Backyard Biology

Nature stories from my backyard and beyond

Nature Stories

The Dreams of a Moth

Sunday, October 3, 2010

I wonder what is running through her mind.  She has had plenty of time to think, buried deep in her hole underground while the winter storms rage overhead. To look at her, you would think her quite idle, since she has not moved in months.  So what else does she have to do but think?  Tobacco Hornworm Pupa

In fact, she is very busy.  Her time is spent breaking down many of her body parts and growing new ones.  Out with the old and in with the new.  Where before she walked, soon she will fly.  Past meals were chewed. Future meals will be sipped.  Her existence was once defined by tastes and smells. Soon her newly grown eyes will reveal a whole new world to her, a world of images and colors.  Even her name will change from Tobacco Hornworm to Carolina Sphinx Moth.  One of the few things she will carry with her into her new life is the name given her by scientists – Manduca sexta.


The question remains.  What is she thinking?  Is she remembering her first life?  Two things - avoiding predators and eating, defined her existence.  Avoiding predators so she could eat.  Eating as much as possible and as fast as possible so she could make her way to the safety of her underground chamber before the predators found her.  Tobacco Hornworm CaterpillarTobacco hornworms have many predators.  As one of our largest, juiciest caterpillars, hornworms make a great snack for a hungry bird or mammal.  They also provide a convenient home for parasites - insects that lay their eggs inside the body of the caterpillar.  These eggs will hatch and the larva will feed off the fluids of the caterpillar, dooming her to a slow lingering death.  She must also avoid me – probably her least efficient predator. 

Perhaps she is not worrying about predators, ignorantly bliss to the dangers surrounding her, instead reveling in the warmth of the sun and the gustatory delights of her tomato plant.  My tomato plant.  She certainly seems to be enjoying her meal for she eats constantly, growing larger and larger, requiring ever more food until very little is left of my tomatoes.


Perhaps she is not remembering the past but dreaming of the future, the new life to come.  If she is dreaming of the future, she must have a fantastic imagination.  She will have to dream of an existence totally unlike anything she has ever experienced.  With her poor caterpillar eyesight, she has probably never seen anything ever fly.  Yet, soon after she emerges from the ground, she will have to figure out what her new wings are for and how to use them.  When hungry, she will no longer be temped by the bitter taste of the leaves of my tomato plants but will be irresistibly drawn to the sugary sweet smell of flower nectar.  How will she know to uncoil her long straw-like tongue and daintily sip the strange liquid?  When a male Carolina Sphinx Moth approaches with mating on his mind, she must decide that it is safe to let this stranger approach when most everything else that approaches is dangerous.  And once they have mated, she must seek out just the right kind of leaves to lay her eggs on.  If her eggs are laid on the wrong leaves, her offspring will starve. 

All of these things she must be able to do instantly, with no guidance from parents.  She has no opportunity to observe others and follow their example.  If she gets any of this wrong, chances are she won’t survive.  Seems a lot to ask of a poor caterpillar turned moth.  When asked to explain how insects can know these things, for it is an experience many insects go through, scientists say it is instinct.  That, of course, just means that they don’t know how insects do it.  So perhaps they do remember the past and dream of the future.  But until scientists can devise experiments to test if they have memories and dreams, no one can say for sure.


In an experiment conducted in 2007, Tobacco Hornworms were made to walk through a glass tube.  They came to a Y in the tube and could go either left or right.  Wafting down one of the arms of the Y was a stream of air.  The air coming down the other arm of the Y was scented with ethylene acetate, a fruity smelling chemical.  The caterpillars showed no preference for either tube; half the time choosing the normal air and half the time the sweet smelling air. 

In the next part of the experiment, every time the caterpillars chose the fruity air, they received a small electrical shock.  As you might imagine, they soon learned to prefer the normal air, avoiding the tube scented with the ethyl acetate.

In the third part of the experiment, the scientists let these trained caterpillars pupate into moths and repeated the experiment.  If the caterpillars were trained early in their caterpillar life, then as adults they showed no inclination to avoid the sweetened air.  Clearly, they did not remember the training from their caterpillar days.  However, if they were trained late in their caterpillar days, then they were able to remember their training and did avoid the sweetened air as adults.  They can remember something from their past.  This memory would be very important to a moth that must lay its eggs on the same kind of plant it fed on as a caterpillar.


A memory certainly is useful for many things in a moth’s life, but what about the rest of it, those things still explained only by instinct?  Now if scientists could just come up with a way to test for the dreams of a caterpillar…