Ant Identification

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General Information

What is an Ant?

While ants are one of the most familiar insects and can be reliably identified by even the youngest naturalist, what exactly is it that makes an ant an ant? There are several traits which will separate them from other insects. First, all ants have either a single small, distinct segment, the petiole, or two small segments, the petiole and postpetiole, between the mesosoma and gaster (click these terms to see their definitions). These separate segments are absent from almost all other insects other than a few groups of wasps. A character which is found only in ants is the metapleural gland. This gland is found on the side of the propodeum just above the hind leg and has a small opening to the outside of the body. It should be noted that while this gland is found only in ants, not all ants have a metapleural gland. A few genera in the subfamily Formicinae (such as Camponotus, Oecophylla and Polyrhachis) have lost the metapleural gland and its associated opening.

In addition to these two characters, there are several other distinctive traits of ants. One is their elbowed antennae. The first segment of the antenna, the scape, is much longer than the remaining segments, the funiculus, and the joint between them is highly flexible. During normal activity the scape is held upright and near the head with the funiculus projecting forward in front of the body. This arrangement allows the tips of the antennae to be positioned near the mouthparts to assist in inspecting nearby objects, or to be extended forward away from the body to investigate more distant items. While a sting is present in many ants, it is absent in several large and common groups (Dolichoderinae, Formicinae) and is of little use in separating ants from many other insects.

Identification Keys

Keys are used to identify and name specimens. On this site there are three sets of keys: a single key to identify subfamilies, a series of keys to identify genera within each subfamily and keys to indentify species within genera (these last keys are only available for selected groups).

These keys are called dichotomous keys. This is because they are composed of paired, contrasting descriptions. Together, the two descriptions (or lugs) comprise a couplet. A specimen is compared with each of the descriptions (lugs) in a couplet and the most appropriate description is selected. In printed versions of keys each description ends in either a number or a name. The number indicates the couplet to proceed to next to continue the identification, while a name indicates that the identification is complete and the specimen in hand belongs to (or is likely to belong to) that group. A typical printed couplet looks similar to this:

40. Eyes small, with at most 4 facets (ommatidia) in greatest diameter, and round or nearly round ......... Anisopheidole

-. Eyes large, with 8 or more facets (ommatidia) in greatest diameter, and distinctly oval or elongate .............. 41

List of all keys

All of these keys contain morphological terms and references. You can go here to find definitions and diagrams that will help you to understand these terms.

When Things Go Wrong

In some cases none of the characters listed in either lug of a couplet will seem appropriate or the characters in a lug only partially match the specimen. In these cases it is likely that a mistake was made earlier in the key and that the specimen was not intended to run to this part of the key. When this happens the previously used couplets should be rechecked to determine if an incorrect lug was selected. A trick used by experienced key users is to note any "problem" or ambiguous couplets during an identification. Later, if the key becomes difficult to use because the characters seem inappropriate, the other lug of the "problem" couplet is tried. If a mistake was indeed made, the other path through the key will often run much more smoothly and the characters used will better fit the specimen in hand.

Confirming an Identification

Keys are only the first step in the identification process. Once a name is found, it is wise to check additional information to confirm that the name seems reasonable and appropriate for the specimen. This additional information can include overall similarity of the specimen to other members in the suspected group, the description listing detailed unique or diagnostic characters, the habitats used by the group and the known distribution of the group. If any of these seem inappropriate, the identification should be treated cautiously until it can be confirmed by comparison with reliably determined material or by a taxonomist experienced with the group.

Using the Keys

The keys are designed to identify workers only. This is because workers are by far the most commonly encountered caste of ants and the best known taxonomically. Because queens, and especially males, are much less common it is often very difficult to identify them, even in cases where the workers are common and well known.

The characters used in the keys are limited to those found on the outside of the body. In addition every effort has been made to select easily observable and unambiguous characters. However, the small size of ants means that reliable identification is only possible with the use of a microscope. The maximum magnification necessary will vary with the specimen being examined, but a range of 10X to 50X will generally be required. In a few cases higher magnification is desirable although not essential. Then using any microscope illumination is important. For example a low quality light source will degrade the image seen through even an expensive microscope. Additionally, some types of illumination will cause bright reflections to appear on the surface of the specimen which will obscure the pattern of the surface sculpturing. Because of this it is important to consider a range of light sources when choosing a microscope and illuminator.