An estimated 3 million insect species all walk using variations of the alternating tripod gait where at any instant these organisms hold one stable triangle of legs steady while swinging the opposite triangle forward.
In this paper, the authors report the discovery that three different flightless beetles use an additional gallop-like gait which has never been associated with any insect before.
Like a bounding hare, three variants of the Pachysoma beetles propel their body forward by synchronously stepping with both middle legs and then both front legs. No aerial phase occurs but the leg coordination is that of a gallop.
Although P. endroedyi, P. hippocrates and P. glentoni can walk using the normal tripod gait these beetles usually employ a unique galloping gait where they move each pair synchronously, stepping alternating with the front and middle legs. The hind legs are dragged behind even if the beetle carries no load and seem to contribute little to propulsion.
The authors found no speed advantage for the Pachysoma galloping gait. On the contrary, when observing beetles on sandpaper or fabric surfaces(to provide grip) the authors measured running speeds in the tripod-walking P. striatium that were significantly faster than the sympatric, similarly-sized P. endroedyi. This is true both in absolute terms(\(9.1 \pm 1.5\) cm/s vs. \(7.6 \pm 1.6\) cm/s) and in relation to body size(\(4.2 \pm 0.9\) bodylengths/s vs. \(3.1 \pm 0.6\) cm/s).
I find this result important because I actually formulated my galloping hexapod hypothesis before learning about this paper. The authors’ observation doesn’t contradict my hypothesis that hexapods, regardless of brain wiring, aren’t suitable for galloping gaits.
10 beetles each of P. striatium and P. endroedyi were used but the authors don’t provide details concerning how their speeds were measured on sandpaper:
a. What devices were used?
From my correspondence with the main author, Jochen Smolka, I learned that the beetles were filmed from above with a high-speed camera. The movement of their legs was manually tracked (by clicking on the position of each leg, frame-by-frame) afterwards using their our own video tracking software).
b. What distance was used?
If we analyse figure 2, the distance appears to be approximately one foot. According to Dr. Smolka, these platforms were also used for some other experiments, where they were introduced into the beetles walking path as ramps that the beetles had to cross; this was a consideration for the size. Moreover, Dr. Smolka claims that in a pilot experiment, they calculated the speed of several beetles that were moving naturally in the field, and found no difference. I’ll assume they used a two-sided t-test here.
c. Assuming the beetles had the same starting position \(A\), how were they motivated to reach the endpoint \(B\)?
According to Dr. Smolka, beetles keep going in a straight line even if you pick them up and place them elsewhere. Smolka claims that this is their natural exploration behaviour as a straight line is the easiest way to make sure you don’t check the same spot twice (for food, mates, etc.). To be honest, I’m actually quite surprised by the exploration behaviour1 of P. endroedyi. I would have expected some variation of Lévy flight search rather than a straight line. I must add that the authors have mainly compared the average speed of exploration. This is probably different from their maximum speeds when fleeing from a predator. On this point Smolka notes that they selected beetle species with the least developed predator avoidance response and are therefore easiest to handle. Moreover he adds that there is usually nowhere to go for these small, flightless desert beetles, especially when they don’t have a burrow, so running away isn’t a good option.
Why is it that of the three galloping Pachysoma beetles, only the galloping speeds of P. endroedyi were measured in a laboratory?
On this matter I just received an email from the main author, Jochen Smolka, and he provided two important reasons. First, the two other beetle species are quire rare and so finding a large enough sample size would have been difficult. Second, P. hippocrates and P. glentoni are extremely skittish so filming from above the sandpaper platform would have been impossible.
Assuming that the authors’ analysis holds and P. endroedyi is representative of all galloping Pachysoma, we may ask why this rare gait evolved. In particular, the authors ask:
Does it provide an advantage in terms of energy consumption or mechanical stress?
Does it make it easier to move straight or stabilise the head and eyes while transporting large loads across shifting sands?
Besides these last two questions, I think it may be interesting to explore a mechanical model of P. endroedyi in a computer simulation and try to figure out whether there are other dynamically stable galloping gaits that P. endroedyi could use which involve the use of all six limbs.
- A new galloping gait in an insect. Smolka et al. 2013.
Dr. Smolka also provided the following anecdotal evidence concerning two phases in P. endroedyi exploratory behaviour. When they emerge from the ground (after feeding/hibernating for many days, weeks, or months), they perform straight-line search until they find a significant food source (dung or, for some species, plant detritus). They then build a burrow nearby (which can take an hour or more). Once they emerge from this new feeding burrow, they show more of a “random walk”-type search, presumably because they do not want to get too far away from this burrow. (Some of the larger species often only forage within a few centimetres distance once they have built a burrow.) ↩