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Minimal Complexity Problem in Prey Detection by the Sand Scorpion

Bijan Nemati
Photo credit: Marshal Hedin from San Diego, CC BY-SA 2.0 , via Wikimedia Commons.

In the introduction to the “Waves” chapter of Halliday, Resnick, and Walker’s Fundamentals of Physics (6th edition), the authors mention a remarkable ability of the sand scorpion. Living in the highly arid and hot region of the Mojave Desert, a sand scorpion must hunt its prey at night. Its visual, olfactory, and auditory abilities are minimal, and not sufficient in the nighttime desert to catch prey. Yet, catch they can, with remarkable efficiency. When a beetle comes within a couple of feet, the disturbance that it creates on the sand is detected by the scorpion first to determine direction, then to determine distance. 

The sand scorpion, like other arachnids, has eight legs. The terminal (“tarsal”) segments of the eight legs form a rough circle. It is at these eight points that the scorpion can detect tiny vibrations, of order 1 Angstrom (the size of a hydrogen atom) in amplitude, that emanate from the prey which is passing by. Detailed studies by Philip H. Brownell of Oregon State University in the 1980s demonstrated that the scorpion detects direction by comparative timing of the disturbance as it passes and is sensed by the legs. The legs that are closer to the prey sense the signal first, by as little as a few microseconds. 

A Spectacular Ability

While this sensitivity is amazing, the most spectacular ability of the scorpion is to detect distance. When the scorpion has established the direction, it will hold completely still. At the next movement of the prey (often beneath the sand at a shallow depth), the scorpion rapidly moves to the location of the origin of the disturbance, plunges its pincers to its estimated location of the prey, and catches it. Once caught, the prey is immobilized by the neurotoxin delivered by the scorpion’s stinger, and its is slowly consumed. 

Brownell showed that two type of disturbances — longitudinal compression waves, and transverse “Rayleigh” waves — with different propagation speeds, propagate effectively over these types of distances from prey to predator, and that the scorpion uses the different arrival times of the pulses to estimate distance. Brownell’s data indicated that at 15 cm or less, the accuracy of its distance estimates was excellent. 

Irreducible Complexity

But how does the scorpion “know” the propagation speeds of the longitudinal and transverse waves? And how does it know how to calculate the distance? This is a simple freshman physics problem if someone gives you the calibrated speeds. But for a Darwinian theory of the origin of species, it presents an incredible minimal-complexity problem. The minimal ingredients are 1) the sensors (atomic level sensitivity of amplitude, sub micro-second timing, the ability to distinguish transverse and longitudinal pulses), 2) the distance-to-velocity equation with the assumption that the two disturbance types were simultaneously originated, and 3) the calibrated propagation speeds for the two types of disturbances (Rayleigh and Compression waves). 

Without all three of these innovations in place, the scorpion cannot survive.