Caterpillar wheel
When danger looms, this caterpillar can transform itself into a wheel, and roll away
Caterpillars ‘inch’ forward notoriously slowly, using their classic motion of wave-like (peristaltic) muscle contractions.
But some species also have a rapid backwards escape strategy, known as ballistic rolling.
The larva of Pleuroptya ruralis, the mother-of-pearl moth, is one such example. When it encounters potential danger, this caterpillar can quickly transform itself into a wheel (see photos A and B).1,2
It can roll for up to five complete revolutions, travelling at about 40 cm (15 inches) per second—about 40 times faster than its ordinary speed of locomotion. The caterpillar can go from flat-and-stationary to rolling in just 60 milliseconds (six-hundredths of a second).
Now there are many passive wheeling animals already known (e.g. salamanders that on a slope curl themselves into a ball to roll away from danger).3 But self-propelled wheeling is much less common. As caterpillar wheel researchers have noted, “self-propulsion is rare and poorly understood.” 4 So the Pleuroptya caterpillar’s launching itself into a rolling wheel is especially notable. And its travel speed makes it “one of the fastest self-propelled wheeling behaviors in nature.” 4
Reinventing the (caterpillar) wheel
The caterpillar has caught the eye of engineers envious of its capacity to morph itself to wheeled or non-wheeled locomotion as the situation demands. “Inspired by this behavior”,4 the engineers have attempted to copy its transformer capabilities in the soft-bodied robot dubbed GoQBot. (The research was funded by a military program to design flexible robots that can gain access to tight spaces.5)
The 10-cm-long robot mimics both the caterpillar’s ‘inching’ movement and its ballistic rolling. (Unlike the caterpillar, GoQBot rolls forwards, not backwards.) The curling part of the procedure is achieved by means of heating with electrical pulses. The robot’s shape-memory alloy coils contract, acting like muscles to curl GoQbot into a round wheel.
But—the robot cannot compete with the fast reaction time of the mother-of-pearl larva. The GoQBot takes 50 milliseconds after electrical stimulation to start curling, whereas the Pleuroptya caterpillar is almost completely curled into a wheel shape by that time.
On a (power) roll
The engineers have estimated from their studies that the mechanical power for caterpillar rolling is comparable to that of a locust jump. But they admit, “How caterpillar musculature produces such power in such a short time is yet to be discovered.” 4
An important lesson from all this is that just as no-one would say that GoQBot was not designed, so too should that apply to the life form that inspired it. Of course that goes against the evolutionary narrative that there was no Creator. But the more that living things are studied and reveal their design intricacies, so all the more the Psalmist’s statement rings loud and true: “O Lord, how manifold are your works! In wisdom have you made them all” (Psalm 104:24).
References and notes
- Brackenbury, J., Caterpillar kinematics, Nature 390(6659):453, 4 Dec 1997. Return to text.
- Brackenbury, J., Fast locomotion in caterpillars, J. Insect Physiology 45(6):525–533, 1999. Return to text.
- To view salamander and caterpillar rolling online: BBC Studios, Weird Nature: Rolling salamanders and caterpillars, youtube.com/watch?v=HmLS2WXZQxU, 31 Jan 2009. Return to text.
- Huai-Ti Lin and 2 others, GoQBot: a caterpillar-inspired soft-bodied rolling robot, Bioinspir. Biomim. 6:026007, 2011. Return to text.
- Matson, J., Flexible, rolling robot copies caterpillar’s escape mechanism, scientificamerican.com, 26 Apr 2011. Return to text.
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