16 September 2013

Evolution delivers: The first mechanical gear found in a living creature

Gears and wheels are not very common in nature. They are mainly encountered at the nanoscale within the membranes of single cells. Here they act as rotary atomic pumps or motors to move ions and wield flagella. Recently however, biologists have discovered a precision gearing mechanism that is built and used by an insect known as a planthopper, which is similar to the grasshopper. The bug employs this device to mechanically synchronize its powerful jumping actuators so that they both fire within 30 microseeconds of each other.

It appears that the planthopper developed this device to circumvent timing errors that would be expected to occur if both sides were under independent nervous control. Just as early fighter planes relied on mechanical synchros to fire their machine guns through their propeller without destroying it, this mechanical timer enables pinpoint directional accuracy when the creature makes leaps of over 100 times its own body length.

If a gearing system only needs to deliver power in one rotational direction, or in one input-output direction, oftentimes certain optimizations can be made in the tooth structure to reduce friction and maximize power transfer. The pitch of the gearing on a worm drive, for example, makes it difficult or impossible for it to be back-driven. Similar optimizations have also been found in the planthopper. It is also worth mentioning that nervous control on the microsecond timescale is not completely off the table in the soft-matter world. It is known that some bats can distinguish nanosecond delays in the timing of auditory stimuli using neurally-based coincidence detectors (devices that amplify timing differences). Many times however, sensing can be done faster than commanding an actual physical motion.

Although the adult still can jump, the gears are only found in the young planthoppers. Biologists would like to understand exactly how they are fashioned by the creature, and also how they might have evolved. We have seen some radical cases of cuticular (shell) modification by insects in order to polish a wide variety of extreme physics, like for example in the mantle of the firefly which is used to couple light from its interior. Rather than top-down design from models and calculations, biological structures like bone, or trees, employ locally adaptive elements that sense stresses over many scales (or wavelengths if you prefer) and lay down additional material only in those areas where the loads demand it.

A good engineer designs structures in such a way that its main failure modes, if unavoidable, are at least predictable. Nature on the other hand, uses predictable failure modes on the front-end — in others words, during the construction phase. For example, the general tooth pattern of the planthopper’s gear can be roughed out in crude form, and than acquire their final form at the edges through use. To take another example, observe how the fingers form in an embryo by the creative dissolution of the initial scaffolding between. Here tissue-digestive gradients are set up much like complex sand patterns can be formed on a vibrating “Chaldni drum.” (see video below)

Clues to the gears’ origins might perhaps be found by considering the largely ornamental creations deployed by beasts like the cog-wheel (spiny) turtle, for example. Once begun, even the smallest rudiment of a tool can be tuned by processes like mate selection and local nervous system feedback over developmental events. The last thing we want to do here is start a silly design argument battle, but it should be noted that yes, elaborate mechanisms like this might be said to be designed — our only clarification is that the “designer” is not some mysterious architect in the sky, but instead a critical consortium of the bug’s peers on one level, and its cells on another.

Now read: Who’s killing the bees? New study implicates virtually every facet of modern farming

Research paper: DOI: 10.1126/science.1240284 – “Interacting Gears Synchronize Propulsive Leg Movements in a Jumping Insect”


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