Some adaptations appear as invention: a creative saltational step well beyond what has done before. Sometimes these leaps are keepers, as defining a lineage. Other times, evolution produces a singular novelty.
Planthopper Leap Gear
Mechanisms previously thought only to be used in manmade machines have evolved in Nature. ~ English zoologists Malcolm Burrows & Gregory Sutton
Gears rarely appear in Nature. The only known life with a working gear is the planthopper: a phloem-sucking plant pest common across the globe, only a bit bigger than a flea.
Planthoppers are camouflaged to look like their meal ticket. To avoid attention, they walk on branches extremely slowly. In case one is spotted, an adult can leap within milliseconds with a force of 500 Gs: flea-like indeed. A human would pass out at such a speed. Other insects simply can’t keep up.
To powerfully jump in an instant, both legs need to move precisely synchronously: otherwise a leap begets an out-of-control spiral.
Nymphs are not as able leapers as adults, so they have an assist: legs in gear. Juvenile planthoppers have a set of matching gears between their legs that coordinates takeoff. Each gear strip has 10–12 teeth, with each tooth 350–400 micrometers long.
Other insects, such as grasshoppers, push their legs straight up to hop. Planthoppers propel themselves with a breaststroke motion: their legs a mirror of each other in splaying out to the sides.
This precise synchronisation would be impossible to achieve through a nervous system, as neural impulses would take far too long for the extraordinarily tight coordination required. ~ Malcolm Burrows
Close registration between the gears ensures that both hind legs move at the identical angular velocity, propelling the body without yaw rotation. The gears are asymmetrical, so that they can only rotate in the direction needed to leap.
Planthopper gear teeth have rounded corners at the point where they mesh: a feature human engineers use for shock absorption, to prevent gear teeth from breaking off.
A nymph might recover from a broken gear tooth in a regenerative stage, but an adult would be doomed if it relied on the same mechanism and had to jump. After molting a half-dozen times, the gears are lost at the final ecdysis into adulthood. Nevertheless, with stronger leg muscles, adults are better jumpers than juveniles.
Owing to a mathematical oddity there are a limitless number of ways to design intermeshing gears. Planthoppers evolved an ideal set for their intended purpose.
Planthoppers are not alone in having mechanical joints that seem more machined than biological. A flightless weevil endemic to Papua (Trigonopterus oblongus) has a nut-and-screw joint connecting its legs to its hips, 0.5 mm in size. (All other known hip-leg joints are either ball-and-socket (as in humans), hinges, or saddle joints.) The unique mechanism allows the weevil to tuck its legs below its body.
Adaptation may arise from opportunity, most notably new nutrient sources. At other times, the habitat demands change if a population is to survive. Such envirotypic demands include ecological associations: whether from parasites, predators, prey, or from one’s own kind: sociality. Group dynamics within a population may itself propel adaptations.