Window on Nature
By Lowell and Kaye Christie, F47246
February 2003
Here’s something sure to drive you buggy “” according to entomologists, there are more species of insects on Earth than any other kind of living creature. As a matter of fact, 95 percent of all known animal species are insects, and although scientists have discovered more than a million species of bugs, 10 times as many may still remain unidentified.
Here’s another fact sure to get under your skin: millions of these creeping critters can exist in a single acre of land. Our house is situated on an acre-sized lot. That means the two of us, plus one dog and one cat, are surrounded by millions of bugs.
Within the insect family are 32 groups, or orders. Of them, the largest and most complex order is that of the beetle (Coleoptera), which has approximately 500,000 species, give or take a few. In fact, it’s estimated that one out of every four animal species on earth is a beetle.
Insects are incredibly adaptable, which is probably why they’ve been able to successfully inhabit every continent. They’ve evolved to live in nearly all of the Earth’s environments, from sun-seared deserts to the frozen Antarctic. The only place where insects are not commonly found is in the oceans (lakes and rivers, yes, but not oceans).
Scientists cite three main reasons for the success of insects: they are very small; most of them can fly; and they have protective shells, or exoskeletons. Their small size and the ability to fly helps them escape from hungry predators. Being so small also allows them to live (and hide) in cracks and crannies, inaccessible to most predators. Plus, insects guarantee the survival of their species by reproducing in large numbers in little time.
A major difference between insects and other creatures is that, rather than having an inner bone structure to give their bodies shape, insects have a lightweight but strong exterior skeleton called an exoskeleton. Not only does the exoskeleton cover and protect the inner muscles and organs, but it also safeguards the insect from weather and its natural enemies. The exoskeleton has sense organs that allow the insect to react to light, pressure, sound, temperature, wind, and scents. Such sense organs can be located almost anywhere on the insect’s body, not just on its head.
The inside of an insect’s body has an open circulatory system. The exoskeleton has a few openings to enable air to enter the body, and an extensive system of breathing tubes. Through these openings oxygen passively spreads to all areas of the insect’s body. Insects have a digestive system and a heart. The rest of the body is filled with fluids that flow around inside the exoskeleton.
Now that your brain is filled with all sorts of insect information, let’s take a look at the cicada, an intriguing insect from the order Homoptera that combines a long life cycle with the ability to make almost deafening calls.
Cicadas are widely distributed throughout the United States and Canada. They can be found nearly everywhere in the contiguous U.S., except for the Northwest. Most cicada species are “annual,” (although they have multiple-year life cycles), because in a given location adults are found every year. But it’s the “periodical” cicadas, genus Magicicada, that get most of the attention. In North America, their life cycle is either 13 or 17 years, with all but the last year spent underground. For reasons entomologists don’t understand, one group spends 13 years in the larval stage before it emerges; the other stays in the stage 17 years. Then, in the spring of the designated year, the cicadas’ internal “clock” tells them it’s time to emerge. It takes them a few weeks to tunnel to the surface and then, as though by a signal, the larvae of a particular cycle emerge from the ground. By this time the cicadas will have grown to as much as three inches long.
Once aboveground, the larvae climb a tree, molt into adult form, and breed. The females lay eggs, and then both sexes die. All this occurs in just a few weeks. Count yourself lucky if you’re around for the show, because you won’t see these bugs perform again for a number of years.
Male cicadas are credited with being the loudest creatures in the insect world. With all the other males around, each is in fierce competition to attract a female. So, male cicadas are well-designed for the task of making noise. Taking advantage of the vibrations from a pair of amplifying cavities in the abdomen, the male cicadas can chirp, buzz, and sing so loud that they can be heard from a half-mile away.
Adult cicadas live a little more than a month aboveground, during which time the males begin calling from late afternoon until well after dark. It’s said that the noise from a large group of cicadas can drown out the loudest lawnmower, and nothing from our experience makes us doubt it.
We’ve found that it takes a couple of nights to get used to the noise of cicadas before we get much sleep, but before long we can rest and enjoy the show, knowing that it will be many years before we hear it again.
Female cicadas emerge and molt at the same time as their male counterparts, and are naturally attracted to the vocalizing males. Once the female decides which male to mate with, they breed, and soon she lays some 400 to 600 eggs in tiny caches in the trees. After the cicada eggs hatch, the ant-sized nymphs drop from the trees and burrow from 1 to 18 inches underground. They will then spend the next 13 (or 17) years munching on the juicy roots of a variety of plants. Silence reigns once again.
During those boisterous days, it’s impossible for the lay bug-watcher to know whether a particular group of cicadas is on an annual, a 13-year, or a 17-year cycle “” but it doesn’t matter. To the untrained eye all cicadas look, sound, and behave alike. You can, however, get a general idea of whether or not you are seeing and hearing periodical cicadas in your location. If you are west of the Mississippi River, you’ll find only annual cicadas. Don’t worry, though; they make every bit as much noise as the periodicals. In the northeastern states and Canada, you will find the 17-year cicadas, while the 13-year varieties typically live in the Midwest and South.
Next month we’ll delve into the lives of a few more insects guaranteed to grab your interest.