SEM of Aleochara insect antenna
Detailed SEM of Aleochara insect antenna
senory hairs on the antenna of the beetle Aleochara
Insect Antennae
The picture above shows the left antenna of the rove beetle Aleochara bilineata. (The right antenna is
identical but a mirror image). This picture was taken through a scanning electron microscope. The whole
antenna is about one millimetre in length. The antenna is made up of segments, called
antennomeres.
Can you count the numbers of antennomeres in the above picture? (There are 11). The antennomere
joining to the head is called the
scape, the next one along is called the pedicel, then there is the
flagellum (literally 'whip') of 9 antennomeres in this case. These last nine form the main sensory organ and
are also called
flagellomeres and labelled from F1 (joining to the pedicel at the base) to F9. The scape
and pedicel serve primarily to move and position the antenna. strictly speaking, entomologists divide all
antennae into three principle segments: the scape, pedicel and flagellum. In this case the falagellomeres
can be regarded as sub-segments. The insect antenna is a highly sophisticated sensory device!

People often think of invertebrates (animals without a backbone) as being simple creatures. Of course, they
are far from simple! Ok, so maybe they are simpler than humans? Well certain mammals, especially
humans, apes and cetaceans (dolphins and whales) do have the most complex brains, and the human
brain is probably the most complex organ in the animal kingdom on Earth. However, other animals may beat
mammals for complexity in other body systems. Also, it depends what we mean by 'complex' - the brain is
complex because of the complex way in which all those microscopic wires connect together, but in terms of
gross structure the brain is not particularly complex - it's a mass of control centres wired together by cables
(bundles of neurones), the complexity is only apparent in terms of microscopic computer network
architecture. Other living systems may possess biochemical complexity, such as the mammalian liver which
carries out a vast multitude of chemical reactions, even though its cells look pretty much the same as one
another and fall into a few distinct morphological types. Some organisms, such as mammals, are also
complex by virtue of their huge cell numbers or their many different types of cells. However, arthropods
come into their own when we look at sensory systems. The arthropod group has more different types of
sensor than any other animal group (on Earth) including the vertebrates (animals with backbones, like
humans).

The picture below is a montage of the same antenna shown above, but at a higher resolution (the details
are sharper, which is why it is a montage - each image was taken at a higher magnification than the one
above - about 10 times higher). (I wish I had remembered to switch the scale bars off! Each bar is 100
micrometres, or one tenth of a millimetre). Click the image to enlarge.
Note that like the insect body, the antennae are hard structures, that is they are encased in tough cuticle
or exoskeletal armour. Notice the many hairs that cover the antenna. Large and small hairs are visible. The
pattern of these hairs is rather precise, that of the larger hairs is exact and each individual has the same
pattern. (There is some variation in the distribution of the small hairs between individuals, but the basic
pattern is very repeatable). Only the seven most terminal segments possess small hairs, giving them a
'furry' appearance. The image below is a higher magnification still.
This close up is typical of the surface of the seven most terminal segments. The large hair in the middle
of the image is one of the large hairs that were so obvious in the lower magnification views. It is
surrounded by
rosette clusters of pores and one single pore to the right. This pattern of pores is
typical, a circle of rosettes and one solitary pore 'beneath the hair, and repeats for each of the large
hairs on the 7 most terminal antennomeres. these pores are the openings of antennal
glands. Note that
the smaller hairs come in two principle types - thin pointed hairs, called
trichomes, and shorter blunter
conical pegs, called
basiconic pegs. The large hair is also called a trichome, so we have two types of
trichomes and basiconic pegs. Closer analysis reveals several other types as well. These hairs do not
serve the same primary purpose as mammalian hairs - they are not there to keep the insect warm, nor
are they there to colour or pattern the animal, their main purpose is sensory - for this reason they are
usually called
sensilla (sensory receptors or sensors, singular sensillum) rather than hairs. Also, they
are not made of the protein keratin, as are mammalian hairs, rather they are extensions of the tough
cuticle and made from chitin.

So why the different types of sensilla?

The larger trichoid sensilla, like the one shown above, respond to touch - they have flexible sockets and
when the antenna touches something the hair gets deflected in its socket and the insect senses this.
However, they have one other function - they are
taste sensors (gustatory chemoreceptors)!
Humans are used to tasting only with their tongue (aided by the olfactory sensors in the nose) but insects
may have taste sensors on their antennae, their palps, or their feet, or all three usually! The ovipositor
(egg-laying tube) of female insects may also have taste sensors. In this way, insects can taste whatever
they touch. The end of the hair contains a tiny pore and when the tip of the hair contacts liquid or a moist
surface, chemicals  in the liquid enter this pore where they are detected by sensory cell processes that
run along the length of the trichoid and end in a cell body beneath the cuticle. This cell body then gives
out a second long process or 'wire' to the brain. Thus, the sensory 'wire' inside the trichoid (hair) tastes
the surface and the signals travel along the hair and then down the antenna to the brain. Another
sensory cell monitors the socket at the base of the hair for deflection. Therefore, these large 'hairs' are
dual chemo and mechanoreceptors. A
chemoreceptor receives or senses chemical information in the
environment, such as taste and smell, while
mechanoreceptors receive mechanical information, such as
movement, touch, pressure and vibration.

The smaller pointed 'hairs' or small trichoid sensilla are mechanoreceptors. They do not taste things, they
have no pore in their tips and no sensory 'wire' inside the shaft of the hair. They do have flexible sockets
and do detect touch when the hair is deflected. Being so much smaller than the larger hairs, these
smaller hairs can probably respond to smaller wind movements and vibration as well as touch. So, you
see that each type of hair is sensitive to a different stimulus or signal from the environment and together
they help the insect to build up a picture of its world.

So what about the small rounded basiconic pegs? How insects smell!

So, we have touch sensors, vibration sensors, wind indicators and taste sensors, but the basiconic
sensilla are smell or
olfactory sensors. They do not have particularly flexible sockets and do not
respond to touch, but they are chemoreceptors that detect odours in the air! So, the insect is feeling,
tasting and smelling with its antennae. As we shall see later, the antennae can sense other things too!

So, how do these olfactory hairs work? The diagram below shows the structure of one cut across (in
longitudinal or lengthwise section) - click the image to enlarge it:
olfactory sensillum to colour in
olfactory sensillum unlabelled
basiconic peg in cross section
Well, the structure is complex! Inside the hair or 'peg' are one or more branching cell processes, called
dendrites. Dendrites are nerve cell processes that transmit signals toward the cell body. An axon also
leaves the cell body. Axons transmit signals away from the cell body. Dendrites and axons are the 'wires'
of nerve cells. The nerve cell consists of one or more dendrites (usually branched), a cell body, and an
axon. The axon travels all the way to the brain (to a region of the brain called the antennal ganglion).

Notice that the basiconic peg is an extension of the tough exoskeletal cuticle of the antenna. The
cuticular wall of the peg is punctured by rows of pores. These pores vary in diameter, depending upon
insect species and sense organ, but are usually between 6 and 65 nanometres in diameter (one
nanometre, or 1 nm, is one millionth of a millimetre!) so they are minute! Various accessory or auxiliary
cells are associated with the nerve cell and the peg - the
trichogen cell synthesises the peg, whilst the
tormogen cell secretes the surounding socket. The tormogen cell encircles the trichogen cell. A
thecogen cell ensheaths the nerve cell (covering part of its dendrite and the cell body) whilst a glial
cell
forms a neurolamella, or neural sheath, around the axon. This provides electrical insulation for the
axon 'wire'. It's difficult to visualise the arrangement of these cells, so perhaps my shaded version of the
diagram below will help you - I could have added a key, but see if you can work out which cell is which
using the labelled diagram above.
olfactory sensillum
Other features are: the sensillum lymph, this is fluid secreted by the trichogen and tormogen cells, and
is very important - the nerve cell will not work without it! Note the crinkly edges of the trichogen and
tormogen cells - these are actually tiny finger-like processes, called microvilli, that increase the surface
areas of these cells, enabling them to secrete more sensillum lymph.

I have included a blank diagram below, for those who want to colour-in their own olfactory sensillum!
Now we need to look at the pores in more detail and relate the structure of this sensillum to its function.
The diagram below shows a cross-section (or transverse section) through the basiconic peg:
Note the pores in the cuticle wall, and the dendritic branches now seen cut across as circles. The
dendrites are bathed by the sensillum lymph. Each pore opens into a tiny expansion, called the
pore
kettle
, and from this pore kettle extend a number of minute pore tubules. These are tubules, presumed
to be hollow, numbering 3-20 per pore (3-4 are visible per pore in these sections) and only 10 to 20
nanometres in diameters. These pore tubules sometimes appear to extend into the sensillum lymph for a
short distance, though they are hard to see, partly because they are so small, and partly because the
sensillum lymph often appears dark under the electron microscope, and partly because they are easily
destroyed during sample preparation (the specimen has been preserved, sectioned and stained for the
electron microscope). However, in the right-hand sections you can see as many as 6 fine tubules per
pore complex extending into the lymph.

Mode of operation - how do insects detect smells?

Now to relate these structures to their function. Odours consist of gases, usually very dilute gases, of
molecules - for example, the smell of garlic is due to molecules like dimethyl sulphide that get carried in
the air. These molecules are extremely tiny - even smaller than the pores and pore tubules and they find
their way into the pores and are thought then to pass down the pore tubules into the sensillum lymph and
then to attach (temporarily) to one of the dendritic branches. The dendrite then generates an electrical
signal that travels down the dendrite to the cell body, and, if the signal is strong enough, it will then travel
down the axon to the brain of the insect.

Things are, of course, not quite that simple. How does an odour molecule find its way in through the
narrow pore and pore tubules? Firstly, odour molecules are much smaller than the pore tubules,
secondly they get knocked about by air molecules and so they zip back and forth at very high speeds (by
the process of diffusion), and so they cover an area as small as an insect's hair very quickly! Sooner or
later one or more of these molecules will pass through one of the pores. (Larger animals have to actively
draw air across their olfactory sensors, as humans do when they breathe in through their noses).
However, without computer models, it is very hard to say whether or not this would be fast enough, or
whether most odour molecules would float past instead of finding the small pores. However, it appears
that the insect can help things along a bit - the hair is covered in a very thin film of oily material. When an
odour molecule strikes the hair it gets trapped in this oily layer. It is now only free to move across the
surface of the hair, it cannot escape, and so sooner or later it will most probably fall into one of the
pores, which also contain oil. Once in the pore it has few places to go and so will often end up inside the
pore tubule. In this way, odourant molecules get funnelled into the pore tubules. At least, that i the
theory! There is some evidence to support this theory - the oil around the pores and pore tubules seems
to bind odour molecules especially strongly, at least in one insect species examined, and so the pores
act as sinks or traps for odour molecules.

Once inside the pore tubules, the molecules have even fewer places to go and dissolve into the
sensillum lymph. Some researchers suggests that the tubules may actually end on or very close to the
surface of a dendritic branch, either way the odourants bind to the surface of the dendrites and are then
detected. Carrier proteins (
odourant-binding proteins, or OBPs) bind tightly to specific odour
molecules, trapping them and then delivering them to the dendritic branches. Different sensilla, which
may otherwise look identical, may have different types of OBPs, each specific to one type of odour, so
that different sensilla specialise in detecting different odours.

Some insects have olfactory pegs sunken into pits, so-called
coeloconic sensilla (coeloconic literally
means 'peg in a hole').
Aleochara bilineata has some of these olfactory sensors on the end-most
antennomere (F9), two of which are shown below:
coeloconic sensilla type 1
The coeloconic sensilla of Aleochara bilineata contain from 1 to 7 basiconic pegs inside a pit in the
cuticle, which may be partially covered by a dome of cuticle with a hole in the top (the one on the left
above has no such dome, but most do). The picture above right shows a cross-section through a similar
sensillum with 4 pegs in a pit. These pegs have the structure characteristic of basiconic olfactory pegs as
described above. Air presumably gets swept into these sheltered pits where it stagnates for a time,
probably giving the sensillum longer to sample the air for odours, before the air is replaced, and so
probably makes these receptors extra sensitive. The scale bar is one micrometre on the left-hand photo
and two micrometres on the right (one micrometre is one thousandth of a millimetre and equal to 1000
nanometres).

Despite the uncertainties in our conceptual understanding of how these insect olfactory sensors work,
they are extremely sensitive! A male moth can detect the tiny amount of pheromone odour given off by a
female moth one mile upwind!

In total, there are 4 different morphologically distinct olfactory sensor types on the antenna of
Aleochara
bilineata
(and also on Aleochara bipustulata, the antenna of which is essentially the same) - two of which
we have looked at here. In total, there are about 1600 olfactory sensors on each antenna! There are
also about 1000 mechanoreceptors, some of which detect touch and vibrations, and some of which
monitor the movement of the antenna and the positions of the joints between antennomeres. The latter
are called
proprioceptors - proprioceptors are mechanoreceptors specialised to detect the positions
and movements of an animal's body parts relative to one another.

Additionally there is another class of peg-like sensillum, which are thought to detect temperature and
humidity changes (and possibly certain odours as well) and so are called
thermohygroreceptors.
Other insect species may have very different types of sensilla - insect sensory systems are very diverse!
All in all, you can see that the insect antenna is a highly complex and very sophisticated state-of-the-art
sense organ!

So, our rove beetle uses its antenna to feel, detect vibrations and wind movements, to smell, to taste and
to detect temperature changes and humidity, but what about those large pores we could see between the
hairs - the rosette pore clusters and large single pores. Click here to find out what these large pores do.

  • Learn more about insect mechanoreceptors and how they work - how do insects detect touch,
    sound, vibration, gravity and pressure?.


  • Learn more about Aleochara's antennal glands.

Click thumbnail to enlarge.
basiconic sensilla section 1
Scale bar = 400 nm (0.0004 mm)
basiconic sensilla section 2a
basiconic sensilla section 2b
Above and right: sections through the basiconic pegs as
seen under the transmission electron microscope.
lareg olfactory basiconic peg
coeloconic sensilla
sensilla occurs on the base of the
tipmost flagellar segment of Aleochara
bilineata. They span about 120
degrees of arc, facing forwards and
towards the side when the antennae
are held at rest. This positioning
makes them ideally placed to measure
differences in stimulus intensity
between the two antennae and so they
are likely used to measure the
direction of some specific chemical
source. In some specimens the domes
appear more like rimmed pits, being
more or less open. These are unusual
for coeloconic senilla in usually
containing more than one sensory peg.
A second type of coeloconic peg also occurs on these
antennae, of unknown function, likely
thermohygroreceptors.
Left: another type of olfactory sensillum
found on the tip of the antenna of
Aleochara bilineata is this large
basiconic peg, of which there are four.
These have extremely large pores
packed with pore tubules and contain a
highly-branched sensory dendrite.
There internal structure suggests that
they are adapted for increased
sensitivity to some unknown chemical
stimulus.
olfactory stimuli, have been carried out and reveal that the antenna is especially sensitive to volatile
substances characteristic of plants which host the root maggots on which
Aleochara bilineata feeds and
grows (see insect life-cycles) and so olfactory receptors help the adults to locate their prey. Insect
olfactory receptors are often
specialists, responding to one or a few chemical stimuli. Some
chemosensilla are reported to be
generalists, although sometimes this results from testing chemicals at
unnaturally high concentrations.