Glossary of Terms

The ant’s body is divided into four main sections. The head is the first section, followed by the large mesosoma, the small petiole and sometimes the postpetiole, and finally the gaster. The head carries the antennae and mouthparts, the three pairs of legs are attached to the lower surface of the mesosoma and the gaster terminates with a defensive structure (often a sting or acidopore). Although the mesosoma and gaster appear to correspond to the thorax and abdomen of most other insects, this is not the case. The rear section of the mesosoma, the propodeum, is actually the first abdominal segment, the petiole is the second segment and the postpetiole, when present, the third segment. The gaster is composed of the remaining segments of the abdomen.

Head
The most important taxonomic structures on the head are the antennae, palps and clypeus. The antennae are composed of two major parts, the long first segment, the scape, which is attached to the head, and the remaining shorter segments, collectively called the funiculus. The important characteristics of the antennae include the number of segments (when counting the number of segments, the scape is always included), the length of the scape (usually in relation to the length of the head), and, in some groups, the position of antennae when at rest against the front of the head.

The palps are small, segmented, sensory organs found on the mouthparts and are visible on the underside of the head behind the mandibles. There are two pairs, the outer pair situated on the maxillae (called the maxillary palps) and the inner pair situated on the labium (called the labial palps). The number of maxillary palp segments varies from 6 to 1 (with 6 being the most common) and the number of labial palp segments varies from 4 to none with (4 being the most common). The palp formula is the standard method used to indicate the number of palp segments and is composed of the number of maxillary palp segments followed by the number of labial palp segments. For example, a palp formula of 6:4 would indicate that the maxillary palps have 6 segments while the labial palps have 4 segments.

The clypeus is the plate on the lower section of the front of the head above the mandibles and below the antennae. Its lower edge (above the mandibles, here called the front margin) is usually convex in overall shape, but can be highly modified with concave regions, teeth or variously shaped projections. The rear section (near the antennae) is usually narrowed, convex or triangular and often extends between the forward sections of the frontal lobes. The central region of the clypeus is usually smooth and gently convex across its entire width, although in some groups it may have a pair of weak to well-developed, diverging ridges (in which case the clypeus is described as being longitudinally bicarinate).

In some groups the shape of the frontal carinae is important. The frontal carinae are a pair of ridges on the front of the head; these ridges start just above the clypeus and between the antennal sockets and extend upwards. Their development varies from being very short, weakly developed or even absent to very distinct and running the length of the head. The lower sections of the frontal carinae are often expanded towards the sides of the head and partially or completely cover the antennal sockets. In these cases this section of the frontal carina is called the frontal lobe.

Other important features on the head include the compound eyes (which vary in size, shape and position, and can be absent), the position of the antennal sockets (the points where the antennae attach to the head), the development of a psammophore (a collection of long hairs on the underside of the head), the presence of antennal scrobes (elongate depressions or grooves on the front of the head which receive the scapes when at rest), and the shape of the mandibles including the number and placement of teeth.

Mesosoma
The mesosoma, sometimes called the "alitrunk" in older literature, is the middle section of the body to which the legs are attached. It is behind the head and in front of the petiole. In workers the mesosoma is relatively simple, with a limited number of sutures and plates. Queens, however, have a much larger mesosoma with many sutures and plates. This additional complexity is required because queens typically have wings during the early part of their lives. The larger mesosoma houses the flight muscles and the additional sutures and plates are used to control the wings during flight.

The mesosoma has numerous structures of taxonomic importance. The upper surface (tergite) of the first segment, immediately above the front legs, is called the pronotum. In most ants the pronotum forms a separate, distinct plate but in some it is fused with the sclerite behind it, the mesonotum, to form a single plate. The mesonotum is the upper surface of the mesosoma behind the pronotum and in front of the metanotal groove. It is essentially the central one-third of the mesosoma and has the middle pair of legs attached to its underside. The metanotal groove is an angle or depression on the upper surface of the mesosoma which separates the mesonotum and the propodeum. In some groups the metanotal groove is lacking and the upper surface of the mesosoma is uniformly arched when viewed from the side. The propodeum (called the "epinotum" in very old literature) is the rear section of the mesosoma, above the hind legs and immediately before the petiole. The metapleural gland, or more correctly, its opening, is located on the side of the propodeum immediately above the hind leg and below the propodeal spiracle, near the attachment point of the petiole. Its small opening is often surrounded by tiny ridges or is located in a shallow, elongate depression. The opening is often protected by a fringe of elongate hairs or setae. In a few groups the metapleural gland is absent and the area above the hind leg is smooth.

The propodeal flanges are found behind (posterior to) the metapleural gland openings. They vary from being narrow and weakly developed to elongate and spine-like.

Legs
The legs are composed of five main segments. The segment nearest the body is the coxa, followed by the very short trochanter (which is seldom used in ant taxonomy), the long femur and tibia, and finally the tarsus. The tarsus is composed of five small segments with a pair of small, curved claws at its tip. The claws are most commonly a single, curved shaft terminating in a sharp point. However, in some groups the claws can have from one to many small teeth along their inner margins. The junction of the tibia and the tarsus is usually armed with a large, stout, articulated, spike-like structure called the tibial spur. The number of spurs can be none, one or two, and they can be simple or comb-like (pectinate). These structures are best viewed from the front with the leg extending outwards from the body at right angles to its long axis.

Petiole and Postpetiole
The petiole is the first segment behind the mesosoma and is present in all ants. Behind the petiole is either the postpetiole or the gaster. The postpetiole is found in only some subfamilies of ants. When present, it forms a distinct segment separate from the gaster. The upper surfaces of the petiole and postpetiole are often high and rounded or angular. This upright structure is called the node. In some cases the node is absent and the petiole is low and tube-like. The narrow forward section of the petiole in front of the node is called the peduncle. This section can be long, short or absent. In many groups there is a subpetiolar process, a projection or lobe on the underside of the petiole near its attachment to the propodeum. This process varies from being absent to thin and pointed to broad and rounded.

The petiole and postpetiole provide a flexible junction between the mesosoma and gaster. This allows an ant to bring the tip of the gaster forward towards the front of its body. In this position the sting or the opening for the defensive system can be used to subdue prey or attack intruders.

Gaster
The last segment of the body is the gaster. In most ants it is smooth in outline, but in some the first segment is separated from the remainder by a shallow constriction, and in a very few each segment is separated by shallow constrictions. A sting is often visible at the tip of the gaster, although it is retractable and may not be visible even when present. In some ants the sting is absent and the tip of the gaster terminates in a small, slit-like or circular, glandular opening. Finally, the upper plate (tergite) of the last segment of the gaster is called the pygidium.

Other Terms
There are a number of terms used for general structures found on ants. A seta is an elongate hair ranging from upright and standing above the body’s surface to appressed against the surface. A spiracle is a small opening in the body which is part of the respiratory system. The most obvious spiracles are generally those near or in the metanotal groove and on the sides of the propodeum. The shape and location of the propodeal spiracle can be of significant taxonomic importance. A suture is a line or impression formed where two body plates or sclerites meet.