CHAPTER 34 - Travelers and Stay-at-homes
As I'm starting this column on Saturday, May 18, I may be wrong, but I think I just saw my first of the 17-year cicadas that will soon be flooding our part of New Jersey. If so, this one had a very short life above ground, thanks to an alert squirrel that pounced upon it and seemed to relish its crunchy texture. I'm hoping that we have a plethora of squirrels and birds that also love to wolf down those tasty treats. Having experienced at least three of these 17-year cicada emergences, I don't look forward to the carpets of these critters covering our lawn, walks and driveway. And the males' incessant "singing" to attract mates can reach 120 decibels, according to a local newscaster I heard this morning.
Actually, I had planned to start this column with mention of another insect with a vastly different lifestyle. Whereas our cicadas and their offspring for generations to come will spend their entire lives in this region of New Jersey, monarch butterflies are travelers extraordinaire. Our monarchs are famous for their well known migrations of over 2000 miles from the Eastern regions of the U.S. and the southeastern parts of Canada down to Central Mexico and the return back. In the Editors' Choice section of the March 15 issue of Science, there was a brief item about a paper by Patrick Guerra and Steven Reppert of the University of Massachusetts Medical School in the February 21 issue of the journal Current Biology. What caught my attention was a statement in the Science article that a single butterfly might fly over 2500 miles to Mexico, enter a hibernation-like state known as diapause, and then head back up north, reproducing and then dying. The offspring then continue the trip back up north. I was under the impression that the round trip migration involved numbers of such reproductive cycles and that no single butterfly ever made anything approaching a 2500 mile segment of the trip.
I was wrong. Visits to such founts of information as Wikipedia and Monarch-Butterfly.com revealed to me that the life cycle of the monarch is much more complex and fascinating than I had thought. The number 4 seems to be the magic number when it comes to monarchs. Their life cycle involves four stages - first, the female butterfly lays a bunch of eggs, typically on a milkweed plant. Let's follow one egg. Within a few days the egg hatches and a caterpillar (or larva) emerges. The caterpillar eats the milkweed as nourishment to grow and, a couple of weeks later, attaches itself to a leaf or twig and starts spinning itself into a silken cocoon known as a chrysalis. For the next ten days, it would seem to the casual observer that nothing is happening. However the fourth stage is evident when, out of the chrysalis bursts a beautiful monarch butterfly. Let's call her Mona.
All this is impressive enough, but here we are more concerned with the history of Mona's offspring and succeeding generations. Let's assume that Mona came from an egg that was laid in March or April by a monarch butterfly or her way up north from Mexico. Her first generation baby butterfly, Mona, will also fly away in a northerly direction but will only live for perhaps 2 to 6 weeks. Before dying, Mona mates and lays eggs that will hatch in May or June, yielding a second generation of monarchs. This second generation of butterflies, Mona's children, will also live relatively short lives and, before dying, spawn a third generation born in July or August. As you might guess, this generation, Mona's grandchildren, will die after mating and producing a fourth generation of monarchs, born in September or October. By this time, this fourth generation is up here in the northeastern U.S. or even in Canada.
Here's where things get interesting. This fourth generation, Mona's great grandchildren, are the lucky ones. They enter a nonreproductive state, diapause, and, unable to withstand the cold weather up north, begin their trip down south all the way to Mexico. Those hardy 4th generation individuals will indeed fly some 2500 miles if they elude predators and other life-threatening hazards. Once in Mexico Mona's great grandchildren will gather with other monarchs in the same trees that Mona's mother occupied. While in Mexico they live a very subdued existence, more or less just sitting around in those trees their ancestors did. It is truly amazing that these insects, unlike birds, which learn from their parents where to go for the winter, arrive in the same areas of Mexico year after year.
In Mexico, the butterflies must experience a period of cold temperatures to prod them to begin to start flying north and emerge from that nonreproductive state to start the cycle all over again. How do the monarchs find their way north and south to their destinations? Especially down to Mexico and the specific trees? Ask Steven Reppert, whose work with Guerra spawned this column. Reppert is a most unusual fellow, to say the least. He's an MD who was a pediatric intern and resident at Mass General in Boston, a professor at the Harvard Medical School, Chair of the Department of Neurobiology at U Mass Medical School and has a host of honors and awards. With this background, who would expect that he would be perhaps the world's authority on a type of butterfly? Reppert and his colleagues have shown that the butterfly employs a "sun compass" and a circadian clock combination to find their way in their migrations. I won't attempt to describe how this works but if you're interested you can just Google Reppert and monarch and you'll be led to various of his Web sites that deal with these subjects.
Well, as I get ready to post this column, our Brood II 17-year cicadas have truly arrived en masse and the males are loudly singing their songs to attract mates. A large number of them have emerged just outside our breakfast room windows and we have gotten close-up views of the nymphs coming out of the ground and climbing up our rhododendrons. The cicadas climb out of their nymph attire, leaving behind the skeletal-like nymph skins. (You can see a cicada emerging from its skin on cicadamania.com.)
After our choristers find willing females and mate, the females will lay their eggs, numbering in the hundreds, in twigs. Some weeks later, the eggs will hatch into critters resembling ants or termites. These baby cicadas will feed for a while on the juices in the twigs and then drop to the ground, where they will dig their way underground to feed on stuff in the roots of trees. They may end up as deep as three feet underground, nibbling away at the roots for the next 17 years before poking their heads above ground in 2030! At 85, I'm pretty confident that I will not be around to witness that emergence. But I wish them well and wonder what kind of a world it will be in 2030.
When I look at a cicada, with its tiny red eyes, I can't help wondering what it sees and whether it has any appreciation for the contrast between its 17 years underground and its few weeks experiencing sunlight, rainstorms and the ability to fly. Similarly, does that fourth generation monarch butterfly appreciate its good fortune in living much longer than its parents and being able to travel thousands of miles over our scenic landscapes. Hey, aren't we lucky to have been born as humans?
Next column, hopefully, will be posted on or about July 1.
Allen F. Bortrum