MAY
2013

 

By
Tom Turpin
 
Professor of
Entomology
Purdue University

 

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Check out these books by Tom Turpin:

Flies in the face of fashion

What's Buggin You Now?

 

 

 

05-23-13

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Caterpillar Charades


The life of a caterpillar is fraught with danger. These babies of the insects - called butterflies and moths - can drown in a rainstorm, be knocked off plants on windy days or die of dessication during droughts. Caterpillars are also a common food item for some insect-eating animals. Lots of birds eat caterpillars. Even some species of predatory or parasitic insects use caterpillars for food.

Sometimes caterpillars are their own worst enemies when it comes to survival. Caterpillars are voracious eaters. A group of caterpillars can literally eat themselves out of house and home, which results in starvation. Just another reason the caterpillar that becomes a moth or a butterfly is a real survivor - most don't make it that far!

The odds for an individual caterpillar to survive to adulthood are a long shot at best. But maybe not as improbable as purchasing the winning ticket for the Powerball lottery, being struck by lightning or getting hit by a meteorite!

Scientists who study insect populations estimate that the number of insects that survive from hatching to reproduction is between 1-2 percent. Human population numbers are just the opposite in terms of survival, because about 98 percent of us survive to reproductive age. The insects that do survive to adulthood make up for population loss by producing a lot of eggs. In the insect world, success is a numbers game - many are hatched so that a few survive!

Even though the cards seem to be stacked against caterpillar survival, these insects employ a few biological tricks to help overcome the odds. For instance, most have extra pairs of legs to help hold onto the plant leaves or stems on which they feed. These extra legs are called prolegs. The last pair of prolegs on the rear end of the caterpillar is called claspers because they can be used to clasp a stem.

Caterpillars are often colored so that they blend into the environment in which they feed. Such camouflage reduces the ability of predators to find a caterpillar meal. While Kermit the Frog lamented that it's not easy to be green, the color is a good thing for caterpillars chowing down on green plants.

Some caterpillars take the opposite approach and try to be seen. These insects are bright colors in reds and yellows and might even have stripes. Such caterpillars usually taste bad because of chemicals from their host plants; they want to advertise this fact to discourage bird predation.

hornworm
Hornworm caterpillar

A few caterpillars are adorned with protrusions that make them look intimidating. Some of the giant silkworm moths such as the Prometheus fall into this category. So does the tomato hornworm, the notorious devourer of tomato plants. It is called the hornworm because of a horn-like projection located on its last body segment. Some people wonder if the horn is a stinger. It's not, but it looks as if it might be!

Those hornworm caterpillars also exhibit a behavior designed to frighten away an animal that accidentally discovers one. The caterpillar strikes a pose: It holds onto tomato stems or foliage with its prolegs and claspers and raises its head and real legs into the air. This action once reminded people of an Egyptian sphinx. That is the reason that adults of such caterpillars are sometimes called sphinx moths.

bird-dropping caterpillar
Bird-dropping caterpillar
(photo credit, T. Gilliam)

Some caterpillars possess markings that look like eyes. When two such large "eyes" are positioned on the front of a caterpillar, it creates the illusion of a snake. Caterpillars of certain swallowtail butterflies pull off another interesting charade. They resemble bird droppings and are appropriately called bird-dropping caterpillars.

So what is a surefire way to avoid becoming a meal for a bird? For some caterpillars, the answer is to just look like something that has already been a meal for a bird. That caterpillar charade speaks volumes about survival!

 

 

Writer: Tom Turpin
Editor: Olivia Maddox