Insects have 6 legs and 3 body parts - a head, a thorax, and an abdomen. The thorax is the major muscle center to which the legs and wings (if present) are attached. Insects, as adults, usually have wings. There are a number of exceptions. For instance, many adult aphids fail to develop wings, females of some beetles look much like the juveniles.
Spiders have 8 legs and 2 body parts - a fused head and thorax called a cephalothorax and an abdomen. (The legs are attached to the cephalothorax.) Similarly, ticks, mites, scorpions, millipedes and such have too many legs to be insects. Ticks, after they have hatched but before they have molted for the first time, do have 6 legs. After they molt, however, they have 8 legs.
Many people are familiar with adult dragonflies and damselflies. Confusion arises when they see adult insects like owlflies and antlions that physically similar. Unlike adult dragonflies and damselflies which are mainly active during the daytime (there are some exceptions), adult owlflies and antlions are mainly active at dusk, dawn or at night (again there are some exceptions).
Dragonflies and damselflies have a 3 stage development - egg, juvenile (sometimes called a nymph, sometimes called a larva) and adult. The juveniles are aquatic insects and look very different from the adults. Dragonflies and damselflies are predators throughout their lives.
Owlflies and antlions have a 4 stage development - egg, juvenile (called a larva), pupa, and adult. The juveniles are terrestrial and look very different from the adults. These insects are predators throughout their lives.
The simplest way to separate these groups is to look for the antennae. The antennae of dragonflies and damselflies are short and are not apparent unless you are up close. Owlflies look like small dragonflies with butterfly antennae. In length, antlion antennae fall in between - they are thick and stiff with knobbed ends.
| damselfly | antlion | owlfly |
|---|---|---|
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| one small thin antenna can be seen crossing the rear eye | the antennae appear thicker due to the shadow | only one antennae is in focus |
The term "bug" is often used in a general sense to refer to an of a number of "creepy crawlies", not all of which are regarded as insects. To the entomologist (a person who studies insects) the term "bug" has a special meaning.
Bugs (also True Bugs or Sucking Bugs) are insects in the order Hemiptera. Bugs have a 3 stage development - egg, juvenile (also called a nymph) and adult. The juveniles look much like adults without wings and use the same foods as the adults. Bugs have piercing, sucking mouthparts. Adults usually have 4 wings (individuals of some species never develop wings). The front wings are divided into two sections - the front portion has a leathery texture often with some pattern / color combination while the back portion is membranous and generally has a uniform color. The membranous portions of the front wings overlap when the insect is at rest. As a result, when viewed from the top, an X-pattern is formed by the wings.
Hemiptera - X pattern indicated
Beetles are insects of the order Coleoptera. Beetles have a 4 stage development - egg, juvenile (also called a larva), pupa and adult. In general, the juveniles do not look much like the adults and they may feed on different things. (The big white grubs often encountered in gardens or lawns are beetle larvae.) Beetles have chewing mouthparts. (Weevils are beetles - the mouthparts are at the end of the snout.) Beetles have 4 wings. The front wings are stiff and uniform in texture. At rest, the front wings do not overlap, instead they butt together above the middle of the abdomen. As a result, when viewed from the top, a T pattern is formed by the wings. In some species of beetles found on the ground, the front wings are fused together (the seam may be hard to see) - the rear wings no longer develop.
Coleoptera - T pattern indicated
"Ladybugs" are beetles. Since they are not true bugs, i.e. members of the order Hemiptera, the common name is written as a single word. There are true bugs commonly called stink bugs and beetles commonly called stink beetles. Sow Bugs and Pill Bugs cause some confusion in the terminology - they are crustaceans not insects.
Butterflies have thin filamentary antennae with knobs (clubs) on the ends. Butterflies are seen during the daytime, and often have brightly colored or patterned wings. At rest, butterflies generally hold their wings spread out flat (basking position) or folded together up over their backs. Some species regularly hold their wings somewhat open, and a number of small butterflies, called grass skippers, often rest with their hind wings more or less parallel to the ground while the front wings are elevated at an angle of 45 degrees or more.
Moths have antennae which are not distinctly clubbed at the ends. Moths are generally seen flying at night or in the time around dusk or dawn. Some moths, like the White-lined Sphinx and others, do fly during the daytime. The front wings of moths are often rather plain. At rest, moths generally hold their wings down flat over their abdomens or along their bodies, so that the rear wings are largely hidden. (Some moths rest with their wings spread out flat.)
Flies have only one pair of wings. Instead of a second pair of wings each fly has a pair of knobbed filaments (see the picture of the crane fly). Flies that are confused with bees usually have short antennae. Flies do not sting although, as an order, they have highly modified mouthparts. (Mosquitoes belong to this order.) At rest, many flies hold their wings out, often giving them a V-shaped appearance when seen from above.
Bees, wasps and ants have 2 pairs of wings. (We don't normally think of ants as having wings, but a winged reproductive generation is produced periodically.) The antennae of bees, wasps and ants are usually fairly long and have many segments. Bees, wasps and ants can sting and they have biting, chewing mouthparts. At rest, most species hold their wings folded over their backs. Some species are social.
Dragonflies (Order Odonata), mayflies (Order Ephemeroptera), alderflies (Order Neuroptera), caddisflies (Order Trichoptera) and some others are not flies, i.e. they do not belong to the Diptera. Hence the general names for these insects are written as single words. While you will often see "honey bee" and "bumble bee" written as one word, this practice blurs the distinction between the insect orders which has been present in common terminology. Incidentally, there is a family of flies called the bee flies because they remind one of bees.
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