The world is full of all sorts of insects. You can find them everywhere you go, whether swimming in a pond, weeding your garden, playing on the playground, or enjoying a picnic lunch. It is said that there are 200,000,000 insects for every person on earth. Scientists have identified more than a million different kinds of insects. And they think there are millions more species still waiting to be discovered.
Insects belong to a group called arthropods. Unlike you and me, with our bony internal skeletons, an arthropod’s body is covered with a tough outer shell, or exoskeleton. This exoskeleton is divided into segments that allow the critter to move. In fact, the word arthropod means “jointed legs.” Arthropods include not only insects, but also centipedes, crustaceans, and arachnids like spiders, ticks, and mites.
So what makes an insect an insect? Despite their great diversity, they all share certain common features. They all have three main body parts: head, thorax and abdomen, plus six jointed legs, and antennae. Most adult insects have two pairs of wings.
Many sensory organs are located on an insect’s head. Depending on the insect, antennae can detect motion, “hear” sounds, determine orientation, recognize changes in humidity, or identify odors, tastes, and a variety of other chemical clues. Insects have two kinds of eyes. They typically have one to three simple eyes, each consisting of a single light-sensing unit that can detect changes in light intensity. Their two compound eyes can have anywhere from 4,000-30,000 facets. These facets provide them with a complex image of their surroundings. Many are able to detect the slightest movement, helping them escape from predators or catch their own lunch. Speaking of lunch, mouthparts vary greatly depending on an insect’s diet. There are two basic types of jaws – those designed for biting and chewing and those that can only sip or mop up liquid food. The latter can range from a mosquito’s piercing/sucking beak, reminiscent of a hypodermic needle, to a butterfly’s coiled drinking straw or even a fly’s absorbent sponge.
An insect’s three pairs of legs are attached to its thorax. Each leg has five joints, allowing for a wide range of mobility. The five leg segments are variously shaped and modified depending on their primary function and form of locomotion. They can range from thickened forelegs for digging or grasping prey, to legs fringed with hairs for swimming, to elongated hind legs for jumping. Legs can even function as sense organs: hearing, smelling, and even tasting. Attached to the thorax are usually two pairs of wings, making insects the only invertebrates that can fly. A closer look at wings reveals their complex venation pattern. The veins bring not only the blood supply to the wings but offer structural support and are an important characteristic used to classify insects into groups. And just as there are many kinds of insects, there are many different wings, ranging from clear and flexible to scaly and colorful. They may hook together or swivel individually in flight, be hardened or leathery for protection, act as thermal collectors, give the insect balance and stability, or even be rubbed together to produce sounds!
The last body segment, the abdomen, contains the main organs and systems in the insect’s body. Insects have an open circulatory system, and blood flows freely throughout the body cavities, pumped forward and back by a simple heart. Their respiratory systems consist of spiracles or pairs of holes along each section of the abdomen and thorax. Gas exchange is achieved as air from outside flows into internal tubes and moves throughout the body by passive diffusion and muscle movement. They have a complete digestive system, which processes and digests food, absorbs nutrients, and excretes waste. Reproductive structures are also located here, the most visible being ovipositors, the egg-laying devices in some females.
While all insects share these same body parts, scientists sort them into groups based on certain characteristic features. You will be surprised at how many insect groups you already know. First, meet the beetles. There are more kinds of beetles than any other animal in the world. They have tough, solid bodies covered by hard overwings called elytra. These meet in a straight line down the center of their backs and protect the delicate, clear, flexible wings underneath. While not used in flight, elytra can be raised, providing additional lift, while the flexible underwings help beetles take flight.
Butterflies and moths are easy to recognize by their large and often colorful or patterned wings. Modified hairs or scales cover their wings. Butterflies and moths can be distinguished from one another. Butterflies have thin smooth bodies, are active in daytime, have slender antennae with knobs at the ends, are brightly colored, and hold wings vertically or folded over their backs. Most moths are active at night, have feathery or threadlike antennae and thick fuzzy bodies, are often dull colored, and hold wings flat or angled like a tent over their bodies when at rest.
Dragonflies and damselflies are winged hunters, with large eyes, long, thin bodies, and outstretched wings, often seen flitting about in search of lunch. They can flap their front and hind wings independently, allowing them to change directions in a flash, hover, and even fly backwards. Dragonflies are more robust and, when at rest, hold their wings perpendicular to their bodies, while delicate damselflies hold their wings above their bodies or only partly extended.
Crickets and grasshoppers are recognized by their large, strong back legs, used for jumping and hopping. They have thin, strong, leathery forewings covering clear, flexible hind wings that fold up like fans. These are the musicians of the insect world, and they can use their legs to both make and detect sounds.
Bees, wasps, and ants are characterized by their four clear, flexible wings. These wings can be hooked together to create a larger wing surface, allowing them to fly for extended periods. Many insects in this group (the bees) are important pollinators, and some live in social groups.
Flies have only two functional wings. The second set is reduced to small knobs called halteres, which are thought to aid in stability and balance. Insects in this group typically have short antennae, often can hover, taste with their feet, and only eat liquid food, which they lap up with their sponge-like mouths.
True bugs have forewings that are half leathery and half clear, while their hind wings are clear and flexible. When at rest, these overlap and create a distinctive X pattern on their back. This pattern and a beak-like mouth are key diagnostic features.
Plant hoppers and cicadas hold their wings tent-like over their backs. They also have piercing/sucking mouthparts and only eat liquid food.
There are many more types of insects, including familiar ones you might see around your house and garden like silverfish, praying mantis, and earwigs. The next time you are outside, see what you can find hiding under rocks, flying from flower to flower, hopping in the grass, or crawling in the sand. Take a closer look at wings, legs, shape, and colors for clues to its identity. The best thing about looking for insects is that you’re always sure to find them – all sorts of insects are just waiting to be discovered.
Mound, L. Eyewitness Books: Insects. New York: Dorling Kindersley, 1990.
Wilsdon, C. National Audubon Society First Field Guide Insects. New York: Scholastic Inc. 1998.
University of Michigan website: www.biokids.umich.edu/critters/Insecta/
BugGuide.net website: http://www.bugguide.net