Onymacris unguicularis

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Onymacris unguicularis
Onymacris unguicularis MHNT (cropped).jpg
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Coleoptera
Infraorder: Cucujiformia
Family: Tenebrionidae
Genus: Onymacris
Species:
O. unguicularis
Binomial name
Onymacris unguicularis
Haag, 1875

Onymacris unguicularis, also known commonly as the fog-basking beetle, head-stander beetle, or the Toktokkies , is a species of darkling beetle that is native to the Namib Desert of southwestern Africa. At night, during foggy weather, these beetles climb sand dunes and stand on their forelegs in order to capture water droplets as they run down their vertical bodies and into their mouths [1] The behavior is called "fog-basking" and is unique to Onymacris unguicularis and Onymacris bicolor.

Contents

Fog basking was first observed in the O. unguicularis in 1976. [1] The mechanism for this ability is still being discovered, but current research points towards the beetle’s elytra containing some hydrophobic component that can catch water droplets being blown in the wind. The idea of harvesting atmospheric moisture from early morning dew using special materials has been widely utilized in many parts of the world. [2]

This species has very often been confused with Stenocara gracilipes, another species of darkling beetle in the Namib Desert, due to their similar coloration and water capturing behavior.

Geographic Range

Onymacris unguicularis inhabits most of the Namib Desert and it is almost entirely restricted to this habitat. [2] [3] The species is confined to the major dune fields within the northern (Cunene, Skeleton Coast) and southern (Namib) sand seas of the desert. [4] The species’ migratory habits correlate very highly with the observed pattern for common fog occurrence in the region. [5]

Habitat

The coastal regions of the Namib Desert serve as the natural habitat of Onymacris unguicularis. The long and narrow desert hugs the southwestern coast of Africa, displaying a subtropical climate. [6] Due to cold water currents, the desert is extremely arid. The Namib Desert annually receives less than 50 mm of rainfall overall and in its western half between 0 and 12 mm. Humidity is low most of the year outside of fog events, with relative humidity daily dropping below 50%, with recorded lows below 20%. [7] A majority of the moisture is brought in by fogs blowing from the ocean, bringing water droplets that condensate across the environment. The phenomenon causes a gradient between “coastal” and “inland” faunas across the region as humidity decreases and temperature increases further inland. [3]

Appearance

Onymacris unguicularis is a fully black beetle, with a fused elytra and no wings. The elytra are almost completely smooth, except for the back half which has large distinct grooves. These grooves are approximately 0.5 mm wide and are divided by narrow ridges. [8] Males have longer legs compared to females, as well as setose brushes on the anterior femora. Females tend to have wider elytra than their male counterparts. [4] There are differences between the two subspecies, with the northern Onymacris unguicularis schulzeae having shorter elytra when compared to the southern Onymacris unguicularis unguicularis. There are also notable differences in pronotal and prosternal shape. [4]

Mating

Copulation

In the events leading to copulation, male Onymacris unguicularis beetles will mount stationary or moving females and remain on top for extended periods of time. Forelegs and middle legs are primarily used to stay clung to the female while hindlegs are left limp and are mostly allowed to drag along. Females may attempt to throw off the mating male, but most efforts are unsuccessful. The whole copulation process may take anywhere from one to ten minutes. [9] Males directly transfer a spermatophore into the female through the insertion of a penile tube extending from the aedeagus into the bursa copulatrix of the female. [10]

Sperm Competition

Female Onymacris unguicularis are known to practice a polyandrous mating system. If the female’s initial sperm supply has not been completely used for fertilizing her eggs before she mates with another male, there is sperm competition. The phenomenon is fairly common in female O. unguicularis, with the average number of matings for a female being three times in 35 days. [10] Aspects of sperm competition such as sperm displacement and sperm precedence have been observed in the species. In terms of the former, during copulation, the newly inserted spermatophore will push the old spermatophore out of the female’s body and into the sand. The old spermatophore, once ejected, appears as a bleb, consisting of the remains of the old spermatophore sac and coiled penile from the last male. [10] The latter phenomenon has been observed through experiments that use sterile male technique, indicating that not all old sperm is displaced during a new mating. In these experiments, females are allowed to mate with fertile males initially and lay eggs before the males are replaced with irradiated sterile males to do the same. The fertility rate between the fertile males and sterile males has shown substantial decrease in fertility rate, showing that the sterile sperm was unable to completely dislodge all of the former sperm. These observations indicate a possible mechanism involved in the long-term storage and maintenance of sperm. [10]

Subspecies [4]

Fog-Basking Behavior

Behavior

One of the most distinguishing features of Onymacris unguicularis is its fog-basking behavior. This behavior is believed to have evolved independently in only two Tenebrionidae beetles, O. unguicularis and Onymacris bicolor [3] [11] O. unguicularis is normally active during the daytime and buries itself in sand dunes when night sets in. During periods of nocturnal fog in the desert, however, they will emerge from the sand and climb onto the slip face of a dune (the side of the sand dune not directly hit by the wind). Positioning themselves against the wind, they conduct a “headstand” posture, angling their heads toward the ground and lifting their dorsal side towards the wind. The angle from resting was observed to be 23 degrees from horizontal. [8] In this handstand position, fog water condenses on the surface of the beetle’s elytra and runs down their body into their mouth. The amount of water that can be collected by an individual beetle relative to their weight can vary greatly, with some beetles gaining little to no water while others have been recorded taking in 34% of their total body weight. [1]

The “headstand” posture is caused by high humidity caused by fog conditions. Low temperatures, observed to be below 20 degrees Celsius during nocturnal fog, [12] may play a role in modulating the behavior, but low temperatures do not seem to be capable of triggering the “headstand” posture without high humidity conditions. [8]

Mechanism

The mechanism for water collection is still debated, but theories have been placed as to how Onymacris unguicularis collects fog water on its elytra. One theory posited for another darkling beetle, Stenocara gracilipes, claims that bumps on the elytra of the beetle function as hydrophilic peaks where water from the fog gathers into droplets before becoming heavy enough to roll off the peak and slide down the smooth hydrophobic regions between peaks and into the beetle’s mouth. [13] The theoretical mechanism, however, has some issues when applied to O. unguicularis. The mechanism requires bumpy elytra for the presence of hydrophilic peaks. O. unguicularis’s elytra is relatively smooth in its entirety, except for the back half which is compressed of grooves and ridges. While these aspects of O. unguicularis’s elytra could have functioned in a manner similar to the theoretical mechanism, hydrophilic regions on the beetle’s elytra were unable to be conclusively found. [8] Another theory posits that O. unguicularis’s smooth elytra is entirely hydrophobic, with no hydrophilic regions. [2] [7]

Function

Fog-basking is clearly an adaptation for obtaining water, but research has shown that it is the primary source of water intake for Onymacris unguicularis. [6] Moisture obtained from foraging detritus during the daytime is insufficient to maintain maximum foraging activity or egg production. Thus, fog-basking is a method that ensures that the beetle is capable of obtaining enough water to maintain an effective water balance that allows for daytime activity even in hot desert temperatures. The water obtained through fog-basking also seems to play a key role in O. unguicularis’s osmoregulation. [5] [14]

Confusion with Stenocara gracilipes

Due to the large variety of darkling beetle species in the Namib Desert, different beetles from the region have been confused with each other. Such is the case with Onymacris unguicularis and Stenocara gracilipes, which in non-scientific literature have both been called the “fog-basking beetle.” The confusion seems to originate from a paper published by Parker and Lawrence [13] detailing a mechanism for S. gracilipes’s ability to capture water using hydrophilic bumps and hydrophobic troughs on the beetle’s elytra during fog-basking. Debate ensued as the only known beetles to display fog-basking behavior were O. unguicularis and Onymacris bicolor. In a notable response, the notion of S. gracilipes’s fog-basking behavior was challenged, and the photograph of S. gracilipes that was used in the original study was stated to have been misidentified, being a photo of another beetle, Physosterna cribripes . Additionally, it was noted that the tilting posture being identified as “fog-basking” was most likely a common alarm response found in many species of the Tenebrionidae family. [12] A later experiment would test the fog-basking behavior and water-collecting efficiency between four Tenebrionidae beetles: O. unguicularis, Onymacris laeviceps, S. gracilipes and P. cribripes. Notably, only O. unguicularis displayed the behavior, and the study supported many of the points made in the response such as re-identifying Parker and Lawrence’s beetle as P. cribripes according to their own observations of the beetles and noting no fog-basking behavior in S. gracilipes. [8]

Related Research Articles

<span class="mw-page-title-main">Beetle</span> Order of insects

Beetles are insects that form the order Coleoptera, in the superorder Holometabola. Their front pair of wings are hardened into wing-cases, elytra, distinguishing them from most other insects. The Coleoptera, with about 400,000 described species, is the largest of all orders, constituting almost 40% of described insects and 25% of all known animal species; new species are discovered frequently, with estimates suggesting that there are between 0.9 and 2.1 million total species. Found in almost every habitat except the sea and the polar regions, they interact with their ecosystems in several ways: beetles often feed on plants and fungi, break down animal and plant debris, and eat other invertebrates. Some species are serious agricultural pests, such as the Colorado potato beetle, while others such as Coccinellidae eat aphids, scale insects, thrips, and other plant-sucking insects that damage crops.

<span class="mw-page-title-main">Namib</span> Desert in Southern Africa

The Namib is a coastal desert in Southern Africa. According to the broadest definition, the Namib stretches for more than 2,000 kilometres (1,200 mi) along the Atlantic coasts of Angola, Namibia, and northwest South Africa, extending southward from the Carunjamba River in Angola, through Namibia and to the Olifants River in Western Cape, South Africa. The Namib's northernmost portion, which extends 450 kilometres (280 mi) from the Angola-Namibia border, is known as Moçâmedes Desert, while its southern portion approaches the neighboring Kalahari Desert. From the Atlantic coast eastward, the Namib gradually ascends in elevation, reaching up to 200 kilometres (120 mi) inland to the foot of the Great Escarpment. Annual precipitation ranges from 2 millimetres (0.079 in) in the aridest regions to 200 millimetres (7.9 in) at the escarpment, making the Namib the only true desert in southern Africa. Having endured arid or semi-arid conditions for roughly 55–80 million years, the Namib may be the oldest desert in the world and contains some of the world's driest regions, with only western South America's Atacama Desert to challenge it for age and aridity benchmarks.

<span class="mw-page-title-main">Namib-Naukluft National Park</span> National park in Namibia

The Namib-Naukluft Park is a national park in western Namibia, situated between the coast of the Atlantic Ocean and the edge of the Great Escarpment. It encompasses part of the Namib Desert, the Naukluft mountain range, and the lagoon at Sandwich Harbour. The best-known area of the park and one of the main visitor attractions in Namibia is Sossusvlei, a clay pan surrounded by dunes, and Sesriem, a small canyon of the Tsauchab. The desert research station of Gobabeb is situated within the park.

<span class="mw-page-title-main">Darkling beetle</span> Family of beetles

Darkling beetle is the common name for members of the beetle family Tenebrionidae, comprising over 20,000 species in a cosmopolitan distribution.

<i>Leucorchestris arenicola</i> Species of spider

Leucorchestris arenicola, commonly called the dancing white lady spider, is a huntsman spider in the family Sparassidae and genus Leucorchestris. It is commonly found in the Namib desert of Namibia. It is often mistaken with the similarly named Carparachne aureoflava, or more commonly known as the wheel spider from the same location. L. arenicola relies on seismic vibrations, called drumming, for communication. It taps its foremost legs on the sand to send messages to other white lady spiders. Male L. arenicola will travel over 50 m in one night searching for a mate. If they find a mate, they must be extremely careful, for drumming the wrong message can be deadly. One of the major features that characterizes its nocturnal behavior is its specialized vision, using eight eyes in different orientations to capture a panoramic view of the surroundings. L. arenicola spiders use temporal summation in order to be able to see dim lighting during night-time wanderings. The species was first described by Reginald Frederick Lawrence in 1962, who described all the species in the genus Leucorchestris.

<span class="mw-page-title-main">Tenebrionoidea</span> Superfamily of beetles

The Tenebrionoidea are a very large and diverse superfamily of beetles. It generally corresponds to the Heteromera of earlier authors.

<i>Pimelia</i> Genus of beetles

Pimelia is a genus of darkling beetles in the subfamily Pimeliinae.

<span class="mw-page-title-main">Namaqua chameleon</span> Species of lizard

The Namaqua chameleon is a ground-living lizard found in the western desert regions of Namibia, South Africa and southern Angola.

<i>Stenocara gracilipes</i> Species of beetle

Stenocara gracilipes is a species of beetle that is native to the Namib Desert in southern Africa. This is one of the most arid areas of the world, receiving only 1.4 centimetres (0.55 in) of rain per year. The beetle is able to survive by collecting water on its bumpy back surface from early morning fogs.

<i>Zophobas</i> Genus of beetles

Zophobas is a genus of beetles in the family Tenebrionidae, the darkling beetles. In Cuba beetles of this genus are known as blind click-beetles.

<i>Eleodes</i> Genus of beetles

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<span class="mw-page-title-main">Confused flour beetle</span> Species of beetle

The confused flour beetle, a type of darkling beetle known as a flour beetle, is a common pest insect known for attacking and infesting stored flour and grain. They are one of the most common and most destructive insect pests for grain and other food products stored in silos, warehouses, grocery stores, and homes.

<i>Bolitotherus</i> Species of beetle

Bolitotherus cornutus is a North American species of darkling beetle known as the horned fungus beetle or forked fungus beetle. All of its life stages are associated with the fruiting bodies of a wood-decaying shelf fungus, commonly Ganoderma applanatum, Ganoderma tsugae, and Ganoderma lucidum.

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Toktokkies are various species of beetles that belong to the large Tenebrionidae family, also known as Darkling beetle. Toktokkies do not belong to a particular tribe or genus of Tenebrionids, but rather a selection of flightless species that make distinct noises by tapping on the ground with the abdomen. The Tenebrionidae family to which these beetles belong is quite large, with almost 3,500 species inhabiting Southern Africa. Nearly 200 species of Toktokkies inhabit the Namibia and 20 have adapted to the extreme temperatures of the Namib Desert. The most common Toktokkies in the Namib Desert are the Fog Basking beetle and the button beetle or trench-digging beetle.

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<i>Physosterna cribripes</i> Species of beetle

Physosterna cribripes, the desert toktokkie or woestyntoktokkie, is a flightless species of desert-dwelling darkling beetle or Tenebrionid found along the West coast of Namibia and Angola. This species has a body length of some 18.4 mm and a mass of 402 mg.

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<i>Meracantha contracta</i> Species of beetle

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References

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