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?


12 thoughts on “The flea’s knees

  1. What an interesting post! This was great! It amuses me that scientists and became so curiously involved in fleas and how they jump. And seeing that video was great.

    Loved this! Never seen a blog like this out there. Cheers 🙂

    • Hi there WFFME. I wanted to send a note of special thanks your way this morning for the kind reply you posted in response to the recent piece on fleas. I have been working on this Pairodox blog for nearly two years and have found it to be a mix blessing with regard to reward and frustration. I have always enjoyed the marriage of words and pretty pictures. But I have always hoped that blogging would be more than that. I had hoped that my blog would make, for a few at least, real connections that were of interest. When I read your comment I was pleased to see that my post had made that sort of connection with you. The smile which spread across my face was genuine and a delight … and I wanted to thank you for taking the time to write those few words of support. They provide me with encouragement. Please come on back some time and scroll through older Pairodox posts … perhaps you’ll find some other topics which appeal. Thanks again. D

  2. So, they use their toes? It appears that they use the entire bottom portion of their hind legs, but then I don’t know how the leg is broken down, where the toes begin, the knees, shins, etc. end. It is fun to consider. I am a flea-hater but I have to respect their adaptations. Animal gears sounds fascinating!

    • The leg is divided into six elements, from the connection to the body, Coxa (nothing similar in humans), Trochanter (knee, I suppose), Femur (like the human femur), Tibia (like the human tibia), Tarsi (there are five of these that telescope together), and finally the little toes. The tarsi and toes have spikes which allow for traction during the jump. So, the push is from these last elements … but the power stroke comes from the resilin pad higher up in the body … and this is transmitted through the other leg elements to down to the tarsi and toes. D

  3. Scale would certainly come into play if that bit of of bioengineering is to be applicable beyond the world of fleas. I recall learning a long time ago about how scales work. If you ‘double in size’ (become twice as tall) you actually increase your mass by a factor of eight since mass (which is proportional to volume) varies with the cube of the scale factor. Your muscle and bone strength, though, only increase by a factor of four since strength depends on the cross-sectional area of the bone or muscle, and, so varies with the square of the scale factor. That means you will be progressively proportionally weaker, and very quickly, as you scale up (and it explains why elephant legs are so blooming massive). It would still be nice to jump that high … except for the landing, of course! Now the gears – that’s something completely new!

    • You have cut through at least two lectures worth of material on the influence of allometry on animal form and function. Sheesh! And (no big surprise) you got all the details correct. There’s also a correlary to all of this and that is the largest animals that have ever existed have been herbivores because there is only enough biomass at the trophic base of ecosystems to support these largest of organisms. So … I always tell my students that Fay Wray didn’t have to really worry much about King Kong … he would have preferred to hit the salad bar rather than consumer her! D PS: This gives me an idea for a post!

  4. Can you imagine if humans were capable of a proportionate jump? Based on the info you present here, that would be about 2 football fields for the average man. It would certainly make day-to-day travel easier, though we’d have to invent some kind of protection for our brains from the g forces!

  5. This is very impressive! I wonder if engineers have ever tried to exploit this – by building macroscopic machines based on related materials? Or did I see too many Transformers movies?

    • It’s an excellent idea! I am willing to bet that many of the economically successful new ideas in materials science will take the form of science imitating nature. That and graphene/carbyne.

  6. More science posts! I love your scientific digressions, I think animal gears should be the next topic 🙂

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