Clambering across uneven terrain, most of us take it for granted that we can stride across breaks in our path. But what if you're an insect faced with a chasm as wide as you are long? Just how do you negotiate an obstacle that`isn't there'? Holk Cruse is fascinated by the ways insects process information about their environment as they clamber about. When faced with a step or barrier, most insects investigate with their antennae before scaling the obstruction. But gaps are a different matter, the insect has to recognise that there's nothing there before attempting to cross. Wondering how insects detect and negotiate gaps, Cruse and Bettina Blaesing challenged stick insects with a variety of chasms, and discovered that insects need to put their foot in it, before they can get across(p. 1273).

After a trip to Borneo to watch Aretaon asperrimus in their natural environment, Blaesing knew that the insects were champion walkers. Foraging at night, the 5 cm long creatures must climb into bushes, clambering between the branches and straddling leaves. But just how do they mind the gap?Back in the lab in Bielefeld Germany, Blaesing began challenging the insect's with gaps ranging from 1 to 5 cm to see how the insects detect and traverse them. At first she couldn't see a pattern to the insect's slow progress across the chasm, but when she began filming their gap crossing antics, Blaesing realised the insect didn't seem to notice the chasm until it had put its foot in it. And when Blaesing blindfolded the insects, they managed to cross just as well as sighted insects. It was only when the insect's foot failed to contact terra firma that the insect noticed the gap, applied the breaks and began investigating the chasm that loomed before it.

Breaking the insect's gap crossing progress into distinct phases, Blaesing realised that the insect first discovered the gap with its foot, and then investigated the chasm with its antennae. It was only when an antenna contacted the other side that the insect seemed to get the go ahead to proceed. In fact this signal was so strong, that Blaesing has watched some insects struggle for over 15 minutes before making it safely to the other side.

Once the insect has taken the first step forward and planted its forelegs on the other side, it ensures that it is stably straddling the gap by taking short steps, before carefully transferring its middle legs over to join the first. Then the insect repeats the whole process with the rear legs. Blaesing also realised that during each stage of the insect's progress, its behaviour could be characterised as five basic movements, which she categorised in an ethogram.

Now that Blaesing has deconstructed the stick insect's gap crossing technique, she's teaching the lessons that she's learned to a simulated insect. She explains that although the cyber-insect was already competent at righting itself and negotiating obstacles, it just kept marching on when faced with a gap. Blaesing is optimistic that the cyber-insect will soon be able to negotiate unexpected breaks in its path as well as Aretaon, and maybe one day, even scale virtual bushes.

Blaesing, B. and Cruse, H. (
). Stick insect locomotion in a complex environment: climbing over large gaps.
J. Exp. Biol.