creatures that live in the desert rely on some sort of special adaptations, and
fog beetles have one of the weirdest ways of finding water.
The Namib desert where these
beetles live, is located on the South-West coast of Africa (21°07´S 14°33´E)
This is one of the most arid
areas of the world, receiving only 1.4 centimetres (0.55 in) of rain per year.
The cold Benguela current runs along the desert creating the most arid habitats
on earth. Water is essential to all living organisms and this harsh environment
presents a major challenges for all life forms. However, the cold coastal
current not only suppresses rainfall over the desert, but is also the origin of
fog that can reach as much as 100 km inland from the coast.
But some species of
tenebrinoid beetles living in the Namib Desert obtain water by drinking water
that condenses from fogs. They may simply take condensate from any surface, but
some species exhibit behavioral adaptations for collecting condensate. (The
insects, structure and function; R. F. Chapman; page 577). The advantage of fog
collection for water intake in extremely arid desert is obvious, and critical
when rainfall is absent over prolonged periods of times. Long term studies on
the population density of darkling beetles in the Namib desert clearly shows
that the fog collecting beetles are still present in great numbers during
periods of low rain fall, whereas the large majority of Darkling beetles that
lack this adaptations disappear or decline to less than 1% of their mean
abundance (Norgaard and Dacke, Frontiers of Zoology, July 20, 2010).
Here, four Darkling
beetles – Onymacris unguicularis,
Onymacris laeviceps, Stenocara
gracilipes and Physasterna cribripes have been discussed.
Some features of fog stand beetles:
Size: length up to 2 cm.
Habitat: The Namib Desert in southwestern
Food: Any plant and animal matter.
often hard to come by in the harsh desert, so fog beetles have adapted to
eating anything they can find. Their sharp jaws can slice up plants and the
bodies of dead animals, and tiny hairs in their mouth absorb moisture from
Long legged: Long legs are essential to these
beetles’ existence. Their legs keep their bodies above the scorching desert
sands. They also allow them to run at a speeds up to 3 feet per second and
cover a lot of ground in the daily search for food.
Shelter: Fogstand beetles need a way to get
out of the scorching sun from time to time. With no natural shelter in the
desert, they burrow into the sand with their front legs, and can disappear in a
matter of second.
Marathon mate: Male beetles chase females during
mating season, but the females can outturn them. If a male loses sight of a
female after she digs into the sand for the night, he will wait and head-butt
any other males that approach to drive them away from his female.
of fog basking desert beetles:
Life cycle of darkling beetles:
like other insects, go through a complete process of metamorphosis in which it
goes through four stages of development.
Eggs: It begins with the female beetle
laying tiny, oval white or yellow eggs. It usually take 4-7 days for the eggs
to hatch. Then they enter into the ‘ larval stage’.
Larvae: At this stage, they will eat a
tremendous amount of food and continue to grow, shedding its exoskeleton many
times while it grows.it takes about 3-7 weeks. Then it enters to the pupal
Pupa: It then enters into pupal stage
which can take up to 7-11 days. After pupating, an adult emerge.
Adult: This beetle will then feed, mate and
if it is a female, she will lay eggs for the beginning of another generation. Usually
their life span is about up to 2 years.
Fig: life cycle of darkling
What is fog-basking behavior?
Fog basking is adapting a characteristic head-down stance on
the dune crests, and facing into the fog-laden
wind; water from the fog condenses on the dorsum and then trickles down to the mouth
where the condensate is imbibed (Hamilton & Seely, 1976; Seely, 1979).
Strikingly, fog-basking frequently occurs outside of the normal activity period
of this species, at ambient temperatures and wind velocities far removed from
their preferences, and they are not known to seek food at these times (Seely et
al., 1983; Louw et al., 1986).
Figure: Fog basking
posture of Onymacris unguicularis.
Photograph of a fog-basking O. unguicularis
inside the fog chamber exhibiting a
characteristic fog-basking head stand. This posture allows fog water collected
on the beetle’s dorsal surface to trickle down to its mouth.
Mechanism of fog basking:
long night, when the air was cooled by the sea breeze , the sun comes up to
warm up the Namib Desert. Turning itself to face the shore, fogstand beetle
uses its long hind legs to prop its rear end up in the air.
fog begins to form after several minutes, and a few drops of moisture appear on
the beetle’s body. After an hour of standing perfectly still, the beetle’s body
is covered by dew, and drops of water drips into its mouth.
mechanism by which fog water forms into large droplets on a beaded surface has
been described from the study of the elytra of beetles from the genus Stenocara
Parker A Lawrence CR. The structures behind this process are believed to be
hydrophilic peaks surrounded by hydrophobic areas; water carried by the fog
settles on the hydrophilic peaks of the smooth bumps on the elytra of the
beetle and form fast-growing droplets that – once large enough to move against
the wind – roll down towards the head.
Here systematic position of four tenebrinoid darkling beetles
Comparative fog basking behavior and water collection
efficiency in these four Namib Desert Darkling beetles:
The fog collecting behaviour of four tenebrionid beetle
species was compared: Onymacris unguicularis (Figure 1A) is known to fog bask
and has a smooth dorsal surface with wide grooves 7. Onymacris laeviceps (Figure 1B) has a similar surface structure,
albeit with finer grooves, and inhabits the same sand dune habitat as O. unguicularis. It is nevertheless, not
known to fogbask but does drink from fog-dampened surfaces Seely et al., 2005.
Stenocara gracilipes (Figure 1C) and Physterna cribripes (Figure 1D) are
found outside the sand dune habitat and have elytra with a more or less regular
array of smooth bumps. It is a matter of debate if either of these two species
or genera fog-bask or not .
Figure 1 Size difference between the four model beetles.
Examples of specimens from each beetle species placed next to each other for
size comparison. A: O. unguicularis,
B: O. laeviceps, C:S. gracilipes, and D: P. cribripes. The dorsal surface area of P. cribripes was found to be 1.39 times larger than O. unguicularis, 1.56 times larger than O. laeviceps, and 2.52 times larger
than S. gracilipes.
Elytra surface structure:
SEM images and photos taken through a dissection microscope
show details of the pronounced differences in elytra structure among the four
beetle species). Whereas the pronotum on all beetles is rather smooth, it is
the elytra that have different structures.
The elytra of O. unguicularis are almost completely
smooth except for the posterior half that has large
distinct grooves, approximately 0.5 mm wide, divided by narrow ridges.
The elytra of O.
laeviceps have much finer grooves (Figures 2B1), approximately 0.1 mm wide,
that cover almost the entire elytra. The valleys of the fine grooves are not as
smooth as those of O. unguicularis
but rather have a coarser surface. In live animals, the posterior half of O. laeviceps has a blue-gray colouration .
The elytra of the
small S. gracilipes are covered in
jagged bumps that form irregular lines, although there are also bumps in
between the lines.
The elytra of the large P.
cribripes likewise have bumps that form irregular rows with additional
bumps in between. The bumps are slightly rounder than those of S. gracilipes and are found over the
entire elytra, with a smooth stripe on either side of the suture of the
beetles’ fused elytra.
Figure: Elytra structures. A)Onymacris unguicularis, B)Onymacris
laeviceps, C)Stenocara gracilipes,
and D)Physasterna cribripes. A1-D1)
Extended Depth Focus images of examples of the experimental animals obtained
with a dissection microscope. Scanning Electron Microscope images of the apex of
Figure: Hydrophobic dorsal surface of Physasterna cribripes
Fog basking behavior:
Out of the four
beetles in the Namib Desert during a fog event only O. unguicularis could be observed to actively collect water from
the fog. In an experimental chamber these beetles positioned themselves on the
top of a sand ridge and assumed a fog basking position after 114.5±9.28 sec.
The starting point of this behaviour was defined as the time at which O. unguicularis had oriented itself with
the back towards the fog and thereafter remained in this static position with
its head tilted downwards for a minimum of 2 min. The ventral side of the
beetle was held at an angle of approximately 23° to horizontal during these
In contrast, the other three beetle species
kept walking around in the arena during the 20 minutes they were observed in the
fog chamber. These three species were consequently excluded from further
behavioural experiments in the fog chamber.
The fog-basking O. unguicularis when again tested in the
fog chamber at temperatures equivalent to what exists under natural fog events,
this time without any fog in the chamber . With no fog present, O. unguicularis did not display any
fog-basking behaviour during the 20 minutes they were observed in the chamber.
However, if the temperature was elevated to room temperature and the chamber
was filled with fog, six out of twelve beetles assume a fog-basking position after
175 ± 21.65 sec. The other six beetles remained active and moved around for the
20 minutes they were observed, but never adopted a static head standing
position. High humidity, rather than low temperature, is thus the critical
condition under which the fog-basking beetles will assume their characteristic
head stand for water collection. However, a combination of fog and low
temperatures is the strongest trigger for this behaviour.
Fog-water collection efficiency:
Irrespective of their ability to actively collect water from
fog in the fog chamber or not, the ability of the four beetle species to passively
collect water from fog was tested by the scientists from dead specimens. Mounted
head down at an angle of approximately 23°. After two hours in the fog chamber, Onymacris unguicularis and O. laeviceps, that have smooth elytra
with grooves had collected 0.16 ± 0.03 and 0.11 ± 0.01 ml of water
Stenocara gracilipes and
P. cribripes , that have elytra with an array of bumps had, during the same
time, collected 0.11 ± 0.01 ml and 0.14 ± 0.03 ml respectively.
Onymacris unguicularis and P. cribripes showed a tendency to harvest more fog water than O. laeviceps and S. gracilipes, but not significantly so. Despite distinctly
different elytra structures and behaviours the four beetles collected the same
amount of water over a 2 hour period in the fog chamber. The four beetle
species do, however, vary in size. The relative sizes of beetles’ dorsal
surface area (the dorsal part of the head, the pronotum, and the elytra) were
established from coloured latex casts of the different beetles used in the
water collection efficiency experiments. Scientists found that the dorsal
surface area of the large P. cribripes on average is 1.39 times larger than the
same region in O. unguicularis, 1.56
times larger than O. laeviceps, and 2.52 times larger than
that of the smallest beetle S. gracilipes.
By applying these relative differences in dorsal surface areas as conversion
factors to the absolute amount of water collected per species ,an estimation have
been gotten of the water collecting efficiency of each species that is
independent of their sizes.
Despite the fact that O. unguicularis is the only beetle in this study that actively collects
water from fog, it does not seem to come equipped with any surface structures
that are superior for this purpose compared to those of the other beetles. In
fact, no significant difference in water harvesting per unit of dorsal surface area can be found between O. unguicularis (0.22 ± 0.04 ml) and O. laeviceps (0.18 ± 0.01 ml), or O. unguicularis and S. gracilipes (0.27
± 0.02 ml). The water collecting efficiency of the big P. cribripes (0.14 ± 0.03 ml) is, however, significantly lower than
that of the fog-basking O. unguicularis.
The small (S. gracilipes) and the big
(P. cribripes) both have elytra with
distinct bumps, but the water collecting efficiency of these two beetles come
out in the high and low end of the spectrum, respectively, with a significant
difference between the two. In fact, S.
gracilipes harvests almost twice as much water per surface area unit (0.27
± 0.02 ml) during the two hours in the fog chamber compared to P. cribripes (0.14 ± 0.03 ml).
Figure 5 Fog harvesting efficiency. Beetles killed by
freezing had their legs and antennae removed and were positioned head down at
an angle of 23° in a fog chamber. An Eppendorf tube for water collection was
placed under each beetle’s head. After two hours in the chamber the total
amount of water captured by each of the four beetle species was measured
(blue). The relative dorsal surface area of each beetle was determined and
normalized to the largest beetle. This conversion factor was used to obtain the
relative amount of water captured per dorsal surface area (red). The columns
show mean ± SE. Columns marked with matching lower-case letters above are not
significantly different at p < 0.05 (Kruskal-Wallis test and Dunn's Multiple Comparisons Test). Discussion: Fog basking behavior in an experimental fog chamber: When the four darkling beetles from the Namib Desert were exposed to fog in a small chamber,(temperature is about 10-12°, which is a temperature range similar to that of a fog event in Namib Desert), the fog basking beetle O.unguicularis readily assumed their characteristic fog basking stance after a little more than 2 minutes in the chamber. The static head stance assumed by O.unguicularis while fog-basking in the chamber was very similar for the same species while fog –basking at the crest of a sand dune during a fog event in the Namib Desert. The other three beetles remained active but did, at no time, assume a similar stance during their 20 minutes in the fog chamber. The lack of a fog-basking behavior in these three species of beetles is in accordance with long term observations of Darkling beetles in the Namib Desert, 5, where only two out of approximately 200 beetle species inhabiting this area have ever been observed to fog-bask - both from the genus Onymacris. O. unguicularis readily and predictably fog-basks in the same artificial environment supports the validity of the experimental setup. However, P. cribripes and many other tenebrionid beetles will also assume a tilting posture as a common alarm response 9. The beetle then sticks its head into the ground, spreads its legs wide, and raises the rear part of its body. This posture resembles fog-basking and could have been mistaken for it in the study by Parker and Lawrence 10. Fog was found to be the triggering factor for O. unguicularis to assume the fog-basking stance. None out of twelve beetles assumed this stance at low temperatures with no fog, but half of the tested O. unguicularis engaged in fog-basking when exposed to fog at approximately 23°C. In contrast, all O. unguicularis placed in a chamber filled with fog at temperatures similar to those under a natural fog event in the Namib Desert 9 assumed a fog-basking stance. This indicates that the temperature is a contributing, but not critical factor, for eliciting this behaviour. The recorded tolerance for variability in the factors that trigger fog collection further supports our finding that other beetle species do not engage in this behaviour. Even if the temperature in the chamber might not have been set at the absolute critical temperature to elicit fog-basking behaviour in O. laeviceps, S. gracilipes or P. cribripes, fog-basking stance when placed in the fog chamber was never observed. Water capturing efficiency by beetle elytra: Experimental results reveal that the small beetle S. gracilipes is as efficient a fog water harvester, when measured per square unit of dorsal surface, as the bigger O. unguicularis, even though it never has been observed to actively fog-bask in nature 5 or in our fog chamber. The high water collecting efficiency recorded for S. gracilipes is most likely a result of its relatively smaller size. Other organisms in the Namib Desert use fog as an important source of water, and small leaves have been shown to be an important factor for plants when harvesting water from fog 12. This is because small or narrow leaves have only thin boundary layers (an envelope of slow moving air around the object) that allow the fog water to collect on the surface of the leaf, rather than being blown around the leaf and away 13,14. Also, a smaller beetle should have a thinner boundary layer and would thus be better at collecting water from the fog laden wind. In the light of this, it is less surprising that the small S. gracilipes proves to be a good fog-water harvester as measured per unit area, and the big P. cribripes the worst. Interestingly, the 1.81 times larger O. unguicularis is as good at fog-harvesting. As S gracilipes, but not the slightly smaller O. laeviceps. This indicates that O. unguicularis - in addition to their fog-basking behaviour - could have structural adaptations on their elytra to improve water harvesting from fog. Part of this favourable outcome for O. unguicularis could of course be influenced by the fact that all beetle species were mounted in the fog-basking position assumed by live O. unguicularis. The finding that P. cribripes turns out to be the worst water harvester of all four beetles, despite its reported hydrophobic and hydrophilic elytra structures for droplet formation 10 does, however, warrant a comparison between the highly different elytra structures of O. unguicularis and P. cribripes. Elytra structures of Onymacris unguicularis and Physasterna cribripes: On a macroscopic scale, the elytra of P. cribripes are covered in an array of bumps, 0.5-1.5 mm apart, each about 0.5-1.5 mm in diameter. This is in accordance with earlier reports on the elytra of this beetle 10. The fog-basking O. unguicularis rather have smooth elytra that, in the back half, are folded into regular grooves that bend towards the apex of the body. The grooves are approximately 0.5 mm wide and approximately 0.1 mm apart. These bumps and grooves could, theoretically, form the basis of the combination of hydrophilic and hydrophobic points to improve water capture from fog 10. Experiments did not reveal any hydrophilic areas on the elytra of any of the beetles . Observations does not agree with earlier reports of P. cribripes having hydrophilic zones on the apex of their elytra bumps 10. Water harvesting in the fog-basking beetle O. unguicularis is not improved by a combination of hydrophilic and hydrophobic points on its elytra. If the comparatively high fog-harvesting efficiency on the smooth surface of O. unguicularis is caused by structural adaptations the effect of these appears to be small. The observation that S. gracilipes - which is covered in bumps, rather than grooves - is an equally efficient harvester of water if placed in a fog-basking position, further suggests that a combination of grooves and smooth surfaces are in no way critical for fog-harvesting in the darkling beetles. It is therefore concluded that water harvesting from fog in the Namib Desert beetle O. unguicularis is primarily a consequence of behavioural, rather than structural adaptations to the utilization of an alternative source of water in an environment where rain is a rare event. Harvesting water as inspired by darkling beetles: For over hundred years, scientists and engineers have been studying ways to effectively harvest fog as a source of water in arid regions. Although some of these man-made systems have proved useful, the plants and insects that inhabit desert are far more efficient dew collectors. After spending a considerable amount of time studying the water collecting mechanism of the Namib Desert beetle, researchers have imitated this astounding method by creating water collection nets and even bottles. Water collection methods: 1. Water collection net: The net is hung between two poles in a vertical position. It has a large surface area to maximize the amount of fog it traps. Fog goes through it whilst leaving behind droplets of water which flow down and are collected. This method has been adopted in countries like Chile where they have a lot of fog, and Israel which is a mostly a desert but is now a leading exporter of agricultural products. 2. Liquid collecting permeable structure: A net is however not the only surface that is used to collect water. A research team led by Kyoo-Chul Park of Harvard, for example, created a surface replicating the beetle's shell. The surface is made up of hundred of grooves that grow wider at the bottom to permit easier runoff. Its slick waxy surface made from a pitcher plant further aids in the runoff. This invention has resulted in increased efficiency in industries that rely on condensation not to mention provision of water to areas that have it in short supply. 3. Dew bank bottle: Another fantastic invention is the Dew Bank Bottle that is placed outside at night to collect water. It is made of stainless steel because of its cold property. Since it gets really cold during nights in the desert, bottle gets colder than the air so water droplets condense on the surface. The droplets immediately collect into the bottle through openings only wide enough for water to penetrate to prevent contamination. Conclusion: Researches shows that O. unguicularis is the only one of our four model beetles that assumes a head standing fog-basking stance in a low temperature environment with artificially produced fog. A comparison of the fog-water harvesting efficiency of the elytra of the fog-basking and non-fog basking beetles reveals that the small S. gracilipes and the fog-basking O. unguicularis were the better fog water harvesters, while the large P. cribripes was the worst. The differences in water collecting efficiency were however minor and it has been concluded that it is the fog basking behaviour itself (i.e. moving to the top of the sand dune ridges and assuming the fog-basking stance) rather than physical adaptations that is the important factor allowing O. unguicularis to exploit fog as an alterna