DO AMAZING THINGS
by Whit Gibbons
August 7, 2005
cease to marvel at how complicated the ecology of some, perhaps most,
animals really is. Insects in particular still have many secrets to reveal,
in part because there are so many species yet to be discovered, and also
because their small size makes certain observations difficult. As more
and more caterpillars appear over the course of the summer and into fall,
the wide variety of shapes, colors, and ornamentation reinforces the idea
that caterpillars may be highly complex in ecology and behavior. The following
discovery supports this conviction. It also highlights some of the amazing
discoveries modern scientists are capable of making.
or caterpillars, of a particular species of moth that lives in southern
Arizona had never been described before. Then, a scientist discovered
that they live on oak trees and have two life cycles during a year. Adults
first become active in late winter and produce eggs that hatch into larvae
in spring. Adults are not seen again until summer, when they lay eggs
and larvae begin hatching soon after. So far, this may not sound particularly
unusual, certainly not that different from a lot of other insects. But
what the caterpillars do at the two different times of year is the intriguing
hatch in the spring feed on the male flowers of the oak trees. These yellow
oak flowers, or catkins, are familiar to anyone who has taken a careful
look at an oak tree in the spring. Not only do the moth caterpillars eat
the catkins, they look almost exactly like them. Thus, when a caterpillar
is having a meal, it looks like the food it is eating. Obviously, this
is great camouflage for a caterpillar that, if it were easily seen, might
otherwise become a nice morsel for a bird.
species of moth has developed a fantastic system of providing its young
with food while at the same time protecting them from predators. But wait.
What about the larvae that hatch out in the summer, a time when all the
catkins are gone? To look like a catkin on an oak tree that has only leaves
and twigs would be like inviting a bird to dinner. Well, sure enough the
moth has a different strategy for that time of year. The caterpillars
eat leaves instead of catkins, and they quickly assume the shape, texture,
and brown color of a tiny oak twig.
from the two different seasons do not vary genetically, so an obvious
ecological question for the investigator to pursue was, how does the caterpillar
know to look like a catkin or like a twig? The investigator did experiments
by raising caterpillars to find out. He captured female moths that were
ready to lay eggs in April and in July. He also collected and froze catkins
and leaves of oak trees. Then, when eggs hatched at the two different
times of year, he was able to feed half the larvae one diet and half the
As it turned
out, the season a caterpillar was born was not the important factor. Instead
its future appearance was determined by what it ate. Thus, caterpillars
that were fed catkins began to look like catkins, even if it were summer,
when catkins would not normally be present. Those fed leaves, even at
a time when leaves would not be present, began to look like twigs.
experiments and chemical testing suggested that caterpillars were responding
to particular compounds, called tannins, that are present in the oak leaves.
Therefore, a larva eating leaves containing tannins assumes the shape
of a twig. If the tannins are absent from the diet, as in the catkins,
the larvae develop into the catkin type of caterpillar.
As we continue
to find out about such intricate schemes of ecology and evolution, we
begin to realize how much we do not know. How many wonderful discoveries
are out there waiting to be revealed? With greater understanding of ecosystems
and all the interconnecting parts, perhaps we will learn to better appreciate
our natural environments.
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