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Cicada Scrapbook, May 2004
In Bloomington we are just over a week into the 2004 emergence of "Brood X"--by all accounts the world's largest brood of periodical cicadas. Brood X covers much of the North Eastern US. It is a 17-year brood containing 3 species of Magicicada cicadas ("Magic Cicadas"). Their last emergence was in 1987. The cicada nymphs have spent 17 years feeding on tree roots. The adults will live a scant few weeks, breed, and die.Emergence
The ground beneath many trees that supported large nymph populations is marked with these holes, left by the emerging nymphs.
This is a live nymph--fresh out of the ground. In images below, most of the nymph-shaped objects on trees are just molted skin (exuvium). The cicadas themselves are long gone.
The adult is just starting to emerge from this nymph.
In the following image the form of the adult is becoming more obvious. When first "born", the adult is white.
A ghostly new adult, surrounded by brood mates.
Within a short time the wings will inflate, and the cicada will acquire its final coloring.
Abundance
The cicadas in some of these images remind me of giant aphids. And that's pretty much what they are--they belong to the same insect family.
Adults
The yellow underside of this large adult's abdomen marks it as Magicicada septendecim. Males, like this one, have elongated, hollow abdomens.
Magicicada cassini is smaller and has a black abdomen. This species now seems to be the most abundant (though I had the impression there were more septendecim last week).
When I photographed the individual in the two images below, I was searching for the rarer third species--Magicicada septendecula. But now I think this is more likely a female septendecim.
I eventually found the individual below, which I think really is septendecula, in Brown County State Park. It has narrow yellow bands on the abdomen, and the orange or brown patches behind the eyes--visible in the septendecim examples above--are absent.
Prime Periods?
Besides the 17-year broods, there are also 13-year broods of periodical cicadas. The 13-year broods also contain multiple species, including close or exact analogues of the 17-year species.Why are these periods prime numbers? Many entomologists think it is a coincidence, but there is an active fringe industry inventing explanations. One idea involves predation by species with "resonant" life cycles---i.e. species that reach maturity after p years, where p is a factor of the cicada period P. The populations of such species would receive a boost every P/p generations, when they reach maturity in cicada years. In some models this raises their populations to a level where they damage the cicadas, driving the evolution towards prime cycles (which don't allow such "resonances"). But this explanation is controversial, and may not be correct.