Insect Basics

What is an Insect?

An insect is a special kind (or "CLASS") of arthropod. To be a member of the Class Insecta, a creature must first be an arthropod (it must have no internal skeleton and it must have paired, segmented legs). An insect will have, in addition, these characteristics:

• three pairs of jointed, segmented legs
• three main body regions (head, thorax, abdomen)
• one pair of antennae

Insects will also OFTEN have wings (not all insects have wings, but NO other arthropods ever have wings). As mentioned above, insects make up most of the arthropod species on earth, with about 1 million species on earth and 15,000 in Kentucky.

Insects include: dragonflies. grasshoppers, crickets, walkingsticks, mantids, roaches, termites, lice, stink bugs, assasin bugs, cicadas, aphids, beetles, butterflies and moths, flies, bees, ants, and wasps, plus many other creatures.

How Do Insects Grow and Develop?

Because their skeleton is on the outside, insects can’t increase gradually in size like humans and other vertebrates. They feed for a period of time, then molt or shed their external skeleton and begin to feed again. Hormones regulate the molting process. Depending on the species, insects may molt four to six or more times as they develop. Only insects in the most primitive orders grow and molt after becoming adults. Most insects don’t grow after they reach the adult stage. Insects are cold-blooded, so their activities are regulated by temperature. An insect goes through its life cycle more rapidly at 85° F than it would at 70° F. Metamorphosis, or change in form during development, is one of the distinctive features of insects. This ability to change has allowed insects to specialize and fill a wide variety of roles in the environment. While most insects begin life as eggs, there are four different paths, or type of metamorphosis, that will allow an insect to reach the adult stage. The types of metamorphosis, with the number of stages in each, are as follows:

• No Metamorphosis - 3 (egg, nymph, adult)
• Gradual Metamorphosis - 3 (egg, nymph, adult)
• Incomplete Metamorphosis - 3 (egg, naiad, adult)
• Complete Metamorphosis - 4 (egg, larva, pupa, adult)

Most insects have either gradual or complete metamorphosis.

No Metamorphosis occurs in the most primitive insects—springtails and silverfish, for example. The only change that takes place in these insects is an increase in size as the insect develops. None of the orders covered in the Kentucky master gardener material have this kind of metamorphosis.

Gradual Metamorphosis. This is a type of development in which the immature stage (nymph) is a smaller version of the adult. In addition to being smaller, the nymph does not have wings and is not sexually mature. External wing buds can be seen on late-stage nymphs. Nymphs have the same type of mouthparts as the adult, feed on the same food, and have the same general behavior. Except for wing characteristics, the basic features of the order can be used to identify nymphs and adults to the order level. The orders of insects covered in the Kentucky master gardener materials include Orthoptera, Mantodea, Blattaria, Dermaptera, Thysanoptera, Hemiptera, and Homoptera. Read more about these orders in Insect Orders I: Gradual Metamorphosis.

The stages of gradual metamorphosis. There is typically an egg stage, multiple nymph stages, and a single adult stage.

Incomplete Metamorphosis. This development type is unique, occurring in three orders of aquatic insects: dragonflies and damselflies (Odonata), mayflies (Ephemeroptera), and stoneflies (Plecoptera). The immature stage is a type of nymph called a naiad. It lives in the water, for which it has the necessary adaptations, including gills and a streamlined body. The adults are flying terrestrial insects, so they have some major structural differences, but the changes are accomplished without a pupal stage. None of these kinds of insects are covered in the Kentucky master gardener material, but dragonflies and damselflies can sometimes be beneficial predators in gardens that are located close to water.  

Complete Metamorphosis. About 75% of all world insect species have this type of development. For these insects, the specialized feeding stage, called a larva, is very different from the adult stage, which may not feed at all. In many cases, it is the larval stage that causes damage in the garden, rather than the adult stage. For example, caterpillars chew their food, while moths and butterflies have siphoning mouthparts to feed on nectar. All moths and butterflies, all beetles, all true flies, and all bees, wasps, and ants go through complete metamorphosis. All of them have larval immature stages and pupal stages. This means that every beetle in the world, for instance, begins life as an egg, lives as a larva for a period of time, goes into a pupal stage, and then emerges as a winged adult which is able to reproduce. Several basic larval forms are specialized. Soft-bodied, legless maggots are ideal for crawling through moist organic matter, while the caterpillar is just right for hanging onto leaves while eating vast quantities of foliage. Recognizing the basic larval types (shown below) can often indicated their role in the garden. The orders of insects covered in the Kentucky master gardener materials include all the species in these orders: Neuroptera, Coleoptera, Lepidoptera, Diptera, and Hymenoptera. Read more about these orders in Insect Orders II: Complete Metamorphosis.

The stages of comeplete metamorphosis, shown for a typical moth. There is typically an egg stage, multiple larva stages, a single pupa stage, and a single adult stage.

All beetle species also undergo complete metamorphosis, including lady beetles.

Basic Larval Types. Shown below are the basic larval types among insects with complete metamorphosis.

a. Predator (some beetles, lacewings). Characteristics include a streamlined body with hard exoskeleton, long thin legs, and big, often sharp, jaws at the front of the head. b. White grub. This type has a distinct yellow-brown head with large jaws and a soft, white, curved body with distinct legs. This type is usually a root feeder, but some larvae of this type live in decaying organic matter. Japanese beetles and green June beetles have this larval type. c. Caterpillar. This type has a distinct head; a long, cylindrical body with three pairs of segmented legs; and two to five pairs of fleshy legs along the abdomen. This is the larval stage of butterflies and moths. Many caterpillars are striped or brightly colored, but caterpillar larvae that bore in plants are usually white or cream-colored. Sawflies are similar but generally have fleshy legs on all abdominal segments. d. Wireworm. This larval type has a round, cylindrical body that is hard and yellow or brown. It has three pairs of short, segmented legs behind the head but no fleshy legs on the abdomen. These larvae may live in the soil and feed on seeds or plant roots; some live in decaying logs. Some beetles have this form. e. Leaf beetles. This type is similar to caterpillars but has no fleshy legs on the abdomen. Many leaf beetles feed on leaves and are camouflaged by color and markings. Some have white, thinner bodies and live in the soil, where they feed on plant roots. f. Maggots. This type is headless, legless, soft-bodied, and white or creamcolored. They are the larvae of flies. g. Legless grubs with distinct heads. Many feed in plants or seeds. Bees and wasps have this type of larva.

Basic insect larval forms: a. predator, b. white grub, c. caterpillar, d. wireworm. e. leaf beetle larva, f. maggot, g. legless grub

Insect Form and Function

Insects are considered to be the most successful group of animals on earth because they are found almost everywhere—from deserts to snow fields—and because so many species exist.

Their success is due to the following:

• small size and the need for limited resources
• short life cycles and high reproductive rates
• wings, which allow them to migrate
• metamorphosis

What is the Basic Insect Body Plan?

The classification of an insect from order to species is based on structures of the body, which also give clues about what the insect does. For example, predators often have large eyes and grasping legs; herbivores (plant feeders) are often rounded with short legs.

Recognition of structural modifications of various insects will help you determine the role of the insects found on or around your plants.

For example, the basic insect body plan can be as simple as that of the familiar grasshopper, but it can also be as finely engineered as the specialized honeybee. A bee has mouthparts that can both sip nectar from a flower’s depths and carefully craft wax to seal the brood cell of a bee larva before it pupates. It also has a stinger at the end of its abdomen that enables it to defend the hive from intruders. A basketlike structure on each hind leg allows it to accumulate pollen to take back to the hive.

Labeled image of a grasshopper, showing the main body parts.

This honey bee is very different from the grasshopper above, but since it is an insect it still has the same basic body parts. See if you can find the head, thorax, abdomen, wings, legs, eyes, and antennae. (Photo: Joseph Berger, Bugwood.org)

Head. The head is made of several smaller segments. It has the antennae, which function much like a nose to detect chemical scents in the air or on surfaces. These scents may help insects locate food or mates. Insects with long or large antennae rely heavily on chemicals or touch, while those with large compound eyes rely on sight. Insect mouthparts are very important in insect identification and determining whether the insect feeds by chewing solids or by sucking up liquids. Recognizing mouthparts can be difficult until you are able to find the segment called the "palps." While chewing and sucking are the basic functions of mouthparts, some insects use mouthparts for rasping-sucking (for example, thrips) and chewing-lapping (such as bees).

Examples of insect mouthparts. a. Basic “no frills” chewing mouthparts of a grasshopper. The arrow points to the palps, structures seen on most insects that chew. b. Coiled, siphoning mouthpart of a butterfly or moth, which is used for drinking nectar from flowers. c. Arrow points to sponging mouthparts of a fly, used to blot up liquids. d. Mouthhooks of a maggot, used to pull the legless larvae across its food and tear away at it. e. The arrow points to chewing mouthparts of the weevil, called a “snout beetle” because of the prolonged head. It’s tricky to recognize because no palps are present. The hard beetlelike front wings are the key to identification. f. The piercing-sucking mouthparts of a stink bug are used to feed on plant sap. (see arrow).

Thorax. The thorax is made up of three smaller segments that are connected to support legs and wings and the internal muscles that move them. A pair of legs is attached to each of these smaller segments. Generally, the wings are attached to the second and third segments (true flies, like house flies, mosquitoes, and crane flies, have wings only the second segment). Only the adult stage of an insect has wings.

Insect legs often are modified for very specific uses. Six legs are almost always present on adult or mature insects and are generally also present in the immature stages, although they may look very different at each stage. In addition to enabling the insect to walk and jump, legs can be flat likeshovel blades for digging, hinged and spiny for grasping, or long and thin for running. The legs of bees are fitted for carrying loads of pollen. This great variation in size and form of insect legs can be useful in identification.

Wing characteristics are also very important in recognizing insect orders. For example, the front wings of beetles are usually very hard and serve as protective covering for the thin, flexible, hind wings when they are not being used. The wings of butterflies and moths are covered with finescales. Wings are also modified for different uses, and their shape, or even their vein pattern, can be used in identifying the insect. The clear wings of flies and wasps are designed for fast or nimble flight, while the flat wings of many cricket species are important in producing the sound used to find mates.

Abdomen. The abdomen is made up eight or more smaller segments, but they may be covered by the wings or may be indistinct. Some insects have a pair of appendages at the tip of the abdomen. These appendages may be short, as in grasshoppers, termites, and cockroaches; extremely long, as in mayflies; or curved, as in earwigs. These are often used like antennae. There may be an egglaying device or stinger on the end of the female’s abdomen on some species.

Insect Identification

All of the world's insect species are divided into about 25 orders.  Learning to identify the common insect orders is the first step in insect identification. In many cases, it is easy to determine the order that an insect belongs to. For instance, all butterflies and moths belong to same order, called "Lepidoptera." But sometimes it's more difficult. For instance, all beetles in the world belong to the order Coleoptera, but there are a few other insects—stinkbugs, for example—that resemble beetles. Stinkbugs belong to a different order called "Hemiptera." As mentioned above, insect mouthparts are very important in identification. One of the ways to tell the difference between a beetle and a stinkbug is by examining the mouthparts: stinkings have piercing-and-sucking mouthparts that resemble a straw, while all beetles have chewing mouthparts that look like scissors or pliers. Mouthparts are often key to determining which order an insect belongs to.

A stink bug (left) and a beetle (right). Stink bugs and beetles can look similar when veiwed from above, but their mouthparts are very different. Stink bugs belong to the Order Hemeiptera. All insects in that order have tubelike or straw-ike mouthparts that are used to suck fluids. All beetles (Order Coleoptera) have mandibles that are built to chew and rip solid food. (Stink bug photo: Susan Ellis, Bugwood.org; Beetle Photo: David Cappaert, Bugwood.org)

Insect Orders

The next sections deal with individual insect orders. For the Kentucky Master Gardener Program, not all insect orders are covered, just those that tend to be important in the garden and the urban landscape. The first section covers those insects with gradual metamorphosis, the second section covers insects with complete metamorphosis.

• Insect Orders I: Gradual Metamorphosis
• Insect Orders II: Complete Metamorphosis