APRIL
2005

 

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?

 

 

 

04-28-05

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Teenage Times Dominate Insect Lives


Biologists like to classify the lives of living things into time periods. Ornithologists refer to birds as juveniles or adults. In this case, the juvenile bird has not reached the age where it reproduces. Many times you can tell the juveniles from the adults because of different plumage.

Such a classification is also used for mammals. We call cattle that are pre-reproductive age calves. We also use the word calf for immature whales. Pups are the young of canines, such as dogs or wolves. Humans appropriate these terms for our own use, such as when someone is called a "young pup."

Sometimes the age of an animal is reflected in the name. Especially among farm animals like horses, cows and sheep individuals in their second year of life are called "yearlings."

Human lives are also divided into units according to age. We use all kinds of words for groups of people according to age. Toddlers, teenagers, and retirees are age-related groups. In education, we refer to programs as pre-K or K through 8. The terms preschool and grade school are older terms that encompass the same group of students. People are also grouped as high school- or college-age even though some students in those categories might well be senior citizens, which is a politically correct term for old geezers!

In mammals, including humans, the length of time spent as a juvenile is generally much shorter than time spent as an adult. Dogs and cats are juveniles for less than a year but may have a 10- to 15-year life span. The same is true of cows. The difference is even greater in horses. Whales, elephants, seals and many fish also live many more years after sexual maturity than before.

Insects, on the other hand, tend to spend more time in the immature form than the adult form. The extreme in this regard is the periodical cicada, an insect that spends 17 years as an immature feeding underground. The adult periodical cicada lives less than a month! The more common dog-day cicada, the one that we hear singing in the fall of the year, lives from 3 to 5 years as an immature. Some June beetles also spend from 3 to 5 years underground before emerging to spend 3-4 weeks as an adult.

Most insects seem to have a relatively long immature life relative to the adult stage. Or maybe it makes more sense to say they have a very short adult life compared to the time spent as an immature. There are exceptions, of course. On one extreme are some aphids that are sexually mature in a matter of days. On the other extreme are the African termite queens that may live for up to 50 years. But as a general rule, insects spend a lot more time as immatures than as adults. 

The biological reason for insects having a long immature life compared to their adult life is a matter of speculation. Many biologists think that it has to do with food. Immature insects gather food to grow and, in some instances, much of the energy needed by the adult insect is gathered by the immature. In some cases, adult insects do not feed at all or feed sparingly.

Some biologists have concluded that the function of immature insects is to eat and grow. That is why caterpillars have been accurately called eating machines. What then is the function of adult insects? The answer is reproduction. Because insects are cold-blooded, the time to mature and reproduce in temperate regions is limited. Thus, the biological survival strategy of many insects is to spend more time growing and less time reproducing. Obviously, based on the number of insects, this is a good strategy.

This might be a good approach for insects, but human parents are probably breathing a sigh of relief that humans don't use the same strategy. I'm not sure how many parents could stand more than 7 years raising a teenage child. Maybe that is why insects generally let their kids grow up on their own!

 

 

Writer: Tom Turpin
Editor: Olivia Maddox