Morphological Terms/General

Anapleural sulcus
See Pleurite/pleuron.

Anepisternum
See Pleurite/pleuron.

Annulus
A simple, unsegmented, ring of cuticle. For example, one of the funicular segments of the antenna, or the torulus (= annulus antennalis) around the antennal foramen.

Apophyseal lines Externally visible lines that mark the internal track of cuticular processes for muscle attachment.

Bulla
See Pleurite/pleuron.

Endophragmal pit
A pit or pit-like impression in a sclerite that is an external indication of the point of attachment of part of the endoskeleton. Endophragmal pits that are universal in ants include the anterior and posterior tentorial pits of the head, and the mesosternal and metasternal pits of the ventral mesosoma. Some groups of ants also have endophragmal pits in other locations. For example, workers of the dorylomorph genera usually have a pit laterally on the mesosomal pleuron, either immediately posterior to the mesometapleural suture, or even more posterior, closer to the propodeal spiracle.

Endoskeleton
As well as the extensive exoskeleton, worker ants have a small but significant endoskeleton that consists of sclerotised internal plates that serve as muscle attachments (the endoskeleton of alate forms remains to be investigated).

The endoskeleton of the head is the tentorium, formed from united anterior and posterior pairs of arms. In ants the fusion is complete and the tentorium takes the form of a pair of longitudinal struts, attached to the head capsule anteriorly and posteriorly. In addition, each tentorial strut has an anterolateral side-branch that extends to the antennal socket. The only visible external indications of the tentorium are the anterior tentorial pits and posterior tentorial pits, which are endophragmal pits that mark the points at which the tentorial arms are attached to invaginations of the exoskeleton.

The mesothorax has a mesendosternite, derived from the invaginated sternum of the segment. At maximum development the mesendosternite is a thin, longitudinal, roughly triangular, sclerite that extends the length of the segment and terminates posteriorly at the externally visible mesosternal pit. In most ant groups the dorsum of this endosclerite extends into a Y-shaped pair of apodemes that are inclined anteriorly and directed anterolaterally and dorsally, and there is usually a reinforcing cross-member between the apodemes, near their base. The apices of the apodemes are usually attached to the pleuron. In dorylomorphs the apodemes are inclined posteriorly, extend laterally from their point of origin and are strongly attached to an invagination of the pleuron; the point of attachment is often visible externally as an endophragmal pit.

At the internal junction of the mesothorax and metathorax, just posterior to the mesocoxal cavities, is a raised ridge or low wall of transversely arched cuticle that appears to be derived from the invaginated fused margins of the two segments, right across their line of junction. Arising from the midpoint of this transverse cuticular wall, fused to its vertical midline anteriorly, and extending posteriorly, is the metendosternite. At maximum development this sclerite is similar in shape to the mesendosternite and terminates posteriorly at the externally visible metasternal pit. In most ant groups the metendosternite has a pair of dorsal apodemes, that may be very long, extending forward into the mesothoracic cavity above the mesendosternal apodemes, or they may fuse to the pleuron far in front of their point of origin. However, in the dorylomorphs the metendosternal apodemes are short and are attached to the posterior surface of the mesendosternal apodemes.

Endosternite
See Endoskeleton.

Exocrine gland
A gland whose secretory cells are located below the cuticle but which has pores, ducts, or one or more orifices, that open to the surface.

Foramen
A natural opening or perforation in a sclerite. Usually an opening in one sclerite that accommodates the insertion of another. For example, the occipital foramen (= foramen magnum) is the posterior hole in the head capsule within which the articulation with the pronotum is accommodated. Similarly, the coxal cavities are the ventral foramina within which the coxae articulate with the thorax, and the propodeal foramen is the posterior orifice in the mesosoma within which the petiole (A2) articulates.

Mesendosternite
See Endoskeleton.

Metendosternite
See Endoskeleton.

Pleurite/pleuron
The lateral sclerites of the thorax proper, excluding the propodeum which is morphologically the tergite of the first abdominal segment.

The propleuron (pleuron of the prothorax) is relatively small in ants and is mostly or entirely overlapped and concealed by the extensive lateral part of the pronotum when viewed in profile, but can always be seen clearly in ventral view (see prothorax). The mesopleuron (pleuron of the mesothorax) is the largest pleurite. It may consist of a single sclerite that extends almost the entire height of the lateral mesothorax or may be divided by a transverse sulcus (the anapleural sulcus) into an upper anepisternum and a lower katepisternum (see mesothorax). The metapleuron (pleuron of the metathorax) is located posteriorly on the side of the mesosoma, mostly below the level of the propodeum in workers but more extensive in queens and males. The metapleuron bears, in the female castes of almost all ants, the metapleural gland (see metathorax). The ventral surfaces of the mesothorax and metathorax are formed by the ventral expansion of the pleurites and their fusion along the ventral midline; the true sternites of these segments are represented only by endoskeletal structures. The abdominal segments do not have pleurites and each consists only of tergite (above) and sternite (below).

Pubescence
Small to minute hair-like cuticular projections which are not socketed basally.

Sclerite
Functionally, a general term for any single plate of the exoskeleton (e.g. pronotal sclerite, abdominal sclerites); more specifically, an integumental plate in which the protein sclerotin has been deposited. In the case of ants, the latter applies to all parts of the exoskeleton.

Seta
Any stout hair-like cuticular process that is socketed basally. Generally, as here, the terms seta and hair are interchangeable, but care must be taken to differentiate between setae and pubescence, as the latter may also sometimes be referred to as hairs.

Spiracle
An orifice of the tracheal system by which gasses enter and leave the body. Adult ants have 9 or 10 spiracles on each side of the body.

The spiracles of the prothorax have been lost, so the first spiracular opening occurs on the mesothorax. This mesothoracic spiracle is situated forward and quite high on the side of the segment and is usually concealed from view by a backward-projecting lobe of the pronotum; only rarely is its orifice open and clearly visible. The metathoracic spiracle may be dorsal (especially in those workers where the metanotum forms part of the dorsal mesosoma), lateral and open, lateral but concealed by a small, sometimes detached, lobe of the mesopleuron (the epimeral sclerite); or the metathoracic spiracle may be absent. Abdominal spiracles are always on the tergite of each segment. The propodeal (first abdominal) spiracle is usually the largest on the body. Behind this, on the metasoma (A2 to apex), spiracles are always visible on abdominal segments 2–4, but those on abdominal segments 5–7 are frequently overlapped and concealed by the posterior margin of the preceding tergite. A spiracle is also present on abdominal tergite 8, but in female castes this sclerite is always concealed; it is internal and forms part of the sting apparatus (the spiracular plate).

Sulcus
Strictly, an external groove or impression that corresponds to an internal ridge-like inflection of the cuticle, which provides mechanical rigidity. The term is also casually used for any linear impression in the cuticle, without any obvious significance, and sulcus is often used interchangeably with suture.

Suture
Strictly, a line of junction between two structural sclerites. The suture may be articulated, where the component sclerites are linked by flexible intersegmental membrane and retain the ability to move relative to each other, or may be fused together and immobile. The term suture is often used interchangeably with sulcus.

Tagma
A fundamental unit of the body; an ancestral section of the body that is distinct from, or separated from, other body units in both form and function. In Insecta there are three ancestral tagmata: head, thorax and abdomen, but in ants the second and third of these have become much modified by secondary evolutionary developments.

Tentorium
See Endoskeleton.