Some 800 years ago, a rice field in China was the site of a brutal murder: a worker had been stabbed by the curved blade of a sickle. The murderer was discovered when blow flies swarmed around his apparently clean tool, attracted to invisible residues of blood and guts on the blade. This is the first documented case of "forensic entomology," the use of insects and arachnids to solve crimes.

Forensic entomology is thriving today. One of its most useful critter-sleuths are maggots, the larvae of flies and other insects. Fly maggots tend to flock to the freshest flesh, and eat the head and vital organs first. They're followed by the maggots of beetles, then those of mites (which feast on dry skin) and moths (which prefer hair). For decades, forensic scientists have relied on which of these critters are currently feasting on a corpse to help estimate the time of death. In the past few years, advancing technology has allowed for more precision. For example, researchers can measure the expression of genes in insect eggs in a corpse to pinpoint exactly how old they are.