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> They are too small to have muscles:
> 
> https://www.quantamagazine.org/too-small-for-big-muscles-tiny-animals-use-springs-20180613/
> 
> "But small animals face problems that muscle power alone can’t solve. When running, for example, the legs of small animals are in contact with the ground for only very brief moments during each step, which constrains how much energy each stride can release.
> 
> "To address this problem, many small animals use flexible structures in their bodies as springs that they can cock and release like an archer’s bow. The spring enables the small animal to store energy slowly and then release it all at once, thus amplifying its power.
> 
> ***
> 
> "The use of springs is not restricted to insects. When a frog crouches, its leg muscles stretch its long Achilles tendon like a spring and store energy in it. The release of that built-up tension propels the frog’s leap, said Christopher Richards, a paleo-robotics researcher at the Royal Veterinary College at the University of London, who is using a combination of robotics, modeling and anatomy to understand how extinct frogs with diverse pelvic shapes and leg proportions used to jump.
> 
> "The latch that the frog uses to release the stored power, however, remains a subject of intense debate: “That’s the million-dollar question,” Richards said. “Nobody has found an anatomical latch in frogs. To my knowledge, nobody has found a latch in a vertebrate animal.”
> 
> "The latches have been figured out for only a handful of insect and crustacean systems. Latches are harder to find than springs because the latch mechanism is usually inside the animal’s body, as opposed to the easily accessible springs made of crustaceans’ outer cuticle or insects’ exoskeleton. Unfortunately, dissection destroys the delicate spring-and-latch systems, making it difficult to determine how they work in living organisms, explained Gregory Sutton, a biomechanics researcher and engineer at the University of Bristol. Usually, researchers end up inferring the existence of a latch from the abrupt release of power from an identified spring. “Something has to switch the system from a mode where the muscles are stretching the springs to a mode where the spring is recoiling and powering, powering that huge motion,” Sutton said.
> 
> ***
> 
> "A drawback of springs is that organisms that push spring-and-latch systems to their performance limits have to worry about breakage. Patek noted that for mantis shrimp, the problem of avoiding self-destruction is severe. “They have to get the energy out of their body [and] try not to have it go back in and tear the muscles in the leg,” she said.
> 
> ***
> 
> "These animals have only one set of springs and latches to use in their limbs throughout their lives. Breaking a spring could be deadly.
> 
> "One secret to their trick of avoiding breakage appears to be that rubbery protein called resilin, which insects have in their wing hinges and tracheal tubes. Resilin is also found in the cuticular springs, where it is configured in thin layers beneath the chitin. In fact, the layering of chitin and resilin resembles the design of archers’ composite bows from hundreds of years ago....The layering of different materials prevents any tiny cracks from spreading, thus limiting damage and giving the animal a chance to repair those cracks before they become catastrophic.
> 
> ***
> 
> "Breakage becomes more of a problem as organisms get smaller. Consequently, there appears to be [b]a sweet spot in body size[/b] — neither too big nor too small — at which animals can make optimal use of latches and springs....the real limitation on fleas’ jumping abilities seems to be that their tiny springs — which are far smaller than the robust ones of mantis shrimp, for example — can tolerate only so much stress. (my bold)
> 
> “'It’s very hard to build a spring that is incredibly small without it breaking,” explained Sutton. “That’s why fleas are not actually terribly good jumpers — because their springs just aren’t big enough to handle the forces involved.”
> 
> Comment: How did chance evolution find resilin protein and the sweet spot? It had to designed.