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The Jump Routine

If you watch a grasshopper closely as it jumps, you will see that it always goes through a three-stage set routine (a motor programme) before it actually takes off.

The Motor Programme

1. Initial Flexion. The first things that happens is that the hind legs flex fully. This is brought about by contraction of the flexor muscle. Full flexion is an essential pre-requisite for jumping (the reason will be explained later) - if it is prevented by, for instance, a lump of dirt getting between the tibia and the femur, the animal simply cannot jump.
2. Co-Contraction. The flexor and extensor muscles contract together. You obviously can't see the actual muscles from the outside, but you can see their effects. The contraction of the flexor muscle keeps the tibia locked in the fully flexed position, so that the simultaneous contraction of the extensor muscle bends the springs in the joint, rather than extending the leg. The extensor muscle contraction is quite slow (it can take up to half a second), and this means that the muscle can contract with maximum force. The energy of the contraction is stored in the distortion of the cuticle springs (and also in stretching the extensor tendon), just like the energy from an archer's arm is stored in the bending of a bow.
3. Trigger Relaxation of Flexor. The leg extension is triggered by a sudden relaxation of the flexor muscle. This allows the leg to extend rapidly and forcefully, using the energy stored in the springs.

A key point is that the tibial extension is not caused directly by contraction of the extensor muscle, since this largely takes place before the jump. The extension is actually driven by the expansion of the spring.

Science stuff : quantitative information on energy storage and power amplification

Take-Home Summary

In essence the 3 stages of the jump can be summarized thus:

1. Move the leg into the pre-jump position and lock it there.
2. Contract the main jumping muscle slowly but with high force, and store the energy in a mechanical spring.
3. Release the leg from the lock, allowing the spring to relax and power the jump.

This summary is worth bearing in mind because there are lots of other small animals that make rapid movements which use the same catapult principle. Some of these are briefly described in the comparative section later.

The Range vs Reaction-Time Trade-Off

The catapult mechanism used by grasshoppers is excellent for producing a jump with a high take-off velocity and therefore a long-range. However, as a mechanism driving escape there is an obvious draw-back - it takes some time for the extensor muscle to store the necessary energy in the semi-lunar processes. After all, it is no good being able to produce a spectacular jump if it takes so long to prepare that a predator can just amble up and eat you!

Having said that, the time required is less than half a second, and the fact that grasshoppers have been around for more than 100 million years suggests that they have got the trade-off more-or-less right. This is no doubt aided by the fact that they have good eyesight, and can usually spot an approaching predator early enough to prepare their jump in good time. But it is also worth noting that each female grasshopper can produce hundreds of offspring, so on balance it is probably a good thing for the rest of us if most of them get eaten before they reach adulthood!

Had Enough?

Things get just a little bit more complicated after this. If you want to keep to the overview, you may want to skip to the comparative stuff at the end. However, if you want to find out some more details about the mechanics and neurobiology, then go to the next page as normal...

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