Naturespeak A naturalist's view of the world

August 23, 2007

Life Lines

Filed under: Uncategorized — wykes @ 4:42 pm

 

  Human beings are one of the few earthlings that are capable of recording their whole life stories with the written line.  The world would be a poorer place indeed without the autobiographies of folks such as U.S. Grant, but even the truly great tend to leave out the bad parts.  There are some among us that do reveal their entire legacy complete with all the blemishes and the scars.  These honest types admittedly don’t have much to tell, but then again neither do all the neophyte presidential candidates either. Our singular writers live out most of their life in-between the layers of a single leaf so are quite small – minuscule would be the word- and very easily overlooked.  I’m talking about tiny moths called Leaf Miners and their life story is a scar.

    You may have come across their writings before, but passed them over as some sort of undecipherable foreign script.  Leaf miners spend their entire larval life feeding on leaf tissue. The course of their development is recorded as a wiggly permanent scar on the home leaf (take a look here and  here.)   That life may only be a month long, so the story is simple. It seems more complicated because several leaf miners are writing at once and their life lines intersect. Try to pick out one of the continuous lines and mentally separate it from the others (think of this as an exercise in concentration).

   You’ll note that the wandering lines are graduated from a very thin trail to a wider road that ends abruptly. These trails are the feeding tunnels excavated by the growing moth caterpillar.  We see them because the larvae eat all the green inner portion of the leaf and leave the transparent top and bottom layers intact.  A leaf is an energy factory composed of a glass ceiling and floor.  All the machinery is sandwiched in the middle.  As the miner moth young eat the machinery, only the transparent outer layers remain. 

  The very thin lines indicate the early stages when the mini moth larva is just starting out.  As the larva grows, so does the diameter of the tunnel that it excavates. This part should be very understandable since the diameter of our pant openings increase as we grow as well. 

  A closer examination of a miner tunnel reveals a line of debris down the middle. These are the droppings.  Essentially, what goes in at one end is deposited out the other.  This is one of the reasons that leaf miners don’t double back on themselves – that would be like us jumping into the latrine (let’s leave that one behind shall we).

  I took the time to measure one the life line trails on the larger leaf shown in the second picture.  This leaf is from an Evening Primrose, by the way.  From birth to the final act, this particular miner journeyed eight inches.  He started down the main rib and looped back along the outer edge.  I suppose it could have been a she, but either way the trail ends at the glass ceiling. Once satisfied that it is going to get as fat as it will ever be (a feat taking only a week or two), our larvae ate out a chamber and disrobed. Underneath that last layer of caterpillar skin was a pupae and the third stage in a four stage life.

  Within the pupae, the leaf miner larva is undergoes some re-wiring to become a winged adult. It converts chewing mouthparts into sucking ones and sprouts wings in place of walkers. Take a look here to see the location of the pupae at the end of the tunnel and here to see two of the tiny pupae themselves. After a few weeks, the adult moth will emerge and punch its head through the glass ceiling.  It will seek out other miner moths and the whole process will begin anew. There can be several generations per summer, with the last one staying with the leaf as it falls and overwinters on the ground. 

  I wish I could show you a portrait of the actual adult moth responsible for this life story, but here the story becomes hard to follow. There is no one moth called the “Leaf Miner Moth” – there are dozens, probably hundreds.  Finding the identity of any single one is difficult.  Because they are so small (members of a group called Micro Moths), no one pays much attention to them. There are very few human produced lines written about them. Actually, they are quite a delicate and jewel-like bunch. Here’s a nice portrait of a typical leaf miner moth.  They seem to stand tall as adults and characteristically prop themselves up on their front legs as if to say “I matter.”

  The minor miner does matter.  Time spent reading their life story is time well spent.

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