occurs on the head, antennae and palps (but mostly the
antennae) of the rove beetles
Aleochara bilineata and Aleochara bipustulata. Similar glands occur on the
antennae of other beetles, though they exhibit interesting diversity of structure. Each of these glands is a
single modified epidermal cell. Epidermal cells make up the antennae by forming a hollow cylinder and they
secrete the cuticle. The hollow interior is occupied by blood, nerves and tracheae (and by another type of
gland as we shall see later). Each of these type 1 glands opens, in these species, via a rosette cluster of
pores, each pore is about 100 nm in diameter, whilst the whole pore cluster is about one micrometre (one
thousandth of a millimetre) so these pores are extremely tiny indeed! This is just as well, the insect cannot
afford to have too many large holes through which to lose water. There are, however, some 320 or so such
pore clusters on each antennae, with the density increasing toward the tips of the antennae, and there are a
few scattered on the head, especially between the antennae.

The function of these glands is not known, but before we discuss what this might be, let's look at their detailed
structure. The diagram below is a labelled diagram of a section through part of the antennae, showing one of
these type 1 gland units (and part of another adjacent to it).
Insect Glands
gland diagram unlabelled
gland diagram labelled
section II
section EE
section HH
section AA
section AA 2
section CC
section BB
section DD
section GG
Section A-A:
Section B-B:
Section C-C:
gland section map
section JJ
Section D-D:
gland pores
Section E-E:
section FF
pores LS 1
pores LS 2
pores LS 3
trichoid sensilla and glands
The gland consists of a vase-like cell, with a hollow cavity inside, and with the bottom of the vase filled with
tiny finger-like projections (called
microvilli, MV). The upper part of the vase sits in a canal that cuts through
the cuticle (both the outermost exocuticle, XC and the innermost endocuticle, EC) and which is covered by a
porous plug of cuticle which opens via
pores (P) on the external surface. Everything above the pores in the
diagram, is outside the insect. The inside of the vase is lined to part-way down by more cuticle (C). Inside
the neck of the vase is a plug of tightly packed tiny tubules (rather like cotton wool in appearance) which
form tight bundles that project midway through the pores. (The function of the dark bodies, d, is not known).
These bundles then give way to larger
pore tubules (PT), one of which travels to each pore. The chemicals
to be excreted are presumably synthesised within the cell (the wall of the flask) and secreted into the The
cytoplasm of the gland cell contains many
mitochondria, which provide the cell with energy. This cell needs
lots of mitochondria and so is expending large amounts of energy, presumably on synthesising and
secreting whatever chemicals it secretes. The secretion presumably travels from the microvilli, where it can
be seen stored as particles, crystals or vesicle-like structures (X, red arrow above). The scretion would then
travel along the inside of the hollow flask to the woolly plug of tubules and along the tubule bundles and
then along the large pore tubules to the outside world. This is shown by the red arrows in the diagram below:
gland secreting
It is also possible that the bottom part of the neck of the vase, which is not lined by cuticle, could secrete
materials (as indicated by the blue arrows). Indeed, some sections seen under the electron microscope do
materials (as indicated by the blue arrows). Indeed, some sections seen under the electron microscope do
could then either spread across the surface of the cuticle or evaporate. (They do not appear in samples
show what could be exocytosis in this region (as shown in section H-H below). The chemicals secreted
prepared for scanning or transmission electron microscopy). Whether the wool of tubules, tubule bundles
or pore tubules actually convey the secretion inside them (they are hollow, but very narrow) or act as
wicks is not certain.
Section F-F:
Section G-G:
Section H-H:
Section I-I:
Section J-J:
This series of photos shows a series of cross-sections through the
gland, as indicated by the red lines in the above diagram. It is
important in biology to be able to visualise three-dimensional
structures and to relate 2D sections to the real 3D structure. This
is usually no easy task! Have a look at these sections (click each
to enlarge it) and see how they relate to the diagram.
tubules. In the bottom-most section, the tubules appear swollen and full of some sort pf secretion
(suggesting that the secretion does travel along the inside of the larger pore tubules at least).
What do these glands secrete - what is their purpose?

Glands have many functions in insects, and we shall look at some of these later on. Antennal glands in
beetles have several possible functions, some of which have been demonstrated in species other than
Aleochara. It has been suggested that glands concentrated on the antennae and at points of articulation
of the joints secrete a
lubricant to reduce friction in moving parts of the cuticle. Some produce enzymes
that seem to have an
antimicrobial function. Another possibility is that these glands secrete pheromones.
Pheromones are chemicals that carry signals, encoded as odours, from one animal to another. Many
insects produce sex pheromones which identify the sex of the individual and their ripeness for
reproduction. Other pheromones are used by social insects to communicate with one another.

So, what about these glands on the antennae of
Aleochara bilineata and Aleochara bipustulata? No
antennal or face-to-face contact is necessary prior to mating in these insects, and the glands are present
in similar numbers in both sexes, so they would appear not to be sex pheromones. However, they could be
released in flight, especially as the antennae vibrate in moving air. These beetles are not social as are
wasps, bees, ants or termites, but they do form communual burrows and antennal-antennal contact occurs
frequently when individuals encounter one another and palpate one another's faces with their antennae.
This would provide an opportunity to exchange pheromones, and perhaps these glands are involved in
species or kin recognition. The photo below shows the position of the glands on the pedicel (the second
segment of the antenna):
glands on pedicel
Notice how the glands open in furrows between the cuticular ridges that sculpt the surface of the
pedicel. These furrows run into the joints of the movable hairs (trichoid sensilla). This supports the
hypothesis that the secretion from these glands is a lubicant to lubricate the sockets of the hairs.
However, these sockets may be most prone to trap micro-organisms (there seem to be two bacteria
trapped in the lower socket shown above) so they could be anti-microbial. Against the lubrication
theory is the fact that these antennal glands are only commonly found on beetles. Beetles do burrow
a lot, however, and perhaps lubricant helps dislodge trapped particles. On the other hand, the joint
between the pedicel and the next segment, visible as the wrinkly membrane, lacks these glands, so
these larger joints don't seem to need the hypothetical lubricant. The sockets of the hairs might also
act as convenient reservoirs for pheromones. Nature is economical, and it may be that the glands
secrete a number of chemicals with different functions. So you see, we really don't know what these
glands in
Aleochara are doing! There is a tremendous amount we still don't know about insects - so
much more research needs to be done!

We have a lot more to say about glands in insects, but there are two more chapters to the
Aleochara
gland story - both coming soon!

Click here to discover the second type of antennal gland that Aleochara has.

Click here to learn about glands that definitely have pheromonal and defense functions in
Aleochara.

Click here to learn about glands in social insects.
gland tubules
Below: a longitudinal section showing the wool-like plug of small tubules
topping the vase-like reservoir of the gland (in white).