Illustration: Brian Bookwalter

From the massive Blue whale to the tiniest plant viroids, size extremes have long fascinated mankind. This is not a trivial pursuit, for size can yield important insights into the physical constraints that govern an organism's evolution, as well as the particular mechanisms that impose a limit at either end of the scale.

Some size limits apply broadly to entire classes such as mammals, while others apply more narrowly to a single species because of its particular ecological niche. Species limitations tend to be imposed by the selected mode of feeding or locomotion, with insects being the most widely studied group in this respect. Recent research is answering various longstanding questions about animal size, such as why insects cannot be too small, and perhaps why mammals cannot get any larger than the Blue whale.

BEATING THE MIDDLE MAN One group whose size is confined within tight boundaries...

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