Insect Scaling

Insects range in size. Bees live towards the middle of this range. The smallest insects are the Ptiliidae or feather-winged beetles. Less than a millimeter in length, these beetles can crawl through the eye of a needle. Smaller yet are the Mymaridae, a family of small parasitic wasps. A male Dicopomorpha echmepterygis lacks wings, is blind and is about one hundred and forty microns long. The largest insects now are fossils. A Phasmid, or walking stick, from the Carboniferous, 375 million years ago, was about nine inches long, and a dragonfly from the Permian, 285 million years ago, possessed a thirty-inch wingspan. In comparison, a modern day African Goliath beetle weighs around one hundred grams. The internal organs of even these diverse insects, however, are quite similar across scales.

Scaling is basic, but we often do not think enough about what scaling means. Our mathematical formulas adjust for scaling effects, but insects and micro-machines live bizarre lives. Peeling a charged surface off another charged surface is sticky but easy when the charged surface is a sweater off a lover, but electrostatic attraction holds our miniaturized parts during production of a MEMS so firmly together that peeling is not usually an option. And stick-ing's not the whole of it. Flimsy micro-parts puncture surface tension with great difficulty. Think of a water strider on a pond. Or consider rice grain-sized wheels that generate too little friction to bear much of a load. Small objects have large surface areas relative to their masses, so the smallest insects having the thinnest cuticles may have difficulty retaining their internal moisture, and so they must live under constant threat of desiccation. A similar relationship exists for heat loss and heat gain. The list is long.

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