A Spring in Their Step

Spittle bugs, (Family Cercopidae) are known for the production of “spittle” a protective froth that surrounds the feeding larva on a plant. They are also known as the champion jumpers, able to leap up to 400 times their length. Why are they such good leapers? Muscle strength is important. In some spittle bug species, the mass of the pair of trochanteral depressor muscles (the muscles that power the jump) are 11% of the total body weight. As discussed in a previous post, however, muscles can deliver either power or speed, but not both. Jumping requires a speedy contraction coupled to a lot of power.

Pine Spittlebug Top Left: Frothy masses of Spittlebug Larvae on Pine Trees Bottom Left: Leaf Flagging Top Right: Larva is barely visible under a cloak of froth Middle Right: Larva Plucked from the Tree Bottom Right: Adult is covered in early morning dew

How does an insect produce both power and speed? An insect must contract its muscles slowly to load power onto a structure that can store energy and release it rapidly. Burrows, Shaw and Sutton* analyze the spittle bugs, Aphrophora alni and Philaenus spumarius and find that the spittle bug ability to leap is due to a structure, the pleural arch, that is a composite of the rubbery protein resilin and a stiffer cuticular protein.

Resilin is known for its resilience. Resilin maintains its original shape even if subjected to severe compression or stretching. However, fully loading the maximum compression force onto resilin alone would only provide 1-2% of the force delivered in a jump. In the pleural arch the resilin is combined with stiffer cuticle that deforms when the trochanteral depressor muscles contract. The bending of the pleural arch stores the energy loaded by the muscles. When the insect jumps, the force loaded onto the pleural arch is released by depressing the trochanter (second leg segment) of the spittle bug. The pleural arch rapidly returns to its “unloaded” position applying both speed and power to the leg movement.

The fine structure of the cuticle will require further investigation. Is the resilin in a layer next to the more rigid cuticle? Or is resilin more uniformly embedded in the composite? Burrows and colleagues compare the composite pleural arch to a composite bow used to shoot arrows. A composite bow is made of layers of materials with differing properties of rigidity and flexibility. Compared to simple, single-material bows, composite bows lose less energy to vibration and have less change of shape after repeated use. Knowing the structure of the pleural arch may be useful for the development of new biomaterials that utilize components of the insect cuticle.

*Malcolm Burrows, Stephen R Shaw and Gregory P Sutton, 2008. Resilin and chitinous cuticle form a composite structure for energy storage in jumping by froghopper insects. BMC Biology 2008, 6:41
doi:10.1186/1741-7007-6-41