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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.
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).
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 . 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.
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 < 0.05 (Kruskal-Wallis test and Dunn’s Multiple Comparisons Test).
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, , 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 . 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 . 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  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  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 . 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  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 . 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 . 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 .
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:
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 alternative.
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