Wormhole Record: Centuries-Old European Woodblock Prints Show Distribution Of Wood-Boring Beetle

Old Wormholes Help Biologist Create Amazing New 'Map'

By Katherine Harmon

Wormholes aren’t just for time travel or teleportation anymore. Some very real and ancient wormholes are now helping to trace the distribution of insect species and artwork.

A biologist found himself in the unlikely world of centuries-old European woodblock print art. There, he discovered that many of the small imperfections in the prints could be identified and traced back to specific species of bugs that had burrowed through the surface of the original woodblock before the print was made. By matching the hole dimensions to the time and locations in which these prints were made, the scientist, Blair Hedges, a professor of biology at The Pennsylvania State University, has been able to paint a historic record of wood-boring beetle distribution across Europe—patterns that had been previously unknown.

Hedges has nicknamed these telltale traces the “wormhole record,” faint trails of these centuries-old animals—all told in hundreds of elegant prints.

Diagram showing beetle larva and exiting adult courtesy of Blair Hedges

Adult beetles lay eggs in the crannies of a piece of wood. Once the larvae hatch, they slowly descend into the wood, spending three or four years living there and feasting on the wood’s cellulose. After these wormlike larvae transform into adult beetles—through a pupal stage—they then burrow out of the wood, creating the noticeable holes that have so textured so many woodblock prints. These holes can also be found in furniture, oak floors and rafters, in addition to the woodblocks. Art historians have used the signs of wormholes in prints and books to place these products in order (if more traces of wormholes appear in one print of the same image than in another, the excess suggests that the original block was infested with the beetles after the print showing fewer holes was made and thus that that the version with more holes is). But they had otherwise considered the holes to be little more than blemishes in the print medium.

For a biological purpose, however, “these tiny errors or interruptions in the print serve as ‘trace fossils,’” Hedges said in a prepared statement. “They aren’t the animals themselves, but they are evidence of the animal’s existence. They show that beetles invaded a particular piece of wood, even if that wood no longer exists.”

The prints actually offer a more precise record of these invasions than the pieces of wood themselves. A beetle can lay eggs on a piece of wood at any point in time, whereas the marks from the woodblock on the paper print offer an indelible clue that the wood was infected before a particular print was made. “Because most prints, including those in books, have publication dates, we know that the wormholes in question were made very close to that date,” Hedges said. “It’s an almost perfect biological timestamp. And in most cases, we also know where the book was printed. So wormholes can tell us when and where a species existed with fairly good accuracy, more than 500 years ago, and that is amazing.”

Detail of a print marked by evidence of wormholes; courtesy of Rijksmuseum, Amsterdam

Hedges studied 3,263 wormholes visible in 473 different prints made between 1462 and 1899. He found that there were two distinct sizes of holes: some were 2.3 millimeters across and others were closer to 1.4 millimeters wide. And there was a distinct pattern of these hole sizes across the European continent; all of the smaller holes were found on prints made in the northeast, and the larger holes came from the southwest.

He was then able to deduce the species of each beetle. “The size of the beetle closely matches the size of the hold made, and most species have preferences for the wood they eat,” Hedges said. “This left two species as the probable hole-makers”: the common furniture beetle (Anobium punctatum) in the northeast and the Mediterranean furniture beetle (Oligomerus ptilinoides) in the southwest. Other types of wood-boring insects don’t share the same preference for dry, smooth-grained woods (such as apple, pear and box) that were used for woodblocks—instead targeting rotting, damp woods or those that are either extremely soft or extremely hard.

The line between these two beetles seemed to be surprisingly steady throughout the study period. “This is surprising because it means that the two species’ ranges were in close contact but, oddly, did not overlap along a precise dividing line,” Hedges said. Local competition and climate differences might have kept these two species apart for centuries, if not millennia.

Map of historic beetle species distribution courtesy of Blair Hedges

The discovery of historical separation is new. “Today and for the past 100 years, because travel, shipping and furniture transport tends to spread insects around, we find both species all over northern and southern Europe and elsewhere in the world,” Hedges said. Indoor controlled climates might have also helped the beetles colonize new ranges.

Hedge’s print-based method could help examine woodborer species distribution and historical ranges throughout the world, indicating changes in local populations and arrival times of invasive species. Traces of worm DNA might also still linger in some of the historic woodblocks, making it possible to support the wormhole species analyses.

The wormhole technique might also help solve some questions in art history as well. “There are some situations in which a book or print’s origin is unknown,” Hedges said. “Now that we know that different species of beetles existed in different locations in Europe, art historians can determine whether a book was from northern or southern Europe simply by measuring the wormholes.”

The findings were described online November 20 in Biology Letters.

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