What Are Insects?

Classifying and Identifying Insects

Praying mantis standing on a patio door sill
Thomas J Peterson/ Photographer's Choice RF/ Getty Images

Insects are the largest group in the animal kingdom. Scientists estimate there are over 1 million insect species on the planet, living in every conceivable environment from volcanoes to glaciers.

Insects help us by pollinating our food crops, decomposing organic matter, providing researchers with clues to a cancer cure, and even solving crimes. They can also harm us, such as by spreading diseases and damaging plants and structures.

Whether you are trying to figure out what’s eating your squash, or just enjoy things that crawl, hop, and fly, learning about the insects in our lives is a worthwhile pursuit.

How Are Insects Classified?

Insects are arthropods. All animals in the phylum Arthropoda have exoskeletons, segmented bodies, and at least three pairs of legs. Other classes that belong to the phylum Arthropoda include: Arachnida (spiders), Diplopoda (millipedes) and Chilopoda (centipedes).

The class Insecta encompasses all of the insects on the earth. It is most often divided into 29 orders. These 29 orders use the physical characteristics of the insects to group similar insect families. Some insect taxonomists organize the insects differently, using evolutionary links instead of physical traits. For the purpose of identifying an insect, it makes more sense to use the system of 29 orders, since you can see the physical similarities and differences between insects you observe.

Here is an example of how an insect, the monarch butterfly, is classified:

The genus and species names are always italicized, and used together to give the scientific name of the individual species.

An insect species may occur in many regions, and may have different common names in other languages and cultures. The scientific name is a standard name that is used by entomologists around the world. This system of using two names (genus and species) is called binomial nomenclature.

Basic Insect Anatomy

As you may remember from elementary school, the most basic definition of an insect is an organism with three pairs of legs and three body regions – head, thorax, and abdomen. Entomologists, scientists who study insects, might also add that insects have a pair of antennae and external mouthparts. As you learn more about insects, you will find there are some exceptions to these rules.

The Head Region

The head region is at the front of the insect’s body, and contains the mouthparts, antennae, and eyes.

Insects have mouthparts designed to help them feed on different things. Some insects drink nectar, and have mouthparts modified into a tube called a proboscis to suck up liquid. Other insects have chewing mouthparts and eat leaves or other plant matter. Some insects bite or pinch, and others pierce and suck blood or plant fluids.

The pair of antennae may have obvious segments, or look like a feather.

They come in different forms and are a clue to identifying the insect. Antennae are used to perceive sounds, vibrations, and other environmental factors.

Insects can have two types of eyes – compound or simple. Compound eyes are usually large with many lenses, giving the insect a complex image of its surroundings. A simple eye contains just a single lens. Some insects have both kinds of eyes.

The Thorax Region:

The thorax, or middle region of an insect’s body, includes the wings and legs. All six legs are attached to the thorax. The thorax also contains the muscles that control movement.

All insect legs have five parts. Legs can be different shapes, and have different adaptations to help the insect move in its unique habitat. Grasshoppers have legs designed for jumping, while honey bees have legs with special baskets to hold pollen as the bee moves from flower to flower.

Wings also come in different shapes and sizes, and are another important clue to help you identify an insect. Butterflies and moths have wings made of overlapping scales, often in brilliant colors. Some insect wings appear transparent, with just a web of veins to identify their shape. When at rest, insects like beetles and praying mantids keep their wings folded flat against their bodies. Other insects hold their wings vertically, like butterflies and damselflies.

The Abdomen Region:

The abdomen is the final region in the insect body, and contains the insect’s vital organs. Insects have digestive organs, including a stomach and intestines, to absorb nutrients from their food and separate waste matter. The sexual organs of the insect are also in the abdomen. Glands that secrete pheremones for marking the insect’s trail or attracting a mate are in this region as well.

The next time you observe a lady beetle or a moth in your yard, stop and take a closer look. See if you can distinguish the head, thorax, and abdomen. Look at the shape of the antennae, and watch how the insect holds its wings. These clues will help you identify a mystery insect, and provide information about how the insect lives, feeds, and moves.