Thursday, February 2, 2012

Mystery solved by new book

We were exploring some new (to us) trails in Scatter Creek WMA in Cross County. The dirt road went through alternating woodland and grassy flats. We began noticing that the branches of young saplings we were walking past were split on the underside and partially ripped open. At first we wondered if it might be damage from browsing deer, but when we looked more closely we could see the regularity of the scarring and our speculation went to some sort of orthopter. Could a katydid do something like that? We thought that some kinds might use their ovipositors to make slits in bark to deposit their eggs. They would have to be large (the branch in the picture is perhaps an inch and a half in diameter), but some katydids are very large. But the more we looked, the more widespread the damage was. In fact it was on almost every small branch we saw over a large area. It would have to be an awful lot of katydids. It would have to be a plague of katydids. We began dropping that theory.

But what could it be? It was a mystery of nature. It was evidently a sign left by something done by a large insect on an incredibly large scale. We both realized at the same moment that the answer was waiting for us in a book we had just purchased, and the moment we got home we took the book out. It would be its first test.

The book is Tracks & Sign of Insects and Other Invertebrates, by Charley Eiseman and Noah Charney, a field guide to all the tiny footprints, burrow openings, carved hieroglyphics under the bark of dead trees and other residua left behind by insects who are no longer there. We looked in the index and found the entry "oviposition scars: cicadas" and there was the answer, the largeish superabundant insects we were looking for. We had completely forgotten the great event of last summer, the biggest, most widespread emergence we had ever seen of Thirteen-Year Periodic Cicadas. Early last summer we couldn't visit woods anywhere in the state that didn't have their persistent calling in the background. We all know the story, the broad blunt larvae with their powerful digging forelegs burrowing under the trees sucking on the juices in their roots for thirteen years before rising to the surface at a synchronized moment in all their millions to shed into adulthood for their brief moment in the sun, spending it as we hope we would, singing and mating. We have heard that they have no fright or evasion reflex, whatever wants to can walk up and gorge on them. There are so many of them all at once that their predators are quickly sated and there are still millions left to insert their eggs into slits on the underside of branches and twigs so hatching larvae can drop to the ground and dig down to the roots.

If you are an inquiring sort of person you might wonder how these insects and their Seventeen-Year cousins got started on this rather bizarre life plan. The best theory I have heard is that originally they were one-year cicadas like the other species but they were beset by a parasite which fed on their tissues. When the cicada larva crawled with the last of its strength up to the surface to try to become an adult, the parasite timed things to also be ready to emerge as an adult. Well, there is always a bit of variability, and a few cicada larvae didn't come up after the first year, but stayed underground for a second year, and when the parasite matured after one year, it was stuck underground and perished. But of course there was variability in the parasite too, and some of them waited until the second year to transform into adults and so survived. But then some cicada larvae waited until the third year, and so on, until some populations reached thirteen years, and some seventeen years, before the poor parasite gave up. This theory can't be proved, naturally, since the losing parasite no longer exists.

1 comment:

  1. The period is also always a prime number, right? So 13 is better than 14 because you can avoid a parasite with a 7 year period and an alternate year period strategy.