Chasing Bookworms: What Missing Art Can Tell You About Insects

"The Rich Man" by Cornelis Anthonisz (1541), courtesy Rijksmuseum, Amsterdam.

Between the 15th and 19th centuries, Europeans illustrated their books mainly with woodcuts. A woodcutter would etch a block of wood with an image so that when the block was dipped in ink and then stamped on a page, the areas that were cut away would leave just the white paper, and the remaining raised parts would pick up the ink and create black lines. (Here’s Albrecht Dürer’s Samson Rending the Lion as woodblock and ink-on-paper).

Those carved-out parts of the blocks and the white spaces on the paper were just as important to the art as the untouched wood and lines of ink. Empty spaces can say a lot. That’s why Blair Hedges, an evolutionary biologist at Pennsylvania State University, is so interested in certain holes that appear in many of these old books.

Bugging Out

These aren’t holes in the plots, but the artwork. Called wormholes, they’re actually the handiwork of beetles which came from eggs laid in trees and then emerged from the wood as adults, sometimes after the trees were turned into lumber—and sometimes even after a piece of wood had been carved with an image for printing. Hiring an illustrator to remake blocks affected by the bugs was expensive, so printers often went ahead and used them anyway, and many woodcut illustrations in older books are pockmarked with small circles that interrupt the ink lines. You can see some in the image above.

To biologists, those circles are trace fossils. Like a tooth mark or a footprint, they provide evidence that an animal was in a given place at one time. In this case, they pinpoint where a beetle once burst forth into the world. Hedges has used wormhole fossils from old books, maps, and art prints to study the distribution of certain wood-boring beetles over the hundreds of years when woodcuts were at the height of their use.

For a recently published study, he examined some 3000 wormholes in woodcut illustrations made between 1462 and 1899. He found that the wormholes in illustrations printed in northern Europe were round and, on average, 1.4 millimeters across. The wormholes from southern Europe were about twice as large, averaging 2.3 mm across. Many southern holes were also pill-shaped, or had “tracks” instead of being a a circle, shaped by the beetle exiting its nursery in a diagonal path instead of digging straight up and out (shown below).

Woodcut (1606) by Giovanni Battista Ramusio, courtesy Library of Congress

Going by the size and shape of the holes and what’s known about beetles’ wood preferences (some, for example, only lay their eggs in damp, rotting wood, which is not something that would be used in printing), Hedges was able to pin the holes in the illustrations on two species. He thinks the common furniture beetle (Anobium punctatum) is the likely culprit for the northern European works, and the Mediterranean furniture beetle (Oligomerus ptilinoides) for the southern ones.

Drawing the Line

The woodcut holes suggest a clear geographic divide between the beetles. Through hundreds of years of European literature and art, the two species’ ranges appear to have butted up against one another, but never overlapped.

This stark division is shocking because, today, both beetles are widely distributed through western, central, and southern Europe. There’s a lot of overlap in their ranges, and no one knew until now how their distribution was in the past, or if or how it had changed.

By looking at where and when the books were printed, Hedges was able to plot the historical dividing line between the two beetles (shown in the map below with each species’ current European range). Characteristics of its shape—like the curve south as it approaches France’s humid west coast—and the northern beetle’s sensitivity to certain environmental factors—like a combination of low humidity and high temperature—suggested to Hedges that the boundary between the two species was partly a matter of climate. As the climate changed over the centuries, though, the border might have held because both beetles prefer the same kind of wood, and they were avoiding competition with each other for it.

Broadening their Horizons

Top: historic range of two wood-boring beetles. Bottom L: modern range of the common furniture beetle. Bottom R: modern range of the Mediterranean furniture beetle. Hedges, 2012

The beetles expanded their range in the late 19th and early 20th centuries, which means that people are one reason for the fall of the dividing line, Hedges says. The beetles' expansion came during a time when increasing global trade, travel, and commerce moved infested wood around Europe and to other continents, and modern homes with carefully controlled climates might have allowed the bugs to acclimate to new areas and eventually colonize them.

And all that comes from some blank spaces in old drawings.

While the books told Hedges a lot about the beetles, he says that the beetles can teach us something about books. In situations where a book’s point of origin is unclear, he says, historians could now use the known historical range of these two beetles to determine whether a book was from northern or southern Europe, just by examining and measuring what the insects left behind.