Morphological Terms/Metasoma

Abdomen
The classical third tagma of the insect body. The abdomen in ants consists of ten segments, of which the first seven (A1–A7, from front to back) are visible in the female castes (workers and queens), while A8 is also exposed in males. The tergites of segments A1–A8 each bears a spiracle, which may be exposed or concealed. In the female castes, segments A8 and A9 are desclerotised, internal, and form parts of the sting apparatus, so that A7, because it is always the last visible segment, is usually referred to as being apical. In males, A8 is exposed but A9 is usually retracted and is the gonosomite, having the male genitalia attached to its posterior margin. Segment A10 is reduced in both sexes, at most a simple tergal arc of cuticle. In males, tergite 10 is sometimes fused to tergite 9 (= syntergite), and in many groups the sclerite bears a pair of pygostyles (= cerci) apically.

The terminologies used to describe the ant abdomen may at first seem confusing. This is because two different systems tend to superimpose, and in places they are not strictly compatible.
 * 1) A terminology based strictly on morphology, which simply numbers the visible abdominal segments from front to back. This has the clear advantage of indicating homologous segments between different ant taxa, regardless of the specialisations of individual segments or groups of segments in different groups of ants.
 * 2) A more casual terminology, based on observed subdivisions of the abdominal segments, which names various specialised segments or groups of segments. The advantage here is that the subdivisions are generally easily visible.

The first abdominal segment (A1) is the propodeum, represented only by its tergite (the sternite has been lost), which is immovably fused to the thorax. The body unit formed by the fusion of thorax and propodeum is termed the mesosoma (in some publications called the alitrunk or truncus, or uncommonly and inaccurately just the thorax).

The second abdominal segment (A2) is termed the petiole, and is always specialised. It is usually reduced in size, always separated from the preceding propodeum by a complex narrow articulation, and is usually separted from the following abdominal segments by at least a constriction. In the vast majority of ants the petiole is distinctly isolated both anteriorly and posteriorly.

Abdominal segments 2 (petiole) to the apical are sometimes collectively called the metasoma (to contrast with the mesosoma = thorax + propodeum). Thus the petiole (A2) may also be referred to as the first metasomal segment, A3 the second metasomal, and so on.

Abdominal segment 3 (A3) is termed the first gastral segment when it is full-sized and broadly articulated to the following segment (A4), but when reduced and distinctly isolated it is commonly called the postpetiole. Abdominal segment 3 articulates with the preceding petiole by means of the helcium, which itself is formed from the reduced and specialised presclerites of A3, which fit within the posterior foramen of A2 (petiole). The anterior surface of the sternite of A3 may bear a cuticular prora, below the helcium.

The one or two isolated segments that follow the mesosoma may be called the waist. An older term, pedicel, should be abandoned, as it is used universally elsewhere in the Hymenoptera for the first funicular (= second antennal) segment.

Abdominal segment 4 (A4) is the first gastral segment when the waist consists of petiole plus postpetiole, but A4 is the second gastral segment when the waist consists of the petiole alone. Abdominal segments 3 to the apex (when petiole (A2) alone is separated), or A4 to the apex (when petiole and postpetiole (A2 and A3) are separated), are collectively called the gaster, the apparent enlarged “abdomen” that comprises the terminal part of the body.

Each abdominal segment behind the first (propodeum) consists of a pair of sclerites (plates), a dorsal tergite (or tergum) and a ventral sternite (or sternum). These may all be similar, or some may be specialised by reduction, fusion, or subdivision into anterior (presclerite) and posterior (postsclerite) portions that are separated by a constriction (cinctus). Tergites and sternites may be referred to as abdominal or gastral, depending on whether an absolute count, or a count relative to the number of separated waist segments, is used. In workers and queens, the last visible tergite, that of A7, is named the pygidium, and its corresponding sternite is the hypopygium. They have individual names because in some groups of ants one or both may exhibit a specialised morphology. In males the sternite of A9 is called the subgenital plate (= hypandrium, = hypopygium) as it shields the genital capsule ventrobasally.

Acidopore
The orifice of the formic acid projecting system peculiar to, and diagnostic of, the female castes of subfamily Formicinae. The acidopore is formed entirely from the apex of the hypopygium (sternite of A7). Often it is plainly visible as a short nozzle, generally with a fringe of short setae (coronula) at its apex. However, in some genera there is no nozzle or setae and the acidopore takes the form of a semicircular to circular emargination or excavation in the apical hypopygial margin. In these taxa the posterior margin of the pygidium may overlap and conceal the acidopore when it is not in use, but the structure is revealed if the pygidium and hypopygium are separated.

Cercus
See Abdomen.

Cinctus
See Girdling constriction.

Fenestra
In general, a thin spot or translucent spot anywhere in the cuticle. In the wing venation, fenestra is the name applied to a translucent spot or apparent break in a vein which indicates the point at which a flexion line or fold line traverses the vein. In the most generalised ant wings, such fenestrae occur in Rs·f2, 2rs-m, cu-a, and Cu2 of the forewing, and in 1rs-m and cu-a of the hindwing.

Gaster
A useful convenience term for the swollen apical portion of the body that forms the apparent “abdomen”. Morphologically the gaster consists of abdominal segments 3 to apex when the waist is of a single isolated segment (petiole, = A2), but of abdominal segments 4 to apex when the waist is of two isolated segments (petiole plus postpetiole, = A2 plus A3). In the second circumstance, the gaster, when of segments A4 to apex, may also be termed the opisthogaster.

Girdling constriction
(= cinctus)

A constriction or sudden and marked narrowing of an abdominal segment, which usually extends around the entire circumference. For convenience it is usually stated in keys that girdling constrictions are present between two segments. This is not strictly correct as the constriction morphologically represents the separation between the presclerites and postsclerites of the more posterior segment. The greater part of the presclerites are usually inserted in the posterior end of the preceding segment and are concealed, leaving only the constriction and postsclerites visible externally.

Helcium
The very reduced and specialised presclerites of abdominal segment 3, which form a complex articulation within the posterior foramen of the petiole (A2). In general the helcium is mostly or entirely concealed within the posterior foramen of the petiole, but in some groups it is partially visible.

Hypopygium
The sternite of abdominal segment 7 in workers and queens; the terminal visible gastral sternite in the female castes. In males the sternite of abdominal segment 9 is sometimes also referred to as the hypopygium, but subgenital plate (or hypandrium) is a better alternative term as it avoids the confusion caused by two non-homologous sclerites having the same name.

Metasoma
A collective term for abdominal segments 2 to apex, regardless of the absence or presence of abdominal constrictions that may occur between any of these segments.

Metasternal pit/ Metasternal process
The ventral surfaces of the mesothorax and metathorax each have an endophragmal pit, located on the midline anterior to the level of the coxal cavities. These pits mark the sites of attachment of the endoskeletal mesendosternite and metendosternite to the exoskeleton. In many groups of ants the pits are associated with a pair of cuticular projections, the mesosternal and metasternal processes. In most groups of ants the metasternal pit is distinctly anterior to the apex of the propodeal foramen, but in taxa where the foramen is extensive the pit may be extremely close to its apex.

Metathoracic spiracle
See Spiracle.

Metathorax
The third segment of the thorax, attached anteriorly to the mesothorax and dorsally and posteriorly to the propodeum, and bearing the metathoracic (third, hind) pair of legs and usually the second spiracle, though this may be lost in some groups. In alate (winged) forms it also bears the posterior (hind) pair of wings.

The tergite of the metathorax is the metanotum. This sclerite is usually distinct in alates but is extremely variably developed among workers of the various groups of ants. At its fullest development in workers the metanotum is a distinct transverse sclerite between the mesonotum and propodeum, separated from each by a transverse suture, the mesometanotal suture anteriorly and the notopropodeal suture posteriorly. In workers there is a very common morphoclinal reduction from this condition, which sees the metanotum become gradually shorter, until it is represented only by a narrow transverse groove (metanotal groove), in which the true metanotum is represented only by the exteme base of the groove. In some groups even this groove is obliterated, so that the posterior margin of the mesonotum meets the anterior margin of the propodeum. This junction may be indicated by a feeble transverse impression (notopropodeal suture), or the sclerites may become fully fused, forming a notopropodeum. Conversely, the metanotum may remain present on the dorsum but become indistinguishably fused to the mesonotum, while retaining a strong suture between itself and the propodeum.

Laterally, the metapleuron in workers forms an oblique sclerite that is usually roughly triangular. This is often separated from the mesopleuron by the mesometapleural suture, though in some the two sclerites are completely fused and no trace of a suture remains. Similarly, the dorsal junction of the metapleuron with the propodeum may be indicated by a suture, but again the two may be entirely fused to form a single sclerite, with no trace of a suture between them. In alates the metapleuron is usually more extensive. It is commonly divided by a short transverse sulcus into an upper metanepisternum, below the articulation of the hindwing, and a lower metakatepisternum. Close to the dorsal apex of the metapleuron is the metathoracic spiracle, which in alates is located between the mesopleuron and metapleuron and often shielded or concealed by the epimeral sclerite. Among workers, in groups where the metanotum forms a discrete dorsal sclerite the spiracle is usually located dorsally or laterodorsally, on the side adjacent to the metanotum. Elsewhere, in workers where the metanotum is greatly reduced or absent, the spiracle is lower down on the side. It is sometimes open, sometimes concealed beneath the epimeral sclerite, and in some groups the spiracle has been lost. The posteroventral corner of the metapleuron, above the metacoxa, has the bulla and orifice of the metapleural gland.

The ventral surface of the metathorax consists entirely of the pleurites, which have expanded across to the ventral midline and fused. The ancestral hymenopterous sternite of the metathorax is internal and represented by the metendosternite. The ventral metathorax commences anteriorly in a curved suture immediately posterior to the mesocoxal cavities and mesosternal pit. On the ventral midline of the metathorax, anterior to the metacoxal cavities, is a metasternal pit, an endophragmal pit that marks the attachment of the metendosternite to the exoskeleton. This pit is sometimes accompanied by a paired, cuticular, metasternal process. The metacoxal cavities are located close to the posterior corners of the ventral surface, and the often extensive propodeal foramen, within which the petiole (A2) articulates, extends forward between them.

Notum
The name applied to any one of the three ancestral thoracic tergites. Hence, pronotum is the notum of the prothorax, mesonotum of the mesothorax, and metanotum of the metathorax. In all worker ants each notum is a single sclerite, but in alate forms the mesonotum is usually subdivided. As the worker caste is derived ultimately from an alate female caste, the simple nota of the workers represent a secondary reversal to a more generalised condition.

Opisthogaster
See Gaster.

Parapsidal groove
See Mesothorax.

Pedicel
An archaic term used in ants for the isolated body segments between mesosoma and gaster, namely the petiole (A2), or petiole plus postpetiole (A2 plus A3). Use of the term pedicel is no longer recommended in this sense, as it is used elsewhere throughout the Hymenoptera as the name for the first funicular segment of the antenna. Abandonment of pedicel as a name for part of the abdomen brings ant morphological terminology into line with the remainder of the Hymenoptera.

Peduncle
(of petiole, = A2)

The relatively slender anterior section of the petiole which begins immediately posterior to the propodeal-petiolar articulation and extends back to the petiolar node or scale. It is very variable in length and thickness, but when present in any form the petiole is termed pedunculate. When a peduncle is absent, so that the node or scale of the petiole immediately follows the articulation with the propodeum, the petiole is termed sessile. If an extremely short or poorly defined peduncle occurs, the petiole is termed subsessile.

Petiole
(= abdominal segment 2 (A2), = first metasomal segment)

Morphologically the second abdominal segment (A2), the segment that immediately follows the mesosoma. Anteriorly, it is always articulated within the propodeal foramen of the mesosoma. Generally the petiole takes the form of a node (nodiform) or of a scale (squamiform) of varying shape and size, but in some taxa it may be very reduced, represented only by a narrow subcylindrical segment that may be overhung from behind by the gaster. The petiole bears the second abdominal spiracle and usually consists of a distinct tergite and much smaller sternite. The tergite may have a differentiated laterotergite, low down on each side and abutting the sternite. The sternite often has a specialised, depressed area anteroventrally, close behind the articulation, that is equipped with numerous, short sensory hairs: the proprioceptor zone. In some groups the tergite and sternite of the petiole are immovably fused together (tergosternal fusion).

Plectrum
See Stridulatory system.

Postpetiole
(= abdominal segment 3 (A3))

A convenience term for what is morphologically the third abdominal segment when it is reduced in size and markedly separated from the petiole (A2) anterior to it and from the fourth abdominal segment (A4) posterior to it.

Postsclerite/poststernite/posttergite
See Presclerite.

Presclerite
A distinctly differentiated anterior section of an abdominal sclerite (tergite or sternite) that is separated from the remaining posterior portion of the sclerite by a constriction, a ridge, or both.

In abdominal segments 3–7 (8 in males) it is usual for the posterior portion of each segment to overlap the anterior portion of the following segment. The overlapped area usually lacks sculpture and pilosity, but the absence of these features alone does not constitute a presclerite: there must be a constriction or ridge that separates the two zones. Presclerites derived from tergites are termed pretergites; those from sternites, presternites. The remainder of each sclerite, posterior to the constriction or ridge, is the postsclerite, posttergite dorsally and poststernite ventrally. A marked constriction that separates presclerites from postsclerites and extends around the entire circumference of a segment is a girdling constriction (= cinctus). The presclerites of abdominal segment 3 are reduced and form a specialised articulation within the posterior foramen of segment A2 (petiole), the helcium.

Presternite/Pretergite
See Presclerite.

Prora
A cuticular process or prominence that projects forward from the anterior surface of abdominal sternite 3, below the helcium. Absent in many groups; when present it takes the form of a U-shaped ridge of cuticle, a tubercle of very variable size, or a distinct prow. In some specialised taxa the prora has become inserted between the tergal apices of the helcium and forms part of the articulation.

Prosternite
See Sternite/sternum.

Pygidium
The tergite of abdominal segment 7 in workers and queens; the terminal visible tergite of the abdomen in these castes.

Pygostyles
See Abdomen.

Sessile (petiole)
See Peduncle.

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).

Sternite/sternum
The lower or ventral sclerite of a segment (the tergite is the upper sclerite on the thoracic segments and the abdomen; the pleurites are the lateral sclerites on the sides of the thorax). The sternite may be a simple, flat or curved plate, or may be specialised or subdivided on some segments. On the prothorax the sternite (prosternum) is small, but visible in ventral view (see prothorax). The sternites of the mesothorax and metathorax are internal (mesendosternite and metendosternite, respectively), the ventral surfaces of these two segments being composed of extensions of the pleurites to the ventral midline, where they fuse (see mesothorax and metathorax). The sternite of the propodeum (A1) has been lost in the course of evolution, but those of the remaining visible abdominal segments are usually distinct, although the lateral margins of some may be difficult to discern because of fusion to the tergite (tergosternal fusion). The sternites of A8 and A9 are membranous in the female castes, internal, and associated with the sting apparatus. In males the sternites of A8 and A9 are visible, and that of A9 is generally called the subgenital plate. Abdominal sternites are usually simple, but may be subdivided or otherwise specialised. The most common modification applies to abdominal sternites 3 and 4 (uncommonly also to A5 and A6), where distinct presternites (see under presclerites) may be differentiated.

Stridulatory system
A sound-producing system present in the female castes of a number of ant subfamilies. The system consists of a plectrum (= stridulatory file), located on the posterior margin of abdominal segment 3 (usually, but not always, on the tergite), and an extremely finely grooved stridulitrum on the anterior portion of abdominal segment 4. Rapid to-and-fro movement of the plectrum along the stridulitrum produces a range of chirping or buzzing sounds.

Stridulitrum
See Stridulatory system.

Subgenital plate
See Abdomen.

Subpetiolar process
A ventral cuticular projection of the sternite of the petiole (A2), either below the node or on its anterior peduncle; sometimes absent but when present very variable in shape and size.

Subsessile (petiole)
See Peduncle.

Tergite/tergum
The upper sclerite of a segment (the sternite is the lower, the pleurite the lateral on the thorax). The tergite may be a simple flat or curved plate, or may be specialised or subdivided on some segments. In terms of comparative morphology each of the three ancestral dorsal plates of the thorax, one for each segment, is termed the notum (pl. nota). Thus, the tergite of the prothorax is composed entirely of the pronotum. This sclerite is hypertrophied in worker ants and extends across the dorsum and down both sides of the segment, mostly or entirely concealing the propleuron (see prothorax). The mesonotum, tergite of the mesothorax, may be separated from the pronotum by the promesonotal suture, or the pronotum and mesonotum may be fused by obliteration of the suture in some workers, to form a single sclerite, the promesonotum. In alate forms the mesonotum is subdivided (see mesothorax). The metanotum, tergite of the metathorax, is usually present across the dorsum as a distinct sclerite in alates, but is frequently reduced and sometimes entirely lost in workers. When the metanotum is extremely reduced, the mesonotum and propodeum are only separated by the metanotal groove, a transverse impression whose base represents the very last vestige of the metanotum on the dorsum (see metathorax). The propodeum is the tergite of the first abdominal segment (A1). The remaining visible abdominal segments (A2–A7 in females, A2–A8 in males) have tergites that are usually simple, but may be subdivided or otherwise specialised. The most common modification applies to abdominal tergites 3 and 4 (uncommonly also to A5 and A6), where distinct pretergites and posttergites (see under presclerites) may be differentiated. In general the abdominal tergites are free and attached to their respective sternites by flexible intersegmental membrane, but in some groups there is tergosternal fusion in segments A2 (petiole), A3 and A4. The petiole (A2) in some groups has a small lower section of the tergite split off from the main part of the sclerite by a distinct suture, on each side, where they flank the sternite. These are called laterotergites, and in some taxa these sections of the tergite are mobile, with repect to both the remainder of the tergite and also the sternite.

Tergosternal fusion
A condition of the abdomen where the tergite and sternite of a single segment fuse together. This may occur in some or all of segments A2 (petiole), A3 and A4; posterior to A4 there is never tergosternal fusion. The absence or presence of tergosternal fusion varies throughout the Formicidae, but in individual subfamilies it tends to be fairly consistent for most of the segments. For example, in workers tergosternal fusion is distributed as follows through the various subfamilies.

Waist
An informal collective term for the one or two isolated and reduced abdominal segments that occur between the mesosoma and gaster. When only the petiole (A2) is isolated the waist is said to be one-segmented, but in those taxa where the postpetiole (A3) is also separated the waist is said to be two-segmented.