Saturday, July 24, 2010

Caterpillar Motion and Memory

Tobacco Hornworm
The tobacco hawkmoth caterpillar, also known as a tobacco hornworm (Manduca sexta) is commonly studied by scientists.  It is large, easily dissected, has a short life cycle and its nervous system is easily accessible for study.  It also can be fed simply on wheat germ.  Good news for science although not always so hot for the caterpillar.

Caterpillars on the Move
 The current Science Daily has some remarkable findings about the hornworm's locomotion.  They discovered that the caterpillar uses its gut independently to move.
"The Tufts-led team reported that the gut of the crawling tobacco hawkmoth caterpillar (Manduca sexta) moves forward independently of and in advance of the surrounding body wall and legs, rather than moving along with them.
Anyone who has ever observed caterpillars knows that they crawl from back to front in waves. But advanced imaging reveals a surprising picture of what goes on inside.
The researchers used synchronized x-ray and visible light microscopy and videos to study the relative timing of movements of the crawling caterpillar's gut body wall and prolegs (unjointed leg-like structures on the mid-body that grip.)
They found that the gut -- essentially a tube suspended at the rear and head of the caterpillar and decoupled from the body wall -- moved nearly a full step in advance of the surrounding structures. In contrast, gut movement was "in step" with motion of the head and rear.
Furthermore, points within the gut moved at different rates, suggesting that the gut was effectively shortening and lengthening during each crawl cycle."
You could say that this gives a whole new meaning to the term "bowel movement".

Moths Remember Early Caterpillar Lessons
Back in 2008,  Science Daily reported a study on the same species. Scientists showed that a caterpillar can learn lessons that its moth will remember after completing metamorphosis.
"The Georgetown researchers found that tobacco hornworm caterpillars could be trained to avoid particular odors delivered in association with a mild shock. When adult moths emerged from the pupae of trained caterpillars, they also avoided the odors, showing that they retained their larval memory. The Georgetown University study is the first to demonstrate conclusively that associative memory can survive metamorphosis in Lepidoptera.
The brain and nervous system of caterpillars is dramatically reorganized during the pupal stage and it has not been clear whether memory could survive such drastic changes.
The findings of the Georgetown researchers suggest the retention of memory is dependent on the maturity of the developing caterpillar's brains. Caterpillars younger than three weeks of age learned to avoid an odor, but could not recall the information as adults, whereas older caterpillars, conditioned in the final larval stage before pupation, learned to avoid the odor and recalled the information as adults.
The results have both ecological and evolutionary implications, as retention of memory through metamorphosis could allow a female butterfly or other insect to lay her eggs on the type of host plant that she herself had fed on as a larva, a behavior that could shape habitat selection and eventually lead to development of a new species."
Tomato Hornworm
Manduca sexta is closely related but separate from the familiar tomato hornworm as described in the  Missouri Extension website.  The tobacco hornworm has a red hook on its tail while the tomato hornworm's hook is black.  Both species feed on tobacco (good!) and tomato (not so good.)
Wasp Larva on Caterpillar
There is some natural biological help available for the tomato hornworm. A brachonid wasp  lays its eggs on the caterpillar.  The larvae hatch and live inside the caterpillar, killing their host naturally.  If you find these infected worms or cocoons in your garden, leave them there.  These infected caterpillars will create the adult wasps to attack the next generation of caterpillars.   This means sacrificing some of your tomatoes.  One strategy is to plant a few tomatoes away from your crop plants and moving the caterpillars to those sacrificial plants.
Sphinx Moth- tomato hornworm
A common question we get at the Bill Roston Butterfly House is "What does the sphinx moth of the tomato hornworm look like?"  Well here it is.  More on the tomato hornworm is on Wikipedia.

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