How to Know Insects

How to Know Insects

Beginning collectors must know something about what insects are, how they live, behave, develop, and reproduce. Although there are many advanced texts written about any one of these topics, this book provides a very basic foundation for beginning entomologists. Understanding the principles of insect growth, development, morphology, and behavior allows us to more easily rear, collect, identify and, in the case of pest insects, control them.

Nearly 75 percent of all animals are insects. Scientists have described and given scientific names to about 920,000 species of insects in the world, which represent almost 85 percent of all known animal species. Just think - there are more different species of dragonflies than of all mammals combined. There are about 9,000 species of birds, but almost twice as many species of butterflies. No wonder you can find insects nearly everywhere.

The fact that more species of insects are known than all other animals and plants combined is phenomenal, but even that number does not represent all of the insects on Earth. Some estimate that the number of named and recorded insects is only a fraction of the total number of living insect species. Many are very small and hard to find. Many live in areas where collection and study are difficult. Still others are right under our noses and are just waiting to be discovered.

Scientists who study the fossil record have estimated that insects have been around for more than 350 million years, much longer than people. Some early insects were huge dragonflies with wingspans in excess of 3 feet; others were very small. Today, the largest insects have bodies that measure 3 to 4 inches in length and may have wings that span 6 or 8 inches, but insects have become very diverse as they have adapted to life on Earth. You'll find them in nearly every type of habitat, with truly amazing features and behaviors that allow them to live and thrive in these conditions. Insects' relatively small size, high reproductive potential, and diverse feeding habits allow them to become very successful animals in our world. They have not only developed ways to live in nearly every habitat, but they have developed the ability to feed on a vast array of foods.

The greatest challenge to learning how to identify insects properly will be learning the names of the various components of an insect's body. Beginners usually compare collected specimens with published photographs or line drawings of the insect. This is sufficient for the most basic identification, but because there are so many insects, a photograph of every one would be very cumbersome. Many are very similar either in color, size, or shape. Often small differences in how an insect is constructed determine whether it is one species or something entirely different. As a result, written keys are constructed that, if followed, will lead a student to the proper identification. The process of identification will be discussed in greater detail in a later section. For now, it is important to gain a working knowledge of the anatomy or morphology of an insect, so that written keys can be followed. For example, some references may be made to particular structures such as tarsi. To answer questions relating to tarsi, you should consult a line drawing of the leg of a typical insect to determine where tarsi are. Similarly, a general recognition of other parts of basic insect anatomy or morphology is required.

Compared to people, insects are constructed inside out and upside down. For example, people have skeletons inside their bodies to which muscles attach and allow movement. Insects have external skeletons (exoskeletons) and their muscles are attached inside to allow for movement. People have nerve cords running along their backs (dorsal), with hearts closer to the front of the body (ventral). Insects have dorsal hearts and ventral nerve cords.

You'll find many more differences between people and insects. A human has four appendages (arms and legs); an insect has six. People breath with lungs, insects breathe through tiny holes along the sides their bodies. The heart and blood of an insect are not important to moving oxygen in the insect body. Insects smell with their antennae, taste with their feet, and hear with special organs in their feet, abdomen, and sometimes antennae. Insects are the only invertebrates that have developed the ability to fly, although they have also mastered running, burrowing, jumping, climbing, swimming, and hopping.

Insects are successful and fascinating, because they are unique in so many ways. Collecting and identifying insects requires a basic understanding of insect anatomy (morphology), development, and physiology (digestion, reproduction, nervous system, circulation, and respiration), as well as behavior. We shall begin with how to distinguish an insect from its close relatives.

Insects and their relatives are all animals (Kingdom: Animalia) that belong to the group or phylum called Arthropoda. These all share many characteristics, including possessing segmented bodies and jointed appendages (legs and antennae). The following five classes of Arthropods are quite common and insect collectors should be able to recognize them. The Latin or Greek translations of their scientific names (provided in parentheses following the name) are helpful in separating these groups.



Arachnida (arachnid=spider-like)
spiders, mites, ticks, scorpions, harvestmen

An arachnid has four pairs of legs, lacks antennae, and possesses a head and thorax that is combined into one part called a cephalothorax.

The arachnids represent the second-largest and (next to insects) second most agriculturally injurious class of arthropods. Arachnids have very diverse life histories. Most spiders are beneficial predators. Most ticks are parasitic and some may transmit important diseases to people. Mites can be beneficial predators, parasites on insects and mammals, or extremely damaging pests on plants.











Chilopoda (chili=thousand, poda=legs)

A centipede has a long, flattened, many-segmented body with one pair of legs attached to each body segment. It also has a pair of moderately long antennae.

Centipedes are often confused with millipedes, but they differ by having more flattened bodies, one pair of legs per body segment, and the ability to move quite rapidly. Centipedes are always predatory upon other small arthropods and each centipede possesses a pair of strong fangs, which it uses to hold and poison their prey.



Crustacea (crust=shell-like)
lobsters, crabs, sowbugs, shrimp

A crustacean has a hardened exoskeleton, at least five pairs of legs and a cephalothorax. Some have two pairs of antennae.

The class crustacea contains many aquatic animals, including crayfish, lobsters, and prawns, which are important human foods. Insect collectors very commonly encounter one order of this class in terrestrial environments. This order (Isopoda) includes the sowbugs and pillbugs. Both sowbugs and pillbugs occasionally feed on vegetable matter, but seldom cause damage. Isopods breathe through gills that must be kept wet and, therefore, must remain in relatively moist habitats. As a result, they are commonly found under rocks, logs, mulch, or other debris on the ground or in wooded areas. Isopods have seven pairs of legs and shield-shaped heads. The pillbug is recognized by most as the tiny animal that rolls itself into a tight ball when disturbed.









Diplopoda (di=two, poda=legs)

Millipedes have long rounded, many-segmented bodies with two pairs of legs attached to each body segment. They each have one pair of antennae.

The millipedes are slow-moving, wormlike arthropods. They eat plants, but seldom cause economic damage to them. They normally feed on dead or decaying plant matter. Millipedes can become nuisance pests in and around homes where they live under patio decks, crawl spaces, and often landscape mulch.



Hexapoda (hex=six, poda=legs)

Insects have six legs, three body parts and two antennae in their adult form.

The largest and most important class of Arthropods is Hexapoda. Hexapods range in size from very tiny (almost too small to be seen with the naked eye) to quite large. As a group, insects are the most damaging pests of human foods, both during production and storage. Certain insects damage ornamental plants, agricultural crops, and stored foods. Some damage structures and buildings, and some transmit very important diseases to people. Other insects are beneficial organisms to the environment, our economy, or human health and welfare. Most add to the beauty and interest of nature.

bees and wasps

Bees & Wasps










Butterflies & Moths

In the adult stage, an insect has three pairs of legs (total = 6) and three distinct body parts. An insect also normally has a pair of antennae, two pairs of wings, and eyes and mouthparts adapted especially for its specific lifestyle.

insect anatomy

An insect exoskeleton is composed of a hardened material called chitin that is similar to human fingernails. This gives the insect the structure to which muscles can attach and operate, allowing movement. The exoskeleton also protects the insect from desiccation, physical injury, and allows for the myriad of colors, shapes, and sizes that make insects so diverse and interesting.

The three main insect body parts are head, thorax, and abdomen. The head contains the antennae, eyes, and mouthparts. The thorax is the middle body part to which the legs and wings are attached. The abdomen contains digestive and reproductive organs internally and often reproductive structures externally. The sides of both the thorax and the abdomen are lined with tiny openings called spiracles, through which an insect obtains oxygen.

insect body parts

Insect mouthparts differ in appearance due to the fact that the diets of insects vary widely. One of the evolutionary marvels in the study of insects concerns the ability of these animals to feed on such a wide assortment of foods. Nearly all organic materials may be consumed by one or another insect. It is no wonder, then, that insect mouthparts are so different. Mouthparts are often used as a basis for separating insects into their respective orders or families. The four most common mouthparts are illustrated below.


Solid foods are consumed by insects with biting-chewing mouthparts. Beetles, caterpillars, and grasshoppers all have biting-chewing mouthparts. These insects leave behind tell-tale signs of feeding, holes in leaves, trunks of trees, or they simply consume the whole plant or animal.


Several different insects, such as mosquitoes, fleas, assassin bugs, and aphids have evolved a piercing-sucking mouthpart in which stylets actually pierce into plant or animal tissue allowing the fluids there to be sucked into the insects. Insects with this type of mouthpart are commonly associated with disease transmission in both plants and animals.


Specialized flies such as, the house fly, exhibit sponging mouthparts. In this group, saliva and regurgitated foods are pumped externally onto the food source to begin the digestion process. The dissolved or suspended food is then sucked up into the alimentary canal of the insect.


Butterflies and moths uncoil their long tube-like proboscis and insert it into the nectaries of a flower to siphon out the fluids found there. These specialized mouthparts are referred to as siphoning mouthparts.


Insect legs can be as different in appearance as the insects themselves and are often referred to in identification keys. They may be modified according to the specific behavior of the insect, whether it is for jumping, digging, swimming, hopping, grasping, or running. All legs typically consist of the segments shown in the diagram.


The essential make-up of all functional insect wings is the same: a thin membrane, which is supported by veins both around and within the margin. Stark contrasts between the wings distinguishes the four largest orders of insects. Minor differences in wing venation or shape further differentiate insect species. The major veins in an insect wing are illustrated below.


Insects grow differently than do other animals. Due to the rigid exoskeleton on the outside of an insect body, it cannot gradually expand in size like vertebrates do. To become larger, an insect must periodically shed the old exoskeleton, expand in size, and then grow a slightly bigger exoskeleton than the one it just shed. This process is called molting. It can be illustrated with an example of an adult damselfly pulling itself out of its old skin.

insect growth

The new flexible skin is expanded by pumping it up with air or water. After expansion, the new exoskeleton will harden and take on color. This process of molting may occur several times during the growth of an insect, depending upon what species it is. Once the insect becomes an adult, however, growth ceases.

without metamorphosis

With few exceptions, insects begin life as eggs. The change in form from eggs through to adults in insects is termed metamorphosis. Authorities use slight differences in metamorphosis to help describe and separate insect groups. For purposes of this publication, the most primitive insects (Collembola and Thysanura) are said to develop without metamorphosis. The insect that comes from the egg appears exactly like the fully grown insect, except that it is not as large.

incomplete metamorphosis

More advanced insects are described as having incomplete metamorphosis. Insects with incomplete metamorphosis change shape gradually as they grow. There are three stages of growth: the egg, nymph, and adult. Grasshoppers, termites, bugs, and lice are all of part of this group.

complete metamorphosis

Insects with complete metamorphosis go through four stages of growth. None of the stages look at all like the others. These stages are referred to as egg, larva, pupa, and adult. Fleas, flies, beetles, bees, and moths all belong to this group.

Circulatory System

Insects possess an "open" circulatory system in which an insect's blood (hemolymph) fills its body (hemocoel) rather than being contained within vessels (closed systems) as in most higher animals. Hemolymph comprises up to 40 percent of an insect's body weight. The only vessel that most insects possess is called an aorta, or sometimes simply referred to as a heart. This long tube lies dorsally inside the insect and serves as a pumping chamber. The insect blood enters through small holes in the aorta and then is pumped forward as the vessel contracts and is dumped into the insect body homocoel again. In this way, hemolymph sloshes around in the insect body and reaches all necessary tissues. Insect blood is clear in color rather than red, because it does not have need for red blood cells to transfer oxygen.

circulatory system

Respiratory System

Oxygen reaches internal tissues by means of tiny tubes (invaginations) of the exoskeleton. The outer holes along the insect's body, called spiracles, allow air to diffuse into the small tracheal tubes, which further branch into progressively smaller tubes that eventually enter into the muscles, internal organs, and tissues of the insect. In this manner, insects are able to get the necessary oxygen to cells for respiration.

respiratory systemrespiratory system

Digestive System

Insects ingest food through one of the several kinds of mouthparts described earlier. Solid or liquid food is taken in through the mouth leading into a complete alimentary food canal. Food is broken down by enzymatic hydrolysis and is absorbed through the gut wall and into the insect. Undigested material is excreted through the anus at the rear of the alimentary canal.

digestive system

Reproductive System

Insect reproduction is almost always sexual (requiring a male and a female) but in a few cases it may be asexual (requiring a female only). Some insects use both. In sexual reproduction, the males copulate with the females, sending sperm into the egg chamber of the female. Eggs there are fertilized and then pass out of the oviduct and are usually deposited either singly or in clusters in the environment.

female reproductive systemmale reproductive system
nervous system
Nervous System

Individual insect nerve cells are constructed much like vertebrate nerves. A sensory structure, such as an eye or antennae, triggers a nerve impulse that travels along the length of the nerve cell. This nerve ends in close contact with other nerves in a junction called a synapse. Specific chemicals are released in this junction and if adequate amounts are picked up by the second nerve, the impulse will again be carried to yet other nerves or to muscle tissue, causing them to contract or expand. Many common insecticides work by disrupting the communication process at this junction.

The nervous system in insects consists of a long nerve cord made up of many nerves bundled together. This cord lays ventral in the insect body and runs the length of the insect. Several nerve masses form a primitive brain toward the anterior end of the nerve cord but, unlike verterates, the central insect nervous system also has smaller centers along the nerve cord in the thorax and even the abdomen. These smaller masses spread along the nerve cord help direct coordinated processes such as reproduction, movement and other life supporting functions, independent of the larger brain in the head area. This is why cutting the head off of an insect often does not stop it from walking or mating.

nervous system

Insects are recognized as cold-blooded or ectothermic animals. This means that an insect's internal body temperature is determined by the temperature of the external environment. Because they have little control over their internal temperatures, insects do not remain active during cold periods and winter months.

Insect behavior consists of active responses to various stimuli in the environment. Strictly speaking, these are motor responses. When a series of motor responses are put together, the actions elicited are referred to as a behavior.

Unlike much of human behavior, insects do not think or reason, but only act. Thus, it is incorrect to assign likes or dislikes to insect behaviors. Responding is the proper term to describe insect behavior. Feeding, defensive, migratory, communication, social, and reproductive behaviors are often complex but exist as a sequence of motor responses to various stimuli.

Insect behavior is a fascinating study. It can lead to an understanding of insects that will not only help us know how they live, and what they do, but also assist in our discovery of ways to manipulate them, which is especially useful in pest management.