OCTOBER
1999

 

 

 

By
Tom Turpin
 
Professor of
Entomology
Purdue University

 

 

 

 

 

10-14-99

Insects Use Clocks to Tell Time

Most animals, including insects, use clocks to time daily and seasonal activities. These clocks are not timepieces like those produced by Timex or Rolex; animal clocks are internal biological clocks.

How biological clocks keep time is not clearly understood. But like early human clocks, they are based on fluctuations in the environment. Sundials captured the daily rotation of the Earth on its axis and calendars the Earth's rotation around the sun.

These rhythms dictate the activity patterns of most living things on Earth. Only animals that live in caves or near the ocean floor do not respond to such daily and seasonal environmental fluctuations. Humans have overcome the need for behavioral changes with artificial light and housing.

However, we still maintain our biological clocks. That is why we wake up before the alarm goes off in the morning and fall asleep in front of the TV at our normal bedtime. It is also the reason why on trips we wake up in the middle of the night local time—a biological time that corresponds to the normal wake-up time where we live.

Human biological clocks can be somewhat useful. They will ring in our heads in time for us to get to work, even if the mechanical alarm doesn't sound. But to other animals, accurate biological clocks are a matter of life and death!

Such is the case for insects. Cold-blooded organisms have to be able to predict the onset of winter or they perish. For many insects, it is a matter of one lifecycle per summer. These insects emerge from the overwintering stage, which varies among species, during the growing season. They feed, mate, lay eggs and generally do what is necessary to get back to the wintering form. For insects that complete several lifecycles during the summer the process is a bit more difficult. These insects have to predict the onset of fall so that individuals are not killed by cold weather before they develop to a stage that can survive the winter. 

Insects, like plants, use the amount of sunlight as an indication of approaching winter. Known in science as photoperiodism, the insect responds to decreasing hours of sunlight as a reliable predictor of the approach of fall.

Thus shorter days will cause yellow jacket and bumble bee nests to stop producing workers and produce queens that will mate and overwinter. Some aphids will stop giving birth to live young and will produce eggs. The eggs will then survive the winter and start the population over again the next season.

That famous migrator of the insect world, the monarch butterfly uses the number of hours of daylight to begin the southward migration each fall. The adults that emerge forgo mating and head south to spend the winter in the mountains of Mexico. Unlike some warm-blooded birds, insects and insect-eating birds can't hang around during unseasonably warm falls. If they do, a killing freeze will be a disaster.

So insects still use a sundial to tell time. Their life depends on it!

 

Writer: Tom Turpin
Editor: Olivia Maddox