Many larval stages of insects that metamorphose – caterpillars, maggots and the like – are shaped like a tube with the gut running through the middle. In a recent hi-tech study, a group of US researchers led by Michael Simon of Tufts University set out to discover how caterpillars move, with surprising results.
First they stuck some crawling hawkmoth caterpillars at the business end of a synchrotron and bombarded them with X-rays, which were then converted to visible light and recorded. The resultant X-ray movies showed that the movement of the caterpillar's internal structures could be measured by following the trachea – the slender tubes that connect the insect's insides with the outside and enable it to respire. Some of these trachea are connected to muscles, others are connected to the gut.
To the researchers' surprise, video analysis revealed that at the beginning of each crawling motion, shortly before the caterpillar's middle prolegs began to move, its gut moved forward within the body, in time with the rear prolegs and ahead of all the external structures with which it had previously been in line. The back half of the body would then catch up, sliding over the gut. At some points in the movement cycle, the middle of the gut would be about 1 s ‘ahead’ of the middle of the external body.
To study whether this bizarre piston-like motion applied to the whole gut, the researchers moved to a smaller, earlier caterpillar stage, which also had the advantage of being transparent, and could be studied directly under a light microscope. They found that although the overall movement of the gut was linked with that of both the head end and the rear of the caterpillar, it was not connected with the movement of the outside body wall. In other words, different bits of the caterpillar stretch and contract at different times, with the outside sliding over the inside.
The fact that the gut swings forward inside the animal as it begins to move might be thought to give added momentum to the caterpillar (the gut and its contents can represent over 1/3 of a caterpillar's body mass). However, this appears not to be the case and it remains unclear why this odd form of locomotion has evolved.
It seems probable that other larval insects and even animals like leeches will turn out to move in the same way. Even more tantalizingly, the authors suggest that studies of biomechanics on animals with stiff skeletons should turn their attention to the soft bits of our bodies – there may be pistons there, too.
