Wormholes in old books preserve a history of insects

By Ed Yong | November 20, 2012 7:15 pm

Absence can speak volumes. The lack of sediment in a flat piece of ground—a track—can testify to the footstep of a dinosaur that once walked on it. The lack of minerals in a solid shell—a hole—can reveal the presence of parasite that was once trapped in it. The world’s museums are full of such “trace fossils”, but so are many of the world’s art galleries.

The image above is taken from a woodcut currently residing in Amsterdam’s Rijksmuseum. It was made by etching a pattern into a block of wood, so that the remaining raised edges could be dipped in ink and used to print an image. These woodcuts were the main way of illustrating European books between the 15th and 19th centuries, and were used for at least 7 million different titles.

But as you can see, the print is littered with tiny white holes. These are called wormholes, and inaccurately so—they’re actually the work of beetles. The adults laid their eggs in crevices within the trunks of trees. The grubs slowly bored their way through the wood, eventually transformed into adults, and burrowed their way out of their shelters. The artists who transformed the tree trunks into printing blocks also inherited the exit-holes of the adult beetles, which left small circles of empty whiteness when pressed onto pages.

The beetles only emerged a year or so after the blocks were carved. The holes they left must have been frustrating, but remaking them would have been expensive. So the blocks were kept and reused despite their defects, unless the beetles had really gone to town. The holes they left behind preserve a record of wood-boring beetles, across four centuries of European literature. These holes are trace fossils. They’re evidence of beetle behaviour that’s been printed into old pages, just as dinosaur tracks were printed into the earth.

Now, Blair Hedges from Pennsylvania State University has used these fossils to study the history of the beetles that made them.

Hedges is an evolutionary biologist, and he has studied the evolution of life right from its very origins through to the rise of back-boned animals. But he also has a fondness for more recent history. He loves Renaissance art and history, and his main website is adorned with pictures of woodcuts. (He has another site with scanned images of 1,100 historical maps from the Caribbean.)

Hedges chose not to study actual woodblocks, even though many are well-preserved. That’s because even now, these can still attract new beetles that leave fresh wormholes in the wood. There’s no way of telling how old a particular hole is. Instead, Hedges looked at prints in actual books. Since the books bear their date and place of publication, Hedges could trace the provenance of each hole to a specific year and country.

He studied more than 3,200 such holes, made between 1462 and 1899. Those from northern Europe, including England, Germany, the Netherlands, and Scandinavia, were round, and just 1.4 millimetres wide on average. Those from southern Europe, including Spain, Portugal, Italy and most of France, were larger, with average diameters of 2.3 millimetres. The southern holes also included many long tracks—these were made when the beetles, rather than burrowing straight out, exited from the wood in diagnonal paths that followed the grain.

These two holes were made by separate species of beetle, and through a process of elimination, Hedges identified the culprits– the common furniture beetle (Anobium punctatum) in northern Europe, and the Mediterranean furniture beetle (Oligomerus ptilinoides) in southern Europe. Only these species make holes of the appropriate size, attack the same types of wood used to make woodcuts, and were found in the right continent.

Until Hedges came along, we had no idea about how either species was spread historically. But the woodcuts revealed that they were separated by an invisible line, which kept the common furniture beetle to the north and the Mediterranean one to the south. For four centuries, where one beetle was found, the other was not. While the common furniture beetle is more sensitive to higher temperatures and low humidity, it seems likely that it was kept out of the south by competition from its Mediterranean cousin.

But this separation is no more. Both beetles are now found throughout most of Europe. As trade and commerce across the continent increased, the beetles expanded—not just in woodblocks, but also furniture, housing materials, and more. The stable indoor environments of buildings would have sheltered them form the climatic predilections, and allowed them to gain footholds in new areas.

Hedges thinks that other woodcuts can tell us about the histories of beetles from other parts of the world too. “There are other regions around the world that have a long history of woodcut use, including in Japan and China, and in the Americas,” he says. “Benjamin Franklin had his own printing house in Philadelphia and made some famous woodcut prints, such as Join or Die. Different species of wood-boring insects have made their mark on those woodcuts as well, and discovering their history in the same way is now possible.”

Reference: Hedges. 2012. Wormholes record species history in space and time. Biology Letters http://dx.doi.org/10.1098/rsbl.2012.0926

Images: print from Rijksmuseum, Amsterdam; others from Hedges, 2012.

More on trace fossils:


Comments (6)

  1. Matt Green

    “The adults laid their eggs in crevices within the trunks of trees.”

    They could easily have laid them in the wood post-harvest and those species do, often. As do Ptilinus pectinicornis and other anobiids of about the same size.

    “The southern holes also included many long tracks—these were made when the beetles, rather than burrowing straight out, exited from the wood in diagnonal paths that followed the grain.”

    That’s one explanation, but it depends on how the blocks are stored. If the blocks are stored on top of each other the larvae would tunnel along the edge of the blocks at the interface giving non-circular exit holes.

    Is ‘g’ Anobium punctatum and ‘h’ Oligomerus ptilinoides? I assume so. Anobium tunnels along the grain too- in make no sense at all for wood borers of any species to spend more energy eating low nutrition late wood (high proportion of cell wall, low proportion of cell contents) than early wood with the opposite composition. On pupation some species with tunnel straight out but equally many tunnel along the grain of the wood- because it’s softer and easier to chew through and they expend less energy.

    Original is behind a pay wall- I haven’t read it. It’s a nice story, but there some very big conclusive leaps. Which is not to say the conclusion is wrong either but it does smack of fitting evidence to a hypothesis.

  2. Georg

    It was made by etching a pattern into a block of wood

    do You know which chemicals were used for that etching process?

  3. Druhim

    Woodcuts are and were typically etched by hand with a chisel or similar tool. Not via chemicals.

  4. In reply to some questions raised by Matt Green:

    If you can’t locate a PDF copy of my article, please email me and I’ll be happy to provide it. The author of the news story was unable to mention all of the details of the study for lack of space. Some points that you make are addressed in the original article. For example, the eggs were certainly laid in the wooden objects, not only post-harvest but post-production of the carved woodblock. In the published supplement I go into detail about criteria used to identify the two species. Besides hole size and shape, they included the type of wood, moisture content, and wood pore size (related to ovipositor size of beetle species).

    Yes, I did consider that tracks might be made by beetles exiting stacked blocks, but that would not explain why the tracks essentially don’t exist in the northern species but are characteristic of the southern species. Also, you say that Anobium follows the grain, but the Anobium expert Hickin (1972) page 70 says it “meanders” (i.e., does not follow the grain). Your explanation for following the grain is logical and I suspect that Anobium probably does some of that too. But after examining thousands of wormholes I can say clearly that the southern species makes tracks and the northern one doesn’t (except very rarely). So they are doing something differently. For my purpose that was sufficient because it gave me better discrimination of the northern and southern species, whatever mechanism was responsible for the tracks. It would be a nice experiment for someone to do in the future: to see if Oligomerus ptilinoides larvae actually exit the wood by mistake, or if the tracks are made by adults.

    Thank you for your comments!

  5. Matt Green

    I’ve tracked down a pdf of the original article. Nicely disguised.

    You don’t really consider the identity of the insects responsible in your paper, sorry. You state they are ‘most probably responsible’ and give a single reference. I would have liked to have seen a little more exposition as to how you had ruled out Nicobium castaneum or the various ‘shot hole’ borers (that really do bore directly straight out of timber and are known for attacking fruit trees). If you were writing about risks to the woodblocks changing over time it would be an interesting aside, but postulating a historic geographical species distribution based on one dated text is an over-extrapolation to me. I don’t think you have proved your case.

    In terms of block stacking- I was suggesting a difference in storage techniques of the wooden blocks might be responsible for differences in tunnelling pattern. I don’t know have these blocks were stored- but I’d be interested in finding out if there were differences in ambient temperature and if there was a difference in the care given to the blocks.

    Hickin’s Insect Factor in Wood Decay is a great reference text. Fig 255, p315 in my copy, pretty clearly shows Anobium preferentially tunnelling along the early wood, rather than meandering across the grain. I think what you have found is genuinely interesting, I just fear the conclusion you have drawn from your data to too much a leap to be presented as fact. Like this:

    “I can say clearly that the southern species makes tracks and the northern one doesn’t”

    No, you can only say clearly that blocks from southern regions have more ‘tracked’ exit holes. You propose that it’s due to species distribution but don’t back it up.

    Anyway. Thanks for responding, it’s an interesting bit of research. I think this should probably go the email from here because I can bore for England if I get started on wood-borers, and will if provoked: em green zero two at gee male dot com Cheers.

  6. The published supplement to the article (I think you missed it; check again in the same place) discusses “Species Identification” and notes, for example, that “Nicobium castaneum has similar habits (preferences) to Oligomerus ptilinoides, although it prefers moist and moldy wood and its attacks on woodwork have been considered minor and unimportant compared with its attacks on paper and libraries [7]”. Ref. 7 is Español F. 1969 Anobiides nusibles en Espangne au bois ouvré. Mem Soc Entomol Ital 48, 25-29.
    Other wood-borers are also considered in that discussion. If you look at the consistency (low variance) in mean hole size among northern and among southern cities (Fig. 2), it infers two species are involved, whatever their identities.
    About stacking: If there were a difference in how blocks were stacked, it would need to be coincidentally correlated with exit hole size, which would be unlikely. An association of exit hole size and tracks suggests that the southern thing is s separate species with different habits (however the tracks were made).
    Please do get a copy of the published Supplement with those details of species identification. I did consider them! But many thanks for you comments.


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