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Ch. 3, pp. 4 - 7

[ Basic Entomology: introduction | anatomy | development | classification | specific groups ]

General classification of insects:

Figure 1

An example of classifying an insect:

Kingdom Animalia
Phylum Arthropoda
Class Insecta
Order Hymenoptera
Family Apidae
Genus Apis
species mellifera
Common name: honey bee
Insects belong to the kingdom Animalia. The Animal Kingdom contains many distinct groups called phyla. Each phylum is divided into a number of classes. Insects belong to the class Insecta in the phylum Arthropoda ("jointed foot"). The insect class is further divided into orders, families, genera, and finally, species. A related class in the phylum Arthropoda is the class Arachnida, the scorpions and spiders. Approximately 1 million species of insects have been identified to date and probably more than 1.3 million different species exist. The greatest numbers of these species belong to the beetle order (Coleoptera), fly order (Diptera) and the wasp-bee-ant order (Hymenoptera).
Insects are, except for a few common structural features, quite different from one another. Size is one factor that varies considerably, ranging from microscopic wasps that are less than a millimeter in length, to some of the long-horned beetles (such as the palo verde borer) which may be as much as six inches long. Some insects have horns and spines which render them rather bizarre, while others may resemble a dead leaf or a stick. Some insects are quite attractive, and have been used as jewelry or in art.
All of this variability makes the insects a fascinating group to investigate, but it also makes the study of insects and their classification somewhat complex. It is important to learn the main differences between insect groups so that we can distinguish one from another. You must know about insects to be able to suggest adequate control procedures and give quality management suggestions.
Figure 2. The three body regions of a typical insect.


Insects, unlike some other types of animals, have no backbones. They have an outer supporting structure called an exoskeleton, rather than the internal support structure (endoskeleton) characteristic of most large animals. The following characteristics separate the insects from other animals:
  • Three distinct body regions - head, thorax and abdomen.
  • Many adult insects have wings, and insects are the only invertebrates capable of flight.
  • Adult insects possess three pairs of legs attached to the thorax.

Figure 3. Life stages of the black cutworm, an example of complex metamorphosis.






All insects change shape during growth by a process called metamorphosis. Related arthropods such as spiders, mites and centipedes also undergo metamorphosis. However, the changes in shape of spiders and lower insects are much less dramatic than the complete change in shape of the more highly developed kinds of insects. This complete change is called complex metamorphosis. Beetles, moths, butterflies, wasps, ants and flies all go through four very different stages in order to complete development: egg, larva, pupa, adult. The larva is usually the damaging stage because it is basically a feeding machine, although feeding by adults may be destructive as well. Additionally, the larval and adult stages of many species feed on different hosts or different parts of the host. The pupal stage does not feed and in most cases is inactive or inert.
The lower insects undergo a slight change of shape which is called simple metamorphosis. True bugs, aphids, grasshoppers, termites, earwigs, many aquatic insects, etc., go through only three stages in order to complete development. These are egg, nymph, and adult. Nymphs and adults closely resemble each other, except the nymphs are smaller in size and lack wings. In these species, the adults and nymphs usually feed on the same host or host parts.
The most primitive insects go through very little change between stages. The adults lack wings, so they closely resemble the nymphs. Most experts refer to this as nonmetamorphosis.


There are several methods of separating or categorizing insects.
Figure 4. Examples of insects with sucking mouthparts (a), and chewing mouthparts (b)

Sucking Mouthparts
Cicada - Sucking Mouthparts

Chewing Mouthparts
Grasshopper - Chewing Mouthparts
1. Scientists use body parts for identification and observe the differences in these parts through the use of a microscope. He or she tracks down the identity by using a written insect "key." The anatomy of an insect will place it into a specific insect group called an order.
2. Insects are also classified by the type of damage they cause, for example: root maggot, twig girdler, wood borer, leaf miner. Identification of the plant and type of injury will often narrow the possibilities and speed up the identification process.
3. Another form of separation is in the manner of feeding mechanisms or mouthparts, for example: chewing versus sucking mouthparts. Although this manner of separation is somewhat helpful for identification, its greatest value is in determining if a certain kind of pesticide will work or not. For example, systemic insecticides generally do not work as well on chewing insects like caterpillars as they do on sucking insects such as aphids or whiteflies.
Proper identification is extremely important. If a beneficial insect or a non-damaging insect is improperly identified as a pest, an unnecessary pesticide application may be made. Pesticide applications are costly, and may cause more harm because they tend to disrupt natural control agents and cause outbreaks of other pests. For example, scientists have shown that the best way to "grow" California red scale insects was to spray insecticides which kill natural enemies of the red scale.
DO NOT MAKE RECOMMENDATIONS based on the verbal description of a pest by the client. Insist on seeing it, or at least its damage, before you volunteer a control method. There are too many beneficial or non-pest species that closely resemble pests, and incorrect identifications lead to ineffective control measures, the expense of unnecessary pesticide applications, or other problems.

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