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#9 Story of 2022: New Mode of Flight Found in Tiny Beetle

Photo: A featherwing beetle (shown at left) is comparable in size to a large amoeba (right), by SKOLKOVO INSTITUTE OF SCIENCE AND TECHNOLOGY, via EurekAlert! (no usage restrictions).

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The following was originally published on February 3, 2022.

Beetles show extraordinary variation in size, habitat and behavior. J. B. S. Haldane’s irreverent quip that God has “an inordinate fondness for beetles” should not deter design scientists from investigating the approximately 400,000 species of beetles, because they are sure to find wonderful surprises. A couple of recent discoveries are highlighted here — including a tiny species that flies by a remarkably effective method not seen in other insects.

Featherwing Flyer

Insect wings are usually thin translucent membranes, but here’s one with “feathers”! The tiny beetle Paratuposa placentiswas brought to scientists’ attention in a paper in Nature by Faresenkov et al., “Novel flight style and light wings boost flight performance of tiny beetles” (open access). Watch this video to see it in action:

This expert flyer is less than a millimeter long. That’s comparable in size to a large amoeba, yet this bug boasts all the requisite body parts of its beetle relatives, complete with the cells needed for each organ, limb, antennae, mouth parts, gut, muscles, and brain.

How can single feather-like wings (a condition known as ptiloptery) produce powered flight? Wouldn’t the air go right through the filaments? It turns out that air feels as dense as syrup at the scale of this tiny bug. The authors explain in the Abstract:

Flight speed is positively correlated with body size in animals. However, miniature featherwing beetles can fly at speeds and accelerations of insects three times their size. Here we show that this performance results from a reduced wing mass and a previously unknown type of wing-motion cycle. Our experiment combines three-dimensional reconstructions of morphology and kinematics in one of the smallest insects, the beetle Paratuposa placentis (body length 395 μm). The flapping bristled wings follow a pronounced figure-of-eight loop that consists of subperpendicular up and down strokes followed by claps at stroke reversals above and below the body. The elytra [wing shields] act as inertial brakes that prevent excessive body oscillation. Computational analyses suggest functional decomposition of the wingbeat cycle into two power half strokes, which produce a large upward force, and two down-dragging recovery half strokes. In contrast to heavier membranous wings, the motion of bristled wings of the same size requires little inertial power. Muscle mechanical power requirements thus remain positive throughout the wingbeat cycle, making elastic energy storage obsolete. These adaptations help to explain how extremely small insects have preserved good aerial performance during miniaturization, one of the factors of their evolutionary success. [Emphasis added.]

Well, let’s just call it a success. The feathery wings, shown in the electron micrograph, even has barbs resembling those in bird feathers, which the narrator says are “key to the beetle’s flight.” The wing achieves lift through its dense aerial environment by pushing its bristly wings against the air in a hummingbird-like figure eight motion, clapping the wings at the top and bottom, and using its elytra as stabilizers. Its “novel flight style” saves energy of its tiny muscles but lets it keep up with insects three times its size. It’s a fascinating and efficient mechanism.

Several unrelated orders of insects contain small species that also exhibit ptiloptery, the authors note. How did that happen? Well of course, by “convergent evolution.” That’s all they needed to say in Nature. Observations explained. Job done. 

Driven by curiosity about the smallest objects, scientific exploration of the microscopic world has facilitated the miniaturization of various industrial products. But miniaturization is not just a human-made artifice: success stories of miniaturization are abundant in the living world. For more than 300 million years, ecological pressures have forced insects to develop extremely small bodies down to 200 μm long without losing their ability to fly.

Evolution drove these diverse species to shrink and learn how to adjust their engineering for new conditions. The environment forced them to “evolve strategies” for dealing with more viscous air. Evolution is not a very clever engineer, but a stern drill sergeant.

Leaping Larvae

Another beetle has an unusual trick. Larvae of the lined flat bark beetle can leap three times their height and travel four times their body length in an instant. The adults are tiny, easily fitting on a finger, and the larvae are only 5 mm long. Strangely, the adults do not leap; this is a kid’s sport. The young ones leap then curl into a circle and roll. It looks like fun.

The Scientist posted an article with video about the “weird” leaping ability of a beetle that lives under tree bark, “a previously unreported behavior in this group of beetles.” Chloe Tenn writes:

Insect larvae are often thought of as worming their way around their environment, legless and slow-moving — or even immobile. But a paper published in PLOS ONE yesterday (January 19) reports a peculiar behavior in the larvae of Laemophloeus biguttatus, commonly known as the lined flat bark beetle: jumping. This rapid locomotion was previously unknown in this insect species, according to the authors of the paper, and has only been commonly observed in fly maggots.

The team figured out the “power amplified” behavior by first filming the larval jumps at 3,200 frames per second. A larva latches onto the ground, arches its back with its claws, and then releases the latch suddenly. Matthew Bertone et al.explain how in PLOS ONE, “A novel power-amplified jumping behavior in larval beetles (Coleoptera: Laemophloeidae).” Co-author Adrian Malone found a colleague in Japan who also observed larvae in this family jumping the same way. The leaps are not the fastest or highest of any insect (consider froghoppers as reported earlier), but they have no obvious latching body parts other than the claws. And they are plenty fast. The larvae seem to vanish before your eyes. Five times the authors attribute this to evolution, but that’s the custom these days.

Catapulting Beetle

One other beetle deserves mention for extraordinary leaps: the flea beetle. Reported two years ago in ZooKeys, the flea beetle cocks a latching mechanism in its femurs. The stored energy is explosively released, launching the animal like a catapult. Its superman-style leaps leave the bark beetle larvae in the dust (but those, remember, are just kids).

The extraordinary jumping ability of flea beetles mainly depends on the metafemoral spring in the dilated femur of their hind legs, which enables them to perform the catapult jump. The jumping of flea beetles is an extremely effective method to avoid potential predators, as it allows beetles quickly [sic] disappear from the leaf surface, where they spend most of their life. Blepharida sacra (Weise) can jump up to 70 cm or 100 times more than its body length, while Longitarsus anchusae (Paykull) reaches a jump of 289 times its body length; the average acceleration of Psylliodes affinis (Paykull) during take-off can be up to 266 times the acceleration of gravity.

Froghoppers beat that record, leaping at 400 g. Those critters, though, are true bugs (Hemiptera) instead of beetles (Coleoptera) like this contender. The authors, once again, are amazed at the creative engineering prowess of evolution.

Flea beetles have evolved an enormous independent spring to aid the storage of elastic potential energy. This is significantly different from many other rapid-moving arthropods which usually only rely on exoskeleton or modified exoskeleton to store elastic potential energy… Moreover, instead of trigger muscles or latches employed in some other insects, flea beetles utilize the elastic plate and triangular plate to control the timing of the instantaneous discharge of a catapult-like jump.

Won’t it be great someday to read scientific papers explaining biological phenomena in engineering terms, instead of falling back on the antiquated habit of attributing creative genius to blind, unguided natural processes? Darwin said that natural selection “can never take a sudden leap, but must advance by short and sure though slow steps.” Since the authors offered no evidence of slow steps to explain the origin of this feat, the flea beetle’s sudden leap illustrates a literal falsification of Darwin’s symbolic assumption.