Spring 2018 Conservationist

Insect, Bug or "Other"?

by Andres Ortega, Natural Resources

Bug, insect, spider, creepy-crawly.  When people see something small with lots of legs crawling or flying around their homes, gardens or forest preserves, calling it by the right name isn’t always their top concern. But believe it or not it’s fairly easy to figure out who’s who.

Let’s start with what these little creatures have in common. First, they’re all invertebrates, which means they don’t have spines. By most estimates, invertebrates make up over 90 percent of all animal life on the planet. (The other 10 percent are the vertebrates — mammals, birds, fish, reptiles and amphibians.) When most people think about animals, they think large and furred or feathered and often forget about invertebrates, the majority of which are no more than a couple of inches long.

If an invertebrate has legs, a segmented body and a hard outer shell, or exoskeleton, it’s an arthropod, a word that comes from the Greek for “joint” and “foot.” Arthropods are the largest group, or phylum, of animals on the planet and have members ranging from butterflies, bees and mosquitoes to spiders, crayfish and lobsters.

Arthropods live worldwide, and some can be relatively large (Maine lobsters can grow to 44 pounds), but as a general rule they’re not able to get as big as vertebrates. Exoskeletons can’t support large, heavy bodies, and the animals can only take in enough oxygen to support smaller frames.

But back to those truly small arthropods that fly and crawl. How can we tell who’s who?

If you look purely at the numbers, there’s a good chance the critter you’re trying to ID is an insect. Ninety percent of all life may be invertebrates, but 80 percent of all life are insects. To be certain, you need to count the body parts on an adult. (Developing arthropods that are still larvae or nymphs don’t always have all the final requisite parts.)

If it has three pairs of legs, three distinct body segments and one pair of antennae it’s an insect. It may or may not have wings, although if it does, it’s definitely an insect. They’re the only arthropods that developed the ability to fly. In DuPage, familiar insects include butterflies, ants, crickets, bees, beetles, houseflies and mosquitoes. Even some of the largest insects in the state — common green darners and black saddlebags, two types of dragonflies, and luna moths — call DuPage home.

If the arthropod you’re looking at has four pairs of walking legs and (usually) two distinct body segments, it’s probably an arachnid. (Some arachnids have another pair of appendages that look like a fifth pair of legs but are used for something other than walking, such as grabbing prey.) Well-known arachnids include spiders, scorpions, ticks and mites.

Harvestmen, also known as daddy longlegs, are also arachnids, but they’re not spiders. They’re anatomically different. Most prominently, the two parts of a harvestman’s body are fused together, creating what looks like a single oval. A harvestman also has just one pair of eyes centered at the front. Spiders have three or four sets (although many people choose not to get close enough to count). Harvestmen don’t spin webs, and contrary to urban legend they’re not the most venomous animals on the planet. Not only do they not produce venom, they don’t even have fangs. Instead their mouthpart consists of a tiny pair of claws used to grasp prey.

Our last well-known group of arthropods are the multilegged myriapods, the centipedes and millipedes. “Myriapod” comes from the Greek for “10,000” and “foot,” and although one Californian millipede does have 750 legs, most have well under 100.
While myriapods in the tropics can grow over 10 inches long, most in DuPage aren’t nearly as big. One myriapod people likely know is the house centipede, which looks frightening but is generally harmless to humans. It only eats other arthropods, and although it can bite, producing something similar to a mild sting, if it comes across a human it’s more likely to scurry away on its 15 pairs of legs.

So the remaining question is, “Then what’s a ‘bug’?” Technically, a bug is a member of a specific order of insects called Hemiptera, the “true bugs.” They’re different than other insects because they have a specific type of biting or sucking mouthpart called a “stylet” that they use to pierce plants or other invertebrates and suck out the juices. Aphids, cicadas, squash bugs and stink bugs are all “true bugs.”

chart describing arthropod featuresUnfortunately, common names can’t always tell you who’s who. Lightning bugs, lady bugs and June bugs are all insects, but none of them are “true bugs.” Neither are pill bugs, which aren’t even insects but crustaceans more closely related to crabs and shrimp than to other arthropods.

So next time you’re out in a DuPage forest preserve and you see something small crawl, climb or fly, take a moment to consider what you call it. Your ability to know the answer may surprise you!


green-darner-sankax.jpgThe Eyes Have It

Insects have what scientists call “compound eyes.” Instead of one lens per eye, each eye has thousands of individual lenses called “ommatidia” that work together to generate an image. The more ommatidia, the more clearly an insect can see. Dragonflies have about 30,000. (They catch their food on the fly, so sharp vision is a must.) Houseflies have so many ommatidia they can almost see 360 degrees around, a big reason it’s difficult to sneak up on one with a swatter!  Image by sankax/CC BY-NC 2.0

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