Rivacindela hudsoni is an Australian species of the family Cicindelinae or tiger beetles and is the fastest running insect. The genus Rivacindela is contentiously treated as a subgenus of the broader Cicindela and are typically found in salty habitats such as dry salt lakes and salt streams. The species was discovered in South Australia and described in 1997, with an adult form of approximately 20–21mm in length and a running speed of 2.49 m/s, or 120 body lengths per second.
Tiger beetles are a unique family of beetles due to their widespread global distribution and abundance. With over 2300 species of tiger beetles, the diversity in their coloration, behavior, and ability to thrive in varying environments has led many scientists to analyze more about their background, history, taxonomy, and ecological relationships with other organisms. Tiger beetles have an interesting classification: they are a unique species within the Coleoptera order and have created their own family called Cicindelidae. However, they have close resemblance and share similar characteristics with another family of ground beetles known as Carabidae. Another point of curiosity is how physically similar tiger beetles can exhibit completely different behaviors and cope with drastically dissimilar ecological conditions.
In the mid 19th century, Carolus Linnaeus, a Swedish biologist and physician known as the father of modern taxonomy, formally named the first tiger beetle species. Later, field naturalist Thomas Say became the first American to analyze tiger beetles in the field during a period when it was dangerous to conduct field work in America. After decent amounts of data collection, beetle specialists then began to publish articles about tiger beetle taxonomy that was disseminated for other researchers specializing in the research area.
Appearance Several physical distinctions regarding the tiger beetle include their elongated jaws with teeth on the inner side, general long body shape ranging from 5mm to 4 cm, and thin legs used for running across a variety of surfaces. While many have dulled-black coloration, some species are brightly colored in shades of blue, red, green, purple, and yellow. Most adults have clear wings right on top of their abdomen that generally only permit short-distance flight ranges.
While it remains unclear why the tiger beetles' wing sizes have reduced, they also go through various physical changes such as rounding of parts of their wing covers (elytra) and shoulder areas (humeri), reducing the middle and lower parts of their body (metasternum, metatergum, and met-episternum), as well as adjusting the positioning of the leg joints (mesocoxae and metacoxae). Since the species only relies on running as their escape mechanism, significant adaptations in their body shape and running behavior can be anticipated.
The hard cuticle that covers the majority of its body is both a crucial survival trait but also a useful marker for researchers to identify them. Their outer layer includes micro engravings of pits and ridges as well as alternating layers of translucent wax and melanin to reflect color and give rise to their dullish pigmentation. Furthermore, the distribution of micro hairlike strands helps with precise sensation and insulation under hot climates. Internally, tiger beetles have a simpler network of internal organs.
Geographic range Tiger beetles are distributed based on limitations in resources and desiccation. Tracing fossil data about the species, it's evident that they were alive from about 250 million years ago and are now found throughout regions worldwide with exception to Antarctica. However, they cannot fly far.
There has been an association found between body-size and habitat preferences in a research paper investigating tiger beetles in Sri Lanka. Upon further investigation, tiger beetles that have a larger bodies and bigger mandibles tend to show preference for coastal areas and reservoir settings that are also characterized by heavy winds, elevated soil pH, and low soil moisture. They tend to be in open areas that are more barren. On the other hand, tiger beetles with smaller bodies and smaller mandibles live in river areas that usually have low wind speeds, low soil pH, and high soil moisture. These smaller species also like urban environments, probably due to the fact they are similar to the characteristics of usual habitats they occupy.
In addition, the areas they occupy also dictate the type of predators they attract. Larger tiger beetles tend to prey on insects, while smaller tiger beetles must avoid common insect and reptile predators like lizards, flies, and spiders. These correlations are likely due to the fact that body size can have a significant effect on the amount of energy an organism needs to acquire and consumes, as well as potential for them to experience predation and parasitism.
Habitat
Saline bodies of water are the most common habitat for Australian tiger beetles as they form a sort of haven for the insects which are surrounded by vast, arid deserts that cannot energetically support them. Rivacindela as a genus is endemic to the Australian Eyrean region and its isolated evolution in this area gave the species its propensity for high speeds which have been shown to increase at higher surface temperatures. The beetle was first recorded at 31°05'40.0"S 135°19'30.0"E by Sumlin in 1997.
Life history
Egg The first life stage of Rivacindela hudsoni is its egg form, inside of which the embryo develops. Although a developmental period is not yet clearly described for this species due to its recent discovery, it can be inferred to be anywhere from 9 to 29 days when following the general trends of the family Cicindelinae.
Larva Once hatched, the organism is in its larval stage. Here, the Rivacindela hudsoni is white and grub-like with two tagmata (specialized body segments): the head and pronotum. Both of these segments are highly chitinized, meaning there is a strong exoskeleton covering the larva which will be shed for metamorphosis and growth to occur. On its lower back, the larva has a pair of large, forward-facing hooks, used to anchor the larva to the environment around itself. Further, it has mandibles under the eyes to capture prey. Not much is known of the pupal stage, as, in the same manner as all members of the Cicindelinae family, this stage of its life is completed entirely underground, since the larvae have to ability to make tunnels. This stage can take anywhere from three to 30 days.
All tiger beetle larvae are relatively similar as they spend their upbringing in the same environment: dark burrows. They resemble grubs with a white body and dark capsule. One defining feature is evident on their lower back where they have two pairs of large hooks used to pull oblivious prey into the burrow. They reside in burrows that range in ecological conditions; some are located in flatter land masses; others include clay banks or even a pile of rotting leaves. The burrow not only is a great place for larvae to await prey, but also for them to quickly retreat in times of danger.
Larval hunting Once the larvae of the Rivacindela hudsoni hatch from the egg, they dig and reside in tunnels, anchoring themselves to the substrate via the pair of hooks on its back. It is here that they move through their three larval developmental stages known as instars. At the end of each instar, the organism moults its chitinized outer layer in order to grow. These stages of growth are facilitated by the energy gained through the larvae's sedentary predatory behaviour.
At this stage of its life, the Rivacindela hudsoni hunts using an ambush method of attack as it lies in wait for its arthropod prey to move in range of its mandibles. Once the prey is in range, the larvae extends its body out of the burrow, grabs the organism with its mandibles and drags it back into the tunnel where it proceeds to kill and eat it. At the end of the third instar, the larvae remain in a chamber situated in the depths of the burrow in which they pupate after blocking the tunnel's entrance. Once the metamorphosis is complete, the adults dig out of the ground to begin the next stage of their lives.
Larva diet Similar to how body size correlates with habitat choice, it also relates to the size of prey they choose. Studies showed that low feeding levels can create significant implications on tiger beetle development, more specifically for larvae. Because of the increased susceptibility and risk of parasitism for larvae, long-term exposure to low feeding levels can greatly increase mortality rate. This is especially worrisome as the vulnerable larval foraging stages are extended when feeding levels are low. Furthermore, the consequences of these low feeding levels can manifest as a domino effect and extend to later stages throughout the life cycle, which result in underdeveloped pupae and adults. Insufficient feeding at a young age can impact the reproductive potential of adult females who similarly experience low feeding levels.
Adult Like all insects, the Rivacindela hudsoni has three tagmata in its adult form. These segments include the head, abdomen, and a patterned thorax onto which six thin, uniramous appendages and two pairs of vestigial wings are attached. The beetle's forewings are hardened to form a protective layer known as the elytra and are fused to the hindwings. The species has two large eyes that together make the head wider than the thorax, underneath which a pair of filiform antennae are attached. Surrounding the mouth is the labrum, onto which sharp projections and maxillae are attached next to a pair of sickle-like mandibles with both compound and simple teeth arranged along its length. The average body length of R. hudsoni is recorded at 20.8 mm.
Adult hunting Adult Rivacindela hudsoni are diurnal, meaning they are active during the day with a period of rest and/or inactivity at night. The hunting practices of these predatory flightless beetles are highly specialized due to the shift from bimodal locomotory status to a unimodal locomotory status, meaning the loss of flight and reduction to only running. They have been observed to run in a zig-zag pattern which is closely followed by long, fast straight lines. When running straight, the R. hudsoni were observed to run faster than any other recorded insect at 2.5 m/s or 125 body lengths every second. However, it has been observed that at high speeds, tiger beetles experience temporary blindness when chasing prey. This is because the beetle cannot gather enough photons that reflect its prey to form an image of it, resulting in a stop-start mode of hunting made affordable by its high speeds. As a predator living in dry, saline environments, the R. hudsoni eats almost anything it can capture such as other beetles, caterpillars and ants while also taking advantage of other, larger meals as scavengers. Their feeding practices are not just to consume their prey whole. The beetles will slowly break down the prey's cuticle to gain access to the soft, internal parts of its body.
Mating