(left) Rijksmuseum, Amsterdam; (right insets) S. Blair Hedges

That'll leave a mark. A Renaissance woodcut print titled The Rich Man (left) shows printed wormholes, made from wood-boring beetle larvae (right) that tunnel through woodblocks and then emerge as adults, leaving circular flaws.

Holey Art Reveals Beetle Boundaries

A biologist with a passion for art history has melded his interests to tackle an unusual question: Which species of beetle created wormholes, or circular flaws, in carved woodblocks that were used for centuries to create paper prints of maps and illustrations? The answer has enabled him to draw an unusual bead on the distribution of beetles in Europe before the 20th century.

If a woodblock contained a wormhole, its print, in turn, reflected that hole as a circular open space in the stamped relief. Wormholes appear in thousands upon thousands of prints in Europe, marring otherwise meticulously illustrated books. "I've always been interested in history and art, and have noticed those wormholes for years," says Blair Hedges, an evolutionary biologist at Pennsylvania State University, University Park. "I thought that, as a biologist, I should figure out what species are making them."

Hedges set out to analyze woodblock prints by traveling to museums and archives around Europe and, in some cases, obtaining images of the prints. From the 15th century—when printing became commonplace in Europe—to the early 19th century, at least 7 million different book titles were printed with hand-operated presses. Hedges soon found that he had more than enough material to work with, accruing nearly 500 woodcut prints spanning 5 centuries.

Voracious beetles were the bane of artists and printers of the time. The insects favored the apple, pear, and boxwood used for woodcarvings. Adult beetles laid eggs on the wood’s surface, and their larvae would burrow into its crevices. The larvae spent up to 4 years munching on the wood's cellulose, until they morphed into adults. The adults then tunneled out of the block, leaving pockmarked exit holes as they emerged into the light and took flight.

Those holes, it turns out, are distinct between beetle species. Hedges measured the shape and width of more than 3000 wormholes that appeared in paper prints made from woodblocks between 1462 and 1899. Through statistical analysis, he found that the holes' sizes and shapes fell into two distinct categories, hinting that two different species were responsible for the damage. Hedges then compared the holes' sizes and shapes and the type of wood that the unknown species preferred to similar data for today's beetles, as he reports online today in Biology Letters. By process of elimination, he identified two likely culprits: the common furniture beetle and the Mediterranean furniture beetle. The former left smaller, rounder exit holes, whereas the latter tended to leave larger, longer marks.

Hedges was able to say more, however. That's because woodblock prints almost always contain a printing date—sometimes down to the day—and can be tied to a location. Using that information, Hedges found that until 1899, an invisible line through Central Europe split these two species’ ranges cleanly down the middle, bisecting France, with the common furniture beetle occurring only in the north and the Mediterranean furniture beetle occurring only in the south. (Woodblocks that traveled around Europe and thus collected holes from both species are the exception.) Today, the two species' ranges overlap.

Hedges hypothesizes that competition between species likely enforced the dividing line for hundreds of years. Then, about 100 years ago, people started moving around the world more frequently, bringing their beetles with them.

"I'm surprised to see that there was a dividing line between north and south, but due to different adaptations of the two insects it sounds reasonable," says Rüdiger Plarre, a biologist specializing in wood-destroying insects at the Federal Institute for Materials Research and Testing in Berlin who was not involved in the study.

Art historians could make use of the method for narrowing down the origins of anonymous prints from the wormholes in them. "For me, this paper shows that there’s still evidence to be gleaned that we overlook as art historians because we don't know it’s there," says Richard Field, the former curator of prints, drawings, and photographs at the Yale University Art Gallery who was not involved in the study but supplied Hedges with some prints and guidance. "No art historian has ever really looked at the kinds of things Blair has looked at."

Hedges hopes that other researchers will pick up on his work and begin their own studies. "It's a fascinating topic, especially if you like art," he says. "In fact, it's almost distracting because you need to measure the holes and get the data points, but then you want to look at these beautiful prints every time you flip the page."

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