Zoology

Tiger Beetles Tiny Terrors



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TIGER BEETLES

Their larvae are predators, as are the adults, although the larvae hunch motionless in vertical tunnels, waiting for prey to come along, while the adults actively hunt, chasing down prey by running and flying. If a military designer needed a model for a perfect combination of jeep and aircraft, able to switch from one to the other instantly, he need look no further than these feisty little creatures.

They're tiger beetles. The name suggests ferocity and savage beauty, although it sounds ironic applied to tiny insects rather than the great striped cats of Asia. The beetles are no threat to humankind, being too small to inflict any sort of injury on human giants, although they're living, mobile nightmares in their own size realm. They're not poisonous; no beetles are actively venomous, although a few species, none of them tiger beetles, wear sheens poisonous to the touch.

Tiger beetles, or Cicindelines, all 4,100-odd species, have a worldwide distribution, excepting Antarctica and a few arctic islands. Around 100 species live in the continental U.S., while 14 have been recorded in Connecticut. The vast majority of species prefer desert or something like it: dry, sandy, open land with a minimum of vegetation cover. Outside of deserts, they're most likely to live in the sandy banks of rivers and lakes, and sandy coastlines. Their main hunting ground in Connecticut is the sandy banks of the Connecticut River, not only in this state but in others sharing the river.

Another characteristic some species of tiger beetles share with their namesake is endangered or threatened species status, as unlikely as this sounds in a world quite literally drenched in insects from subsoil to miles overhead. Like tigers, the cats, tiger beetles are losing habitat, already relatively rare in the Northeast because the Cicindelines are confined by adaptation to a limited habitat, the sand beaches of fresh-water rivers in the northeastern U.S., among them the Connecticut River. Even within the sands, individual species are further confined by their favoring specific textures of sand, found in only a limited number of locales, other textures being useless to their purposes. There are fourteen known species of tiger beetles in Connecticut, including three species listed as extinct in the state, no specimens having been found since 1980. The rest are in various stages of being relatively plentiful to rare. Seven species are listed as endangered, threatened, or "of special concern" within the state, according to Connecticut environmental laws. One species, ironically named the puritan tiger beetle, is rare throughout its small range in the Northeast, mostly along the Connecticut River, and is listed as Federally endangered and protected, as well as endangered under Connecticut laws.

A closer look at a tiger beetle may give you cause to feel thankful they can never get big. One the size of a dog would be most unwelcome company, unless you could train it as you might train a rotweiler or pit bull. The beetle has a signature predatory look, with a lean, efficient shape, protruding, bowling-ball eyes and a formidable set of barbed, caliper jaws. The beetles prefer hunting on the ground, and can run at an impressive clip over the sandy floors of their realm, which for them must be like sprinting over an endless plaza of cobblestones. For flying insects who might be prey to the beetles, landing on tiger beetle turf is never a wise move, since instant flight isn't necessarily a lifesaver. Tiger beetles can take off instantly and rapidly from the ground, a rare feat among beetles, most of which tend to be more pokey, slow-reacting creatures. Tiger beetles eat anything they can catch, which is nearly everything in their size range: other beetles, leafhoppers, ants and caterpillars. They'll also scavenge on dead or waste organic material if nothing alive is at hand. Cornell entomologists studying tiger beetle vision have found that when tiger beetles chase prey at high speeds they have momentary fits of blindness. A tiger beetle will alternately halt and run 3 or 4 times when chasing prey, reorienting itself toward the prey at each freeze. The beetle seems to compensate for delay by its phenomenal sprinting speed. Apparently, as the beetle steps up its chase speed, their eyes can't collect enough light to form an image of the prey animal, so they stop, note the position of the prey, and renew the chase. "If the tiger beetles move too quickly, they don't gather enough photons (light waves) to form an image of their prey," explains Cole Gilbert, Cornell professor of entomology. "Now, it doesn't mean they are not receptive. It just means that at their speed during the chase, they're not getting enough photons reflected from the prey to make an image and locate the prey. That is why they have to stop, look around and go." The beetle seems to compensate for delay by its phenomenal sprinting speed.

To appreciate how fast these little creatures can truck, Gilbert compared an average tiger beetle to Olympic superstar Michael Johnson, the world-record holder in running speed, at 200 meters in 19 seconds, or 10.35 meters per second. "The top speed for my tiger beetles is 0.53 meters per second'" says Gilbert. "This is not very impressive, but the beetles are a lot smaller than Michael Johnson. If we scale the speed for body length, we get a much different picture." Johnson is about 6 feet tall (1.83 meters). His 10.35 meters per second becomes 5.6 body lengths per second. The tiger beetle has a body length of only 10 millimeters, but its running speed of 0.53 meters per second becomes 53.87 body lengths per second, or relatively 10 times faster than a human's top running speed. Gilbert adds, "there is an Australian species (of tiger beetle) which is 20 millimeters long and can run 2.5 meters per second. This translates into a relative speed of 125 body lengths per second." Gilbert sees application of the tiger beetle's peculiar visual system to artificial tracking systems, including those on roving robot vehicles sent to Mars. "The Mars Rover needs optical sensors to look around Mars, but it also needs to move around. With speed of movement, there's a trade-off. You want to move quickly to explore a large area, but if you move too fast for the optical sensors to gather enough information to form an image, the exploration is fruitless," said Gilbert. "Through knowledge of biological tracking systems, we can learn how nature has coped with this trade-off, and we may then design better systems to see what is going on around us."

Tiger beetles put most other temperate beetle species to shame with their tropical exuberance of colors and patterns, another suggestive link for their common name. Somewhat like tigers, some tiger beetle species sport stripes on their wingcases, and others, even more elaborate patterns. As bright and loud as the colors look on a hand-held tiger beetle, on sandy ground they nearly disappear. The varied pattern breaks up the insect's visible shape and outline, and it seems to fade into the randomness of sand grains. The upper surface of a Cicindelid's abdomen, beneath the wing cases, is generally brightly colored, but when grounded, the colors aren't visible, concealed by the folded wingcases. When a tiger beetle takes flight the bright colors are suddenly flashed, serving to startle and spook other predators. All in all, Cicindelids are a unique and unusual take on a common insect type.

Like all beetles, tiger beetles cycle through larval, pupal, and adult stages. The different Connecticut species (and any Cicindeline species sharing a range) apportion out the months of a year among themselves for life activities like feeding, mating, egg-laying hibernation, and metamorphosis. Some species are active from spring through summer into autumn, others are active spring and fall but become inactive in summer, still others get going in mid-summer and keep busy into autumn. All Northeast species hibernate at least once per generation. Tiger beetle larvae are long and look like crosses of grubs and caterpillars, with powerful jaws. They're predatory, but in a more passive way. A larva waits, hunched and braced into a S-shape, within a vertical tunnel it has dug into the sand, either out of sight or with its upper body hunched forward, blocking the entrance and passing for a pebble. The larva keeps motionless but alert for telltale vibrations from the foot-taps of a passing creature hiking over the sand, in most cases insects. Instinct drives the rest. As a luckless insect skitters too close to the lair, the larva springs out at the exact right moment, snags the victim in tonglike jaws, then drags it in and down to an unenviable death. The action is too fast to clearly see. Special adaptations, hooklike protrubrances on its lower back and another set on its hind end, allow the larva to brace itself against the wall of the tunnel, and provide levers for its ultrafast spring-and-grab. The larvae are perpetually wary and will quickly drop to the bottom of the burrow if they're spooked. They are sensitive to motion sensed through ground vibrations, but are quite insensitive to sound as air vibrations. A larva needing privacy for rest or protection will plug its burrow with a cap of sandy soil.

Tiger beetle larva feed on many types of small arthropods. The larvae go through three growth stages, called "instars." Following the third instar, the larva digs a special pupal chamber to one side of the tunnel, then changes into the pupa, which matures into the adult form.

Adult tiger beetles are themselves preyed upon. Dragonflies capture tiger beetles in mid-flight and eat them at leisure on the wing, while robber flies will also attack beetles in flight. Birds, amphibians, reptiles and mammals also regularly help themselves to tiger beetles. Dr. Derek S. Sikes of the University of Connecticut's Department of Ecology and Evolutionary Biology recently conducted two statewide surveys of tiger beetle populations for The Nature Conservancy-Connecticut Branch in 1996 and for The Connecticut Department of Environmental Protection in 1997-1998. His Connecticut DEP report includes all the information from the 1996 report to The Nature Conservancy, along with new data he recorded following the earlier report. The lengthy, descriptive title of the DEP report is "Connecticut Tiger Beetle Status Survey (Carabidae: Cicindelinae) 1996-1998:status, Taxonomy and Ecology."

According to Sikes, "Cicindelines in general are predisposed towards endangerment, most likely due to the tight associations of many tiger beetle species with 'ephemeral, patchy, dis-climax communities,' " meaning environments that don't last long, exist in small patches rather than extensive areas, and are types of ecologies that will soon give way to other types. These sandy, desert-like mini-habitats, with little vegetation, are "dependent on periodic disturbances such as fire, erosion or flooding" to keep them more or less as they are, though not forever. Sandy habitats are generally rare in Connecticut and are often dismissed as junk ecologies unworthy of conservation. Despite their drab, useless appearance, these ecologies are rich and diverse, filled mostly with creatures adapted to their special conditions. Tiger beetles can thus be valuable as "flagship species" for highlighting the protection of such habitats.

Even along the lengthy Connecticut River, bordered by ribbons of sand nearly as long, tiger beetles are uncommon. Add to that, of course, you and me. Human presence and building on the shores of the river has buried much tiger beetle habitat, although, ironically, the beetle thrives in some picnic areas. People using the sand banks for picnicking have actually improved or at least maintained tiger beetle hunting ground by keeping these areas mostly clear of vegetation, since the beetles prefer open ground. In time, sandy ground tends to give way to dry-adapted ground vegetation, which in turn prepares sandy areas for takeover by forestland, which is useless to most species of tiger beetles.

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