The flea’s knees
If Harold Russell’s 1913 contribution to The Cambridge Manuals of Science and Literature represents the first major inquiry into the incredible jump of the flea then this phenomenon has been a scientific puzzler for a century. What’s so special about the jump of the flea, you ask? Well, for one thing, an average flea weighs 0.7 milligrams and is less than 2 millimeters long yet its jump occurs at a velocity of 1.9 meters/second (~ 4 miles/hour) and at fully 160 g forces (for comparison consider that a rider on the roller coaster known as Shock Wave experiences just 5.9 g forces). And what if I told you that the Cat Flea is capable of jumping a horizontal distance of more than 100 times its body length and can achieve a standing leap of nearly twice that? Check out this short video clip which captures the phenomenon at 5000 frames per second. [A Google search yielded all of the images below. The head-on SEM was found at ferrebeekeeper’s post entitled Flea Portraits.]
Animal physiologists were quick to reason that, by themselves, muscle contractions could not account for the truly explosive nature of the launch of the flea and in 1967 H. C. Bennet-Clark suggested that the elastomeric protein, resilin, might be involved. Imagine a relaxed and tangled rubber band with cross-links holding the thing together in three-dimensions. In this configuration the band occupies a low energy state and is disordered. Compress and straighten the band; it is more ordered and now occupies a higher energy state. When the force keeping the band reconfigured is removed the cross-links will ensure that it returns to its disordered state and the energy required to order the system will be released. This is just how resilin works as a biological (entropic) spring. It stores energy and then releases it with nearly perfect efficiency. In 1973 Miriam Rothschild and others detailed the anatomy of the flea with special regard to muscles and tendons of the massive hind legs. In particular they: (1) confirmed the location of a small pad of resilin; (2) described the mechanism whereby (epipleural and trochanteral) muscles compress the resilin thereby loading both it and the stiff body walls of the flea with energy; (3) described a remarkable series of catches which engage to maintain the system in its loaded state and to allow the epipleural and trochanteral muscles to relax; and finally, they (4) described the relaxation of the levator muscles which have the effect of disengaging the catches thereby releasing energy stored in the resilin. These actions are similar to those which describe the workings of a common a mouse trap. The hammer is the analog to the epipleural and trochanteral muscles and the action of pulling it back stores the compressive energy of potential in the spring (the analog of the resilin pad). The analog to the holding bar is the tendon of the trochanter and the catch of the trap finds its analog in the socket and coxa-abdominal catches of the flea. Energy release in the trap is achieved by perturbation of the catch while energy release in the flea is achieved by the relaxation of skeletal musculature. And finally, of course, the transformation of energies of potential into energies of movement both snap a trap and launch a flea. The video below documents several more launches and the accompanying narration discusses the recent resolution of a long-standing difference of opinion concerning its particulars.
I mentioned in a previous post that I derive great pleasure from learning and from figuring stuff out. Composing this post has been fun, I wonder how many readers have found it of interest? Perhaps I’ll write about animal gears next?