1. The energy requirement of a jump of 3.5 cm. height by the rabbit flea, Spilopsyllus cuniculi, is 2.25 ergs. This must be delivered in 0.75-1.0 msec. over a distance of 0.37-0.5mm., which is not compatible with direct muscular action.

2. The anatomy of the jumping mechanism has been re-examined and its action studied by high-speed cinematography. The following explanation is offered. The main impulse comes from the depression of the metathoracic femur, whose depressor muscle originates on the notum. In the resting position the attachment of the depressor tendon to the femur is over-centre with respect to the trochanter-femoral joint. When the muscle contracts energy is stored in a resilin pad located between notum and pleuron. This energy is released by a second muscle which pulls the depressor tendon away from the over-centre position, thus enabling it to depress the femur.

3. Operations on live fleas have provided evidence in support of the proposed mechanism.

4. The depressor muscle is capable of providing 1.96 ergs on each side.

5. The resilin pad is capable of storing 4 ergs on each side.

6. Species which are better jumpers have larger resilin pads.

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