Morphological Terms/Head

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Antenna

Plural: Antennae

The antenna in ants is made up of a number of discrete segments, the antennomeres (sing. antennomere). The antennomeres consist of an elongate basal segment, the scape (= scapus), that is followed distally by 3–11 shorter segments in workers and queens, and 8–12 in males, which together constitute the funiculus (= flagellum), giving a total antennal segment count (= antennomere count) of 4–12 in females and 9–13 in males. The scape and funiculus meet at an angle so that in life the entire antenna appears bent (geniculate) between the two sections. The basal (first) funicular segment, the one that articulates with the apex of the scape, is sometimes called the pedicel. The scape articulates with the head in the antennal foramen (= antennal socket), a foramen located dorsally on the head, posterior to the clypeus. The funicular segments may all be simple (= filiform), or the segments may gradually enlarge towards the apex (= gradually incrassate), or a number towards the apex may be expanded into a distinctly differentiated clava (= club) that is usually of 2–3 segments but sometimes may be more. When a club is present the antenna is said to be clavate or claviform.

The antennal foramen itself is encircled by a narrow annular sclerite, the torulus (= antennal sclerite, = annulus antennalis), and the socket may be overhung and concealed by the frontal lobe. At the base of the scape is a roughly ball-like condylar bulb (= articulatory bulb, = bulbus), which is the part of the scape that actually articulates within the socket. Just distal of the condylar bulb is a short constriction or neck, which may be straight or curved, distal of which the scape shaft proper commences.

Antennal fossa

Plural: Antennal fossae

The antennal fossa is a depressed area in the cuticle of the dorsum of the head, present in some ant taxa, that surrounds and contains the torulus and antennal foramen, which consequently appear to be somewhat sunk into the surface of the head.

Antennal scrobe

(= scrobe, = scrobis)

A longitudinal groove, impression or excavation in the side of the head, which may extend above, below, or in front of the eye, that can accommodate at least the antennal scape but sometimes may accommodate the entire antenna when scape and funiculus are folded together. Scrobes vary in development from simple broad, shallow grooves to extensive deep trenches, but they are absent from the majority of ant genera.

Antennal socket/foramen/insertion

See Antenna.

Antennomere

See Antenna.

Anterior tentorial pits

A pair of endophragmal pits or impressions located anteriorly on the dorsal surface of the head capsule, at or very close to the posterior clypeal margin (epistomal suture) and usually close to the antennal sockets. The pits are small invaginations of the exoskeleton that indicate the points of attachment of the anterior arms of the internal skeleton (tentorium) of the head to the head capsule. The termination of the posterior arms of the tentorium are marked by a pair of posterior tentorial pits, which are located close to the occipital foramen.

Buccal cavity

(= oral fossa)

The anteroventral cavity in the head capsule, which contains the labium and maxillae. It is bounded anteriorly by the labrum, posteriorly and laterally by the ventral cuticle of the head (the hypostoma or the genal bridge). Within the buccal cavity the median appendage is the labium, which is flanked by a maxilla on each side of it.

Bulbus

See Antenna.

Canthellus

See Trulleum.

Cardo

See Maxilla.

Cephalon

See Head.

Cervix

Strictly, the flexible intersegmental region between the head and the prothorax. It is usually shielded from above by a neck-like projection of the anterior pronotum, and from below by fused anterior extensions of the propleuron. Sometimes the anterior portion of the pronotum, that covers and protects the true cervix, is also termed the cervix, or the cervical portion of the pronotum.

Clava/ clavate/claviform

See Antenna.

Clypeus

Anterior sclerite of the dorsal head, bounded posteriorly by the epistomal suture, which is very commonly referred to as the clypeal suture, or posterior clypeal margin. The median section of the epistomal suture, immediately anterior to the frontal carinae and antennal sockets, is sometimes called the frontoclypeal suture. The anterior clypeal margin usually forms the anterior margin of the head in full-face view (but a projection of the labrum may be anterior to the clypeus in some taxa). The body of the clypeus consists of a pair of lateral portions, or narrow bands of cuticle, on each side of a shield-like median portion. The median portion of the clypeus may be equipped with one or more longitudinal carinae, or may be variously specialised in shape. Posteriorly, the median portion of the clypeus may end in front of the antennal sockets/frontal carinae or lobes, or may project backwards between them. In some taxa the clypeus is extremely reduced and very narrow from front to back, bringing the antennal sockets very close to the anterior margin of the head.

Condylar bulb

See Antenna.

Epistomal suture

See Clypeus.

Filiform (antenna)

With the antennal funiculus thread-like, the segments all of approximately the same width. The contrasting antennal form is clavate (club-like or clubbed), where the apical segments of the antenna are disproportionately enlarged.

Flagellum

(= funiculus)

See Antenna.

Frons

The area of the dorsal head posterior to the frontoclypeal suture, above the antennal sockets and including the median ocellus. As ocelli are so often absent in worker ants the posterior limit of the frons can be referred to only in a very vague way in these taxa. Ancestrally, the anterior ocellus arises on the posteriormost part of the frons, while the pair of posterior ocelli arise on the vertex.

Frontal carinae

Singular: Frontal carina

A pair of longitudinal cuticular ridges or flanges on the head, located dorsally behind the clypeus and between the antennal sockets. They are very variable in length and strength of development in the various ant taxa, frequently being short and simple but sometimes extending back to the posterior margin of the head. In some groups the frontal carinae are vestigial or absent, but elsewhere they may be very strongly developed or form the dorsal margins of extensive antennal scrobes. In many groups the frontal carinae anteriorly, especially between the antennal sockets, are expanded into laterally projecting lobate extensions, the frontal lobes, which overhang and partially to entirely conceal the antennal sockets themselves. Frontal lobes may be the only expression of the frontal carinae in some groups. Sometimes the portion of the torulus closest to the cephalic midline is also raised and expanded into a small lobe that projects laterally. This may be visible below the frontal lobe, or be fused to the frontal lobe.

Frontal lobes

See Frontal carinae.

Frontal triangle

(= supraclypeal area)

A small, usually triangular patch of cuticle located medio-dorsally on the head, immediately behind the clypeus and approximately between the antennal sockets or the anterior parts of the frontal carinae. Not apparent in many ant taxa and reduced or very narrow in some. In groups where the frontal lobes are medially very closely approximated, the frontal triangle may be compressed and longitudinal, appearing posterior to the frontal lobes as a narrow median sclerite.

Frontoclypeal suture

See Clypeus.

Funiculus

(= flagellum)

See Antenna.

Galea

See Maxilla.

Gena

Plural: Genae

Area of the side of the head which is bounded in front by the anterior margin of the head capsule, and extends to the posterior margin of the head, below the eye when eyes are present. In ants the gena is expanded ventrally and forms the extensive ventral surface of the head, the genal bridge.

Genal bridge

On the ventral surface of the head the space between the buccal cavity and the occipital foramen is occupied by an extensive area of cuticle, which is divided by a mid-ventral longitudinal line or groove. This surface is formed by the ventral expansion and mid-line fusion of the genae and is called the genal bridge. The extreme anterior margin of the genal bridge, that surrounds the buccal cavity, is the hypostoma.

Geniculate

Bent like a knee joint. The term is usually used to describe the shape of the ant’s antenna in life, where the funiculus is carried at a marked angle to the scape.

Glossa

See Labium.

Gula

Use of this term when referring to the ventral surface of the head capsule in ants is incorrect. Morphologically, the gula is a separated medioventral sclerite of the head, which is bounded anteriorly by the posterior tentorial pits. In consequence the posterior tentorial pits are distant from, and considerably anterior to, the occipital foramen. In ants the posterior tentorial pits are located adjacent to the occipital foramen; no gula sclerite is present.

Head

(= cephalon, = cranium, = prosoma)

The classical first tagma of the insect body. The head capsule is the result of the fusion of six (or perhaps seven) embryonic segments. In adult ants these segments are completely fused and indistinguishable, but most retain appendages that are conspicuous. The appendages of cephalic segment 1 are fused and form the labrum; the appendages of segment 2 are the antennae; the appendages of segment 3 are embryonic only, unrepresented in adults; the appendages of segment 4 are the mandibles, of segment 5 the maxillae, and of segment 6 the labium. Some morphologists argue the presence of a seventh embryonic segment, which would occur between 1 and 3 in the preceding list and bear the eyes as its appendages.

Hypostoma

The narrow U-shaped strip of cuticle immediately behind the buccal cavity, that forms its posterior and lateral margins on the anteroventral surface of the head. In ants the hypostoma is often indistinct, sometimes indiscernible, and usually merely forms the anterior margin of the genal bridge, to which it is entirely fused. However, in some taxa a narrow suture remains detectable between the hypostoma and the genal bridge.

Hypostomal teeth

One or more pairs of triangular or rounded teeth that project forward from the hypostoma, towards or slightly into the buccal cavity.

Labial palp

(= labial palpus; plural: labial palpi).

A sensory palp, with a maximum of four segments, that arises anterolaterally on each side of the prementum sclerite of the labium. A count of the number of segments in a maxillary palp and a labial palp, in that order, is called the palp formula.

Labium

With the head in ventral view the labium is a longitudinal appendage, situated medially within the buccal cavity; it is flanked on each side by a maxilla. The labium is formed from an ancestral pair of appendages, now indistinguishably fused together along the midline to form a single structure. The main sclerite of the labium is the prementum. Basal to the prementum, and attaching it to the head capsule, may be a small sclerite, the postmentum, but this area is frequently entirely membranous. The prementum bears a pair of labial palps, one at each side. At the distal end of the prementum is a lobe, the glossa, which may be simple or bilobed apically. The base of the glossa is sometimes flanked by a much smaller lobe on each side, the paraglossa, but in ants these are usually extremely reduced or absent. The glossae and paraglossae together are sometimes termed the ligula.

Labrum

Mouthpart sclerite that hinges on the anterior margin of the clypeus and usually folds back and down over the apices of the maxillae and labium, shielding them when the mouthparts are not in use. In most ants the labrum is a bilobed plate that is not visible in dorsal view, but in some taxa a part of it projects forward beyond the anterior clypeal margin even when the mouthparts are retracted. Occasionally it is modified into one or more long, prominent labral lobes.

Lacinia

See Maxilla.

Ligula

See Labium.

Malar area

(= malar space)

The area of the head capsule between the base of the mandible and the anterior margin of the eye. Strictly a part of the gena but sometimes its relative length is a useful concept in the taxonomy of some groups.

Mandalus

See Trulleum.

Mandibles

The appendages with which ants manipulate their environment. They are extremely variable in shape, size and dentition, and have great importance in ant taxonomy. Sometimes, but not always, the mandibles in the female castes are more strongly, or somewhat differently, developed than in the conspecific males.

Margins

In full-face view, with the mandibles closed, the longitudinal margin or border of each mandibular blade that is closest to an anterior extension of the midline of the head, is the masticatory margin (= apical margin), and is usually armed with teeth. The base of this margin, close to the anterior margin of the clypeus, usually passes through a basal angle into a transverse or oblique basal margin. The two margins may meet through a broad or narrow curve, or meet in an angle or tooth. When the mandibles are narrow or linear, the distinction between masticatory and basal margins may be lost by obliteration of the basal angle. The external margin (= lateral or outer margin) of each mandible forms its outer border in full-face view and may be straight, sinuate or convex.

In many groups the dorsal base of the body of the mandible bears a number of specialised structures, just distal to the mandibular articulation and proximal of the basal margin of the mandible, the mandalus, canthellus and trulleum; see under the last named for a discussion of these.

Shape

In the vast majority of ants the mandibular margins form a triangular or subtriangular shape in full-face view, but may be drawn out anteriorly while retaining the basic triangular shape and become elongate-triangular. In several discrete lineages the mandible has become linear; the blade is long and narrow and the external and masticatory margins are approximately parallel or taper gradually to the apex; the whole blade may be straight or curved. Linear mandibles may evolve in one of three ways.

  1. The base of the mandible narrows and the basal angle is obliterated, so that the basal and masticatory margins form a single margin.
  2. The masticatory margin becomes elongated and the basal margin contracted.
  3. The basal margin becomes elongated and the masticatory margin contracted.

Extremely curved mandibles, usually quite short and slender, and with few or no teeth on the masticatory margin, are termed falcate.

Dentition

The masticatory margin of each mandible is usually armed with a series of teeth or denticles (short or very reduced teeth), or a mixture of both, which generally extend the length of the masticatory margin. If teeth alone are present, or a combination of teeth and denticles, the mandible is dentate. If only tiny denticles occur on the margin the mandible is denticulate, and if the margin lacks any form of armament it is edentate. A natural gap in a row of teeth (as opposed to a site where teeth have been broken off or completely worn down) is a diastema (pl. diastemata) and an elongate mandible with an uninterrupted series of teeth may be described as serially dentate. Individual teeth are usually sharp and triangular in shape but may be rounded (crenulate), long, narrow and spine-like (spiniform), or peg-like (paxilliform). Reduced teeth or denticles that occur between full-sized teeth are termed intercalary.

In general the first, distalmost, or apical tooth, the one farthest away from the anterior clypeal margin, is the largest on the masticatory margin, although in some taxa median or even basal teeth may be the largest. The tooth immediately preceding the apical is usually called the preapical tooth (= subapical tooth). Sometimes the term preapical teeth may be loosely applied to all the teeth that precede the apical. The tooth immediately distal of the basal tooth may be termed the prebasal (= subbasal) tooth.

In some genera teeth may also be present on the basal margin of the mandible, but in most this margin is unarmed. Some taxa also have a basal lamella on the mandible. This is a thin cuticular outgrowth from the margin, proximal to any teeth that may be present.

Maxilla

Plural: Maxillae

With the head in ventral view the maxillae are situated within the buccal cavity, one on each side of the central labium. The basal segment of the maxilla, that attaches the structure to the head capsule, is the cardo. Attached distally to the cardo is the main sclerite of the maxilla, the stipes, which towards its apex bears a maxillary palp. The inner surface of the stipes bears a lobe towards its distal end, the lacinia, which usually has its free margin irregular, minutely dentiform, or even pectinate. At the distal apex of the stipes is a terminal hood or lobe, the galea, which often folds over the lacinia.

Maxillary palp

(= maxillary palpus; plural: maxillary palpi)

The segmented sensory palp on the maxilla, articulated to the stipes. The palp may have at most 6 segments but these are variously reduced in number in different ant groups; only very rarely are the maxillary palps absent. A count of the number of segments in a maxillary palp and a labial palp, in that order, is called the palp formula.

Mouthparts

The feeding appendages, located anteriorly and anteroventrally on the prognathous head capsule. The mouthparts consist of the mandibles, maxillae and labium, the last two are located within the buccal cavity, on the underside of the head capsule.

Nuchal carina

A ridge situated posteriorly on the head capsule that separates the dorsal and lateral surfaces (vertex and genae) from the occipital surface.

Occipital corners/ margin (of head)

See Posterior corners/margin.

Occipital foramen

(= foramen magnum)

The foramen located posteromedially in the head capsule, within which the membranous cervix articulates the head to the prothorax.

Occiput

(= occipital surface)

The posterior surface of the head capsule, immediately above the occipital foramen. The occiput is usually vertical or nearly so above the foramen, and is separated from the vertex of the cephalic dorsum by the transverse posterior margin of the head capsule.

Ocellus

Plural: Ocelli

A maximum of three simple, single-faceted eyes, which when present (absent in many worker ant taxa) are located in a triangle on the cephalic dorsum. Morphologically the anterior (median) ocellus marks the posterior limit of the frons, and the posterior (lateral) pair are on the vertex.

Ommatidium

Plural: Ommatidia

A single optical component (facet) of the compound eye.

Palp Formula (PF)

A standardised way of indicating the number of segments in the maxillary and labial palps. The number of maxillary palp segments is given first, the number of labial palp segments second. Thus PF 6,4 indicates that the maxillary palp has six segments, the labial palp four.

Paraglossa

See Labium.

Posterior tentorial pits

See Anterior tentorial pits.

Postmentum

See Labium.

Posterior corners/margin (of head)

With the head in full-face view, the rounded to acute posterolateral angles, where the sides of the head curve into the posterior margin; the latter extends transversely between the corners. Earlier frequently referred to as occipital corners and occipital margin, these terms are not strictly accurate because the true occipital surface (occiput) is not involved.

Prementum

See Labium.

Prognathous (head)

Among the ants the long axis of the head is horizontal or nearly horizontal, so that the head more or less continues the line of the long axis of the body, and the mouthparts, particularly the mandibles, are at the front of the head capsule. The ventral surface of the head has an extensive cuticular area (genal bridge) that widely separates the buccal cavity from the occipital foramen. This prognathous condition is in marked contrast to almost all the other families of Hymenoptera, where the long axis of the head is vertical or nearly vertical, the mandibles are ventral, and the buccal cavity and occipital foramen are closely approximated or even confluent; a condition termed hypognathous.

Because of the prognathous condition of the head, references to its orientation differ from what is usual in Hymenoptera. Apart from the mandibles being anterior, what is referred to as the frontal or anterior surface of the head elsewhere in the order, is dorsal in ants.

Prosoma

See Head.

Psammophore

A basket-like series of long, and usually stout, curved setae that arise from the ventral surfaces of the head and mandibles in some deserticolous ants, used for carrying sand grains. In some publications the setae of the psammophore are called ammochaete hairs.

Scape

See Antenna.

Scrobe/scrobis

See Antennal scrobe.

Stipes

Plural: Stipites

See Maxilla.

Supraclypeal area

See Frontal triangle.

Torulus

Plural: Toruli

(= antennal sclerite, = annulus antennalis)

The small annular sclerite that surrounds the antennal socket (antennal foramen). The torulus may be horizontal, or the part closest to the midline of the head may be elevated, in some taxa to such an extent that the antennal socket is almost vertical. The upper arc of the torulus may be indistinguishably fused to the inner wall of the frontal carina, or may remain discrete, or may even project laterally as a small torular lobe below the frontal carina. In some groups where the frontal carina is very slender, the torulus projects laterally beyond the outer margin of the frontal carina, and becomes visible in full-face view.

Trulleum

A basin-shaped depression near the dorsal base of the mandible, close to its articulation. It is bounded distally by the basal margin of the mandibular blade and, in those groups where it occurs, is visible just in front of the anterior clypeal margin when the mandibles are open. The trulleum in many groups is closed along its inner (medial) border by a raised ridge of cuticle, the canthellus. The canthellus may extend to, and even fuse with, the basal margin of the mandible (canthellus closed), or may fail to reach the basal margin (canthellus open). Proximal to both these structures, in the cuticle of the extreme dorsal base of the mandible, is a small, unsclerotised impression of variable shape, the mandalus. It contrasts strongly with the fully sclerotised surrounding cuticle of the mandibular base.

Vertex

The portion of the cephalic dorsum that lies immediately in front of the occiput. In those groups where ocelli are absent the area can be only vaguely defined, but in those which possess ocelli the vertex is the area from immediately behind the anterior ocellus to the occiput, that contains the posterior pair of ocelli. Ancestrally, the anterior ocellus is on, and marks the posterior limit of, the frons.