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INTRODUCTION behavioral adaptations for collecting condensate. (The insects,

            
  INTRODUCTION

 

Many
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.

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                  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
Africa.

 

Food: Any plant and animal matter.

           Food is
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
food.

 

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.

 

Distribution
of fog basking desert beetles:

Life cycle of darkling beetles:

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
stage.

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
beetle

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:

·      
After
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.

·      
The
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.

                                      The
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
are given:

 

           

                                          

 

 

 

 

 

 

 

 

 

 

 

 

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 .

 

Size differences:

  

 

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
the elytra.

                              
                             

                     
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
events .

         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
respectively.

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

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