Pheidole Working Group - Characters

The goal here is not an exhaustive list of every possible character and character state, but rather a restricted set of primary characters that seem most useful for separating Pheidole species. These are being applied to a Lucid key to New World Pheidole.

There is also an excellent and more extensive description of Pheidole characters by Katsuyuki Eguchi. He made this document as part of the ANeT Ant Course held in Malaysia in 2005. It is designed for Indo-Malayan species but works very well for New World species too.

Antennal Count

The great majority of species have 12-segmented antennae (including the scape). A few species have 11, and a few have 10.

Measurements

A standard set of measurements go a long way toward narrowing the search when identifying Pheidole species.

Lucid keys handle measurement data with four values for each metric variable: outer minimum, inner minimum, inner maximum, outer maximum. Longino is building a dataset for New World Pheidole measurements. The data are a combination of the measurements listed in Wilson (2003) (received as a datafile from Piotr Naskrecki) and new measurements by Longino and students. At this point most species have single or few values for each variable, but Lucid needs bracketing or range values. Longino and students have used an indirect approach for obtaining the bracketing values. Two species, a large one and a small one, were characterized by measuring many replicates. We obtained the Coefficient of Variation (CV, ratio of standard deviation [SD] to mean) for each species and each variable. Results were similar for the large and small species, suggesting CV is stable with size. Many measurement variables had CV between 5-10%, and these were classified as having CV = 10%. Some measurements were more variable, with CV in the 10-20% range, and these were classified as having CV = 20%. Indices CI and SI were less variable and were classified as having CV = 5%. For each species, the single value available (or the mean of the few values available) was multiplied by 0.05, 0.10, or 0.20 (depending on variable) to obtain SD. The outer bracketing values were set at plus or minus 2 SD, the inner at plus or minus 1 SD. Lower bracketing values less than or equal to 0.01mm were set to 0.

The following measurements apply to minor and major workers:

HL: head length; in full-face view, maximum length of head, measured along midline, from line tangent to anteriormost projection of head capsule or clypeus to line tangent to posteriormost projection of vertex margin (including nuchal collar, if present).

HLA: anterior head length; in full-face view, perpendicular distance between two horizontal lines, one tangent to anteriormost projection of clypeus, one tangent to lowermost margin of compound eye.

HW: head width; in full-face view, maximum width of head capsule not including eyes (if eyes protrude beyond margins of head, measured above or below eyes, depending on which is widest).

SL: scape length; length of scape shaft from apex to basal flange, not including basal condyle and neck.

EL: eye length; maximum length of compound eye.

ML: mesosoma length; in lateral view, distance from base of anterior face of pronotum (at inflection point between downward-sloping anterior face and flange-like anteriormost projection of pronotum [the latter extending to foramen and usually partially hidden by head capsule]) to posteriormost extension of metapleural or propodeal lobes.

PSL: propodeal spine length; distance from inflection point between dorsal face of propodeum and base of spine to tip of spine.

PMG: depth of promesonotal groove; measured by establishing a line tangent to dorsalmost convexities of pronotum and mesonotum, measuring perpendicular distance from this line to bottom of promesonotal groove.

SPL: propodeal spiracle width, measured from the outer edge of cuticular ring that surrounds orifice.

PTW: petiole width; maximum width of petiole in dorsal view.

PPW: postpetiole width; maximum width of postpetiole in dorsal view.

The following measurements apply to major workers only

IHT: distance between inner hypostomal teeth.

OHT: distance between outer hypostomal teeth.

When hypostomal teeth are curved laterally (either inward or outward), the approximate midpoint of the base of the tooth is used, not the tip.

Measurement indices

Indices can narrow a search more than knowing just the ranges of the variables that make up the index, as illustrated in the figure below. Knowing just the ranges of two variables circumscribes a box. Knowing just the ranges of an index of the two variables circumscribes the area between two lines. Knowing both circumscribes the area in gray, a smaller subset of morphology space.

CI: cephalic index; 100*HW/HL.

SI: scape index; 100*SL/HW.

PSLI: propodeal spine index; 100*PSL/HL.

PMGI: Promesonotal groove index; 100*PMG/HL.

SPLI: Propodeal spiracle index; 100*SPL/HL.

PPI: Postpetiole index; 100*PPW/PTW.

HTI: Hypostomal tooth index; 100*IHT/OHT.

Major worker hypostomal teeth

The ventral surface of the head in ants is formed by the genal bridge. In major workers, the anterior margin of the genal bridge forms a nearly right angle, and a thin, shelf-like hypostomal sclerite extends inward to the buccal cavity. The juncture of the genal bridge and the hypostomal sclerite is the hypostomal margin. There are nearly always a pair of teeth at the inner margin of the hypostomal sclerite, bordering the bases of the mandibles. These are the Outer Hypostomal Teeth. Less regularly present are a pair of teeth on the hypostomal margin itself. When present, these teeth are always more closely spaced than the outer teeth, and are called the Inner Hypostomal Teeth. There may also be a median tooth on the hypostomal margin. The median tooth, when present, often seems intraspecifically variable in size and distinctness. The following figures illustrate hypostomal teeth.

Above: hypostomal margin of Pheidole fiorii, viewing ventral surface of head capsule perpendicular to long axis of head. In this species, the inner hypostomal teeth are widely spaced, closer to the outer teeth than to the midline.

Above: Pheidole fiorii in anterior view, perpendicular to hypostomal sclerite.

Above: hypostomal margin of Pheidole amabilis, a species with more closely spaced inner hypostomal teeth.

Above: Pheidole amabilis in anterior view.

Above: hypostomal margin of Pheidole ajax, a species lacking inner hypostomal teeth.

Above: Pheidole ajax in anterior view.

Minor worker, characters of the head

Minor worker occipital carina

The occipital foramen is surrounded posteriorly and laterally by a differentiated cuticular rim: the occipital carina (Gauld and Bolton 1988) [in some Old World species the carina extends anteriorly as the genal carina]. In most species the occipital carina is not visible in full-face view. In some cases it is visible as a narrow rim. But in a few species it is highly developed as a flaring collar that encloses (and presumably protects) the articulation of the head and mesosoma. In these cases it is easily visible in face view. It is termed the "nuchal collar" in Wilson (2003). For purposes of a genus-wide key, species are divided into two categories: those with strongly flaring occipital carina and those with occipital carina small or invisible. Most species can be placed unambiguously in one group or the other, but it is a continuum with some intermediate species. The intermediate species are scored so that they can be interpreted either way. Further refinement of characters associated with the occipital carina and the posterior shape of the head in face view permit additional separation of closely similar species, but are not used for the genus as a whole.

Above: weakly developed occipital carina.

Above: strongly developed occipital carina.

Minor worker face sculpture

Minor face sculpture is highly variable, in some cases intraspecifically, but there are two common patterns: completely smooth and shining, and uniformly foveate (foveae are small pits). Less often the face is rugose. Often faint rugulae may overlie foveolate sculpture. For purposes of a genus-wide key, species are divided into three categories: smooth and shining, uniformly foveate, and other. Images below show the two common patterns and a sampling of "other."

Upper left: face smooth and shiny. Upper right: uniformly foveate.

Upper left: foveate with shiny patches. Upper right: shiny with arcing striae.

Upper left: foveate with weak rugae. Upper right: foveate with stronger rugae.

Upper left: foveate with even stronger rugae. Upper right: strongly reticulate rugose over shiny surface.

Minor worker, characters of the mesosoma

Pronotal Tubercles

The anterolateral pronotal margins (the "shoulders") are usually smoothly rounded. This condition grades into various degrees of development of pronotal tubercles or spines. For the key, species are categorized as with or without pronotal tubercles, recognizing that some species are intermediate. These species can be scored either way.

Above: oblique view of minor worker mesosoma, showing pronounced pronotal tubercle.

Sculpture on Side of Pronotum and Katepisternum

The side of the mesosoma may be uniformly foveate, or there may be smooth shiny patches on the side of the pronotum, on the katepisternum, or both. These sculptural features may also be overlain with rugae of varying distribution and strength. For the key, the lateral pronotum and katepisternum are treated as two different characters. Each may be scored as uniformly foveate, foveate with a median shiny patch, completely smooth and shining, or "other." Other refers to the presence of rugae or any other sculptural condition that does not fit one of the first three.

Above: pronotum uniformly foveate, katepisternum foveate with median shiny patch.

Above: pronotum foveate with median shiny patch, katepisternum uniformly foveate.

Above: pronotum and katepisternum uniformly foveate.

Pronotum entirely smooth and shining, katepisternum uniformly foveate.

Dorsal Pilosity

Dorsal pilosity varies greatly among species and is an excellent character for species-level distinctions. However, it varies so continuously among species that it is difficult to establish discrete categories, and it is not used in the genus-wide key.

Above: dorsal pilosity abundant, long, flexuous.

Above: dorsal pilosity abundant, short, woolly.

Above: dorsal pilosity abundant, short, stiff, forming stubble.

Above: dorsal pilosity sparse, short, stiff.

Above: hairs are occasionally be branched at the tip.

Tibial Pilosity

The pilosity of the hind tibia (mid tibia often or always the same), often overlooked, provides excellent species-level characters. The tibia may appear completely bare, with very short, fully appressed pubescence and no erect setae. It may be densely clothed in short, subdecumbent to suberect pubescence that appears very uniform in length, and with no erect setae. It may have one of the above states of underlying pubescence, but in addition several long erect setae (presumably sensory). It may be covered entirely with long erect setae, with no differentiation of long setae and underlying pubescence. It may have an underlying pubescence grading into suberect setae of varying length, so that it blurs the distinction among the previously described conditions. Tibial pilosity sometimes very clearly differentiates species that are otherwise difficult to tell apart. For the genus-wide key, tibial pilosity is divided into two categories: (1) lacking erect setae, appearing bare or with short pubescence of uniform length, or (2) with erect setae, which can be few in number over differentiated pubescence, or abundant and long, or of irregular lengths.

Minor worker, characters of the metasoma

Ventral Margin of Postpetiole

Above: ventral margins of postpetiole. The ventral margin of the postpetiole is usually flat (above center), although there may be varying degrees of an anterior shelf or tooth (above right). But a distinctive condition is to have much of the ventral surface bulging and evenly convex (above left). In the key, species are scored as having the uniformly bulging condition (above left) present or not.

Sculpture on Metasomal Dorsum

A common condition is for the first gastral tergum (fourth abdominal tergum) to be completely smooth and shining. In some species a portion of the tergum is sculptured, usually a shagreening (very fine granular microsculpture) that makes the surface dull instead of shining. The entire tergum may be sculptured, or some portion anteriorly, grading to smooth and shining posteriorly. The sculpture may be faint, requiring that the specimen be tilted at certain angles to see it. Also, one can be fooled by dirty specimens with surface films covering the gaster. For example, when specimens are collected with greasy baits like tuna or cookies they may acquire a grease layer that obscures surface sculpture.

For the genus-wide key, gastral sculpture is divided into two categories: (1) completely smooth and shining, or (2) other. Other comprises any surface sculpturing.

Major worker, characters of the head

Antennal Scrobes

Scrobes are depressions or grooves beneath the scapes. Some majors have evenly convex faces, with no hint of a scrobe. Many species have shallow scrobes, ranging from a barely discernable flattening beneath the scape to a conspicuous depression. Often the sculpture in a scrobe is weaker than the sculpture around it, to the extent of forming a smooth shiny patch surrounded by foveate or rugose sculpture. In some cases the scrobe is very strongly developed, forming a deep channel with distinct medial and lateral margins, such that the scape can be completely recessed and protected within it.

For the genus-wide key, the antennal scrobe character is divided into two categories: (1) scrobe absent to moderately developed, or (2) scrobe strongly developed, forming a deep channel.

Above: antennal scrobe shallow, weakly developed.

Above: antennal scrobe deep, forming margined channel.

Face Sculpture

The face sculpture of Pheidole majors varies dramatically among species and is an important species-level character. However, it is difficult to quantize into discrete character states at a genus-wide level and so is not included in the genus-wide key.

Pilosity on Sides of Head

When in full-face view, the appearance of pilosity on the sides of the head is often of species-level diagnostic value. Some species show no projecting pilosity at all. Some have dense decumbent short or long pubescence. Some have abundant long or short erect setae. These conditions are continuously variable and are not used in the genus-wide key.

Antennal Scape Base

The scape base may be terete (round in cross section) or strongly flattened, and it may be gently or strongly curved where it attaches to the head. These characters vary continuously and are not used in the genus-wide key.

Transverse Face Excavation

Above: major worker showing deep transverse excavation on face. There are intermediate cases, and some species have a strongly bulging anterior head and strongly compressed posterior head. In scoring the key, only species with a strongly pronounced excavation, such that there are both anterior and posterior convexities, are recorded with the trait present.

Face Phragmotic

Above: major worker with phragmotic head. Phragmosis is the use of a body part to form a plug with which to block a nest entrance.

Species with phragmotic soldiers have the front of the head semi-cylindrical and abruptly truncate anteriorly. The mandibles, clypeus, and part of the anterior head capsule form a single disk.

Major Worker, Characters of the Mesosoma

Tibial Pilosity

Tibial pilosity of major workers usually but not always parallels that of the minor workers. For the genus-wide key, tibial pilosity is divided into two categories: (1) lacking erect setae, appearing bare or with short pubescence of uniform length, or (2) with erect setae, which can be few in number over differentiated pubescence, or abundant and long, or of irregular lengths.

Major Worker, Characters of the Metasoma

Shape of Postpetiole

The postpetiole in dorsal view assumes a variety of shapes. In some cases the sides are smoothly rounded, in some the shape is trapezoidal, and in others the sides are produced as acute, angular projections. These characters grade into each other and are partially captured by PPI (Postpetiole index). Postpetiole shape is not used in the genus-wide key.

Ventral Margin of Postpetiole

Above: ventral margins of postpetiole. The ventral margin of the postpetiole is usually flat (above center), although there may be varying degrees of an anterior shelf or tooth (above right). But a distinctive condition is to have much of the ventral surface bulging and evenly convex (above left). In the key, species are scored as having the uniformly bulging condition (above left) present or not.

Sculpture on Metasomal Dorsum

Gastral sculpture is like the minor workers. In many cases the first gastral tergum is completely smooth and shining. In others, part or all of the tergum is shagreened or foveolate. When partially sculptured, the sculpture is anterior, grading to smooth and shining posteriorly.

For the genus-wide key, gastral sculpture is divided into two categories: (1) completely smooth and shining, or (2) other. Other comprises any surface sculpturing.

Pilosity on Metasomal Dorsum

Pilosity on the metasomal dorsum is highly variable. Some species have the first gastral tergum entirely bare and with sparse, short, fully appressed pubescence. Others have dense long subdecumbent pubescence and no erect setae. The majority of species have erect setae of some form, either short and stiff or long and flexuous. Metasomal pilosity has great species-level diagnostic value, but is not used in the genus-wide key.

Workerless Social Parasites

A few species are workerless social parasites, with no known worker caste. Selecting any character for a worker will eliminate the workerless parasites from the key.