Understanding Arthropod Classification and Identification

by Gary A. Dunn, M.S., F.R.E.S., Director of Education

PRINCIPLES OF ANIMAL CLASSIFICATION

All living organisms can be divided into groups (classified) for purposes such as identification. The creation of artificial categories (based upon similarities and differences in morphological or other features) also helps us achieve a better understanding of the relationships among the many types of living organisms.

Let's look at the concept of insect classification by drawing an analogy with human classifications. In our society individual people are classified in numerous ways. A good example is the use of a person's mailing address to identify and separate that person from other people. Determining where an insect fits into the animal kingdom is much like your mail carrier determining who you are from a well-addressed letter. Let's compare the two. A complete address would include your name (which really has two parts - a surname and a first name), street number and street, city, and state. But, when a mail carrier sorts mail, he/she reads the address from bottom to top looking first for the state, then the city, street and so on, ending with delivery to a single individual.

Similarly, an entomologist classifies insects by the process of elimination, ending with the identity of one species. In classifying an insect, an entomologist gets clues from an insect's "address". The categories of the scientific "address" are kingdom, phylum, class, order, family, genus, and species. Let's compare postal classification with insect classification, using two examples.

Postal Classification        Insect Classification

Nation ............United States      Kingdom ............ Animalia

State ............... Michigan           Phylum ............... Arthropoda

City ................ Lansing              Class ................... Insecta

Street ............. Peggy Place       Order .................. Diptera

Number .........1915                   Family ................. Muscidae

Surname ........ Dunn                  Genus .................. Musca (fly)

First Name ..... Gary                  Species ................ domestica (house)

In both postal and insect classification, the last two categories give us the complete name of the individual - Gary Dunn and Musca domestica (house fly) in our example. In the case of the insect we have given both the scientific name (which is always italicized or underlined) and the common name.

The species is the basic unit of the classification system. A species is a group of morphologically similar organisms that can and do interbreed, thus sharing the same gene pool. Closely related species (those sharing one or more significant morphological features) are grouped together in the next highest category, the genus. Related genera make up a family (note that family names always end in "idae"); related families are grouped into an order; related orders are grouped into a class; and related classes are grouped into a phylum.

ARTHROPOD CLASSIFICATION

All of the members of the class Insecta belong to the phylum Arthropoda (commonly referred to as arthropods). Arthropods are nature's joint-footed animals that possess a hard, armor-like outer skeleton. The arthropods comprise about 95% of all known animal species. In fact, all but 5% of the arthropods are insects, so the insects outnumber all other animal species by 9 to 1! Non-insect arthropods include the classCrustacea (water-dwelling crabs, shrimp, crayfish and lobsters, and land-dwelling isopods or sowbugs), the class Diplopoda [dip-low-POH-dah] (many-legged, worm-like millipedes), the class Chilopoda [ki-low-POH- dah] (centipedes), the class Arachnida (mites, ticks, spiders, scorpions, and harvestmen), the class Xiphosura(horseshoe crabs), and the extinct class Trilobita (trilobites).

The classes and orders of arthropods can be distinguished from one another through the characteristics used in the keys on the following pages. For example, the class Insecta is distinguished from other arthropod classes (and other animals) by the following characteristics: (1) exoskeleton divided into three parts, (2) three pairs of legs, (3) one pair of antennae, (4) compound eyes, and (5) many adults are winged.

PRACTICING ARTHROPOD CLASSIFICATION AND IDENTIFICATION

One of the best ways to learn about and practice arthropod identification is by using insect and spider replicas ("rubber bugs"). Various "rubber bugs" are available from a variety of stores, or by mail from the Young Entomologists' Society see the (Plastic and Plush Critter Palace in the Minibeast Merchandise Mall).

There are, in fact, two different ways that you can learn about arthropod classification and identification using "rubber bugs". First, you can practice making your own original dichotomous (binary) keys to arthropod groups. This is not as difficult as it might seem. Start with one representative from each arthropod orders in the previous section. Examine the arthropods and come up with an initial yes or no question that will serve to separate out a subset of specimens based on a unique characteristic. For example, the question might be "Are there more than six legs"? After the first "subset" is set aside from the main group, continue breaking the subsets down by asking additional yes or no questions. All that remains to be done is to "map out", or diagram, the questions and the yes or no "pathways" that were used to separate the arthropods into different groups. The resulting diagram is actually a dichotomous (binary) identification key! (Later this diagrammatic key can be converted to a word key if you so desire.) Keep in mind that there are many ways to "sort", or classify, arthropods; therefore, many correct solutions to this classification exercise are possible. (Just like using a roadmap to navigate from one city to another, there are many possible routes that you could choose and all of them will get you to your destination - some may be more direct and efficient than others, but they will all eventually lead you to your objective.)

Secondly, you can use "rubber bugs" to practice using the identification keys to arthropods (classes and orders) included here. This will help you better understand the physical features of various arthropod groups, while at the same time becoming more familiar and comfortable with dichotomous (binary) keys. For an additional challenge find dichotomous keys to families of the insect orders and see if you can identify any of your insect replicas to the family level.

For more information on identification of insects, spiders and other arthropods (including keys to orders and families), see The Digital Guide to Insects, Spiders and Arthropods of North America by Gary A. Dunn (published by the Young Entomologists' Society, available at the Minibeast Merchandise Mall).

A Key to Major Classes of Adult Arthropods

1a. Typically with three pairs of legs, with or without wings, and three distinct body regions..........................insects (INSECTA)

1b. More than three pairs of legs and wings never present................. 2

2a. With four pairs of legs, no antennae, and head and thorax fused into one large cephalothorax.......................arachnids (ARACHNIDA)

2b. More than four pairs of legs present ............................................3

3a. Body broadly oval with stout spinelike tail ..............horseshoe crabs (XIPHOSURA)

3b. Body not as above .....................................................................4

4a. Usually with 5 to 7 pairs of legs and two pairs of antennae.............. crustaceans (CRUSTACEA)

4b. With more than ten pairs of legs and only one pair of antennae; body long and wormlike ..................................................................5

5a. Body cylindrical; two pairs of legs on each segment; antennae short ..................................................................millipedes (DIPLOPODA)

5b. Body more-or-less flattened; one pair of legs per segment; antennae long ...........................................centipedes (CHILOPODA)

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This educational resource was prepared by the Young Entomologists Society, 6907 West Grand River Ave., Lansing MI 48906-9131, phone/fax 517-886-0630, e-mail YESbugs@aol.com.  Support minibeast youth education - join Y.E.S. today!