Those of us who have been called—and consider ourselves—“bookworms” rather like the tactile feel of a printed book. But another type of bookworm prefers the taste of a good book—and, it turns out, these “worms” provide a detailed record of certain species of insects. The “bookworm”—of the entomological, not intellectual, variety—is not actually a worm at all, but is rather the larvae of any of a variety of beetles. Actual “bookworms” are pretty rare; it’s not easy for beetles to get inside books, lay eggs, and have the larvae emerge within the pages. Some larvae, if they happen to be near books, have been known to bore through them, but it’s the exception rather than the rule (and we’ve all read a boring book, but book boring is actually quite rare). However, what some beetle larvae do enjoy are woodcuts. Back in the early days of printing, illustrations were reproduced using carved blocks of wood, and the type of wood favored for woodcut illustrations also happened to be favored by certain types of European beetles, who leave their larvae in the wood which then grow into proper beetles and gnaw their way out. This happened particularly to woodcuts left in storage. So when they were retrieved later and reprinted, they left telltale white blank spaces on the paper where the beetle holes marred the image. So looking at books from certain periods, researchers can examine the number and pattern of these white dots, plus where the books were printed, and get good information on beetle distribution. Says LiveScience:

By surveying medieval tomes in library collections and in online high-resolution digital archives, Hedges was able to measure the white spots. In 473 prints dating from 1462 to 1899, he found thousands of spots, including 3,263 perfectly round holes created when beetles exited the wood block and 318 meandering "tracks" created as beetles chewed their way along the wood grain. This kind of left-behind evidence of living organisms is called trace fossils.

By measuring the holes, and noting other distinctive markings found in books printed with affected woodcuts, biologists could identify which species were distributed in which areas. Try that with a Kindle!

These measurements reveal that in the north, the woodcut-chewer was the common furniture beetle (Anobium punctatum). In the south, the Mediterranean furniture beetle (Oligomerus ptilinoides) was the culprit. Surprisingly, the two never met. They stayed on either side of a line that cut across France, hugged the border between Switzerland and Germany and then followed the boundary between Italy and Austria.

The research appears in the November 20 issue of the journal Biology Letters.