Morphological Measurements

Size and shape characters are quantified and reported as lengths or indices. Measurements are normally recorded in thousandths of a millimetre but reported to the nearest hundredth as a range from minimum to maximum across all measured specimens.

Within ants there are a "core" set of measurements and indices that are used across most groups. In addition there are specialised measures for structures and features that are only found in certain taxonomic groups. These are listed separately below.

Core Measurements

• EL: Eye length measured in one of two ways: (1) Maximum eye length measured along the maximum diameter or (2) maximum eye length measured in full face view.
• EW: Measured one of two ways:
  • Maximum eye width.
  • Maximum eye width measured in full face view.
• HL: The length of the head capsule excluding the mandibles, measured in full face view in a straight line from the mid-point of the anterior clypeal margin to the mid-point of the posterior margin. In species where one or both of these margins is concave the measurement is taken from the mid-point of a transverse line that spans the apices of the projecting portions.
• HW: The maximum width of the head in full face view, excluding the eyes. (Maximum width is usually posterior to level of eyes. Lateral tubercles and projections are generally ignored.)
• MandL. The straight line length of the mandible at full closure, measured in the same plane for which the HL measurement is taken (i.e. full face view), from the mandibular apex to the anterior clypeal margin, or to the transverse line connecting the anterior-most points in those taxa where the margin is concave medially.
• ML (Mesosomal or Weber's length). The diagonal length of the mesosoma in profile from the point at which the pronotum meets the cervical shield to the posterior basal angle of the metapleuron.
  • Note that often the posterior point of measurement for ML is the propodeal lobes rather than the metapleuron.
• PronW. The maximum width of the pronotum in dorsal view. (Projecting spines, tubercles or other cuticular prominences at the pronotal humeral angles are ignored.)
• SL: The maximum straight-line length of the scape, excluding the basal constriction or neck that occurs just distal of the condylar bulb. (In taxa with a hypertrophied subbasal lobe on the scape SL is measured from the tip of the subbasal lobe to the scape apex.)

Core Indices

Note that in about 70% of cases indices are multiplied by 100 (giving a range of 0-100) rather than being expressed as simple ratios (with a range of 0-1).

• CI (cephalic index): HW/HL x 100.
• Eye size indices:
  • EI (eye index): EL/HW x 100.
  • REL (relative eye length) EL/HW x 100.
• MandI. MandL/HL x 100.
• SI (scape index): SL/HW x 100.

Specialised Measurements

• Antennal segment measurements:
  • A11L, A10L, A09L, A08L: length of 11th to 8th antennal segment.
  • A11W, A10W, A09W, A08W: width of 11th to 8th antennal segment.
• ATL: Abdominal Tergum IV Length. Maximum length of fourth abdominal tergum measured with anterior and posterior margins in same plane of focus.
• ATW: Abdominal Tergum IV Width. Maximum width of fourth abdominal tergum with anterior, posterior, and lateral borders in same plane of focus.
• CDD: Clypeal Denticle Distance. Distance between clypeal denticle apices, measured in full-face view.
• CFW: Clypeal fork width measured between the anterior-most points of the clypeal teeth.
• CW: Clypeus Width. Distance between the apices of the frontal lobes across the clypeus.
• HFL: Maximum length of hind femur, measured in anterior view.
• HLA: Head Length, Anterior. Distance between the anterior edges of the eyes to the mandible bases in full-face view.
• HTL: Maximum length of hind tibia, excluding the proximal part of the articulation which is received into the distal end of the hind femur.
• MTL: Maximum length of mid tibia, excluding the proximal part of the articulation which is received into the distal end of the femur.
• PetH: Petiolar height in lateral profile measured as the perpendicular distance from the ventral margin to the highest point of posterolateral tubercles; if ventral margin is concave upward then measured from a line tangent to the uppermost portion of the curve and oriented as close as possible to the long axis of petiole.
• PetI (petiolar index): PetW/PetL x 100.
• PetL: Measured one of two ways:
  • Maximum length of the petiolar node in dorsal view.
  • When viewed in lateral profile and measured in the same plane as the anterodorsal face, the distance from the inflection point marking the juncture of the posterolateral lobes and the cylindrical posterior portion of the segment to the anterior inflection point where the petiole curves up to the condyle or, if the inflection point is not visible, where the petiole is obscured by the posteroventral lobes of the propodeum.
• PetW: Maximum width of the petiolar node in dorsal view.
• PetNL: Maximum length of petiolar node, measured longitudinally from the level of the anterolateral angle/corner to that of the posterior-most extension of the petiolar tergum, where it surrounds the gastric articulation.
• PML: Promesonotum Length. Maximum length of promesonotum from posterior spine/denticle apices to anterolateral denticle apices; all four apices in same plane of focus. (= PMD, Schödl 2007)
• PnL: Pronotal length measured from the anterior edge of the pronotal collar to the pronotal-mesonotal suture parallel to the measuring axis.
• PPH: Postpetiole Height, measured one of two ways:
  • Measured from sternal process base to postpetiole apex in lateral view.
  • Maximum height of propodeum measured tangentially to the line measuring PpL.
• PPL: Postpetiole Length. Measured from anterior to posterior inflections of postpetiole node in lateral view.
• PpL: Measured one of two ways:
  • Propodeal length measured from the metanotal groove to the posterior-most point of the propodeal lobes or metapleural flanges measured in lateral (side) view.
  • When viewing at an angle that maximizes length (approximately parallel to fourth abdominal tergite), perpendicular distance from line tangent to anterior inflection point (narrowest point when postpetiole is hourglass-shaped where it joins the helcium, point immediately anterior to node when helcium is sharply differentiated from node as a distinct cylindrical stem) to line tangent to posteriormost lobes if bilobed, to posteriormost point if globular.
• PpW: Postpetiole width measured as the maximum width of the postpetiole, in same view as and perpendicular to postpetiole length.
• PronW: Maximum width of the pronotum in dorsal view.
• SPL: Propodeal spine length measured from the tip of propodeal spine to the closest point on outer rim of the propodeal spiracle.
• PWA: Promesonotal Width, Anterior. Maximum width of promesonotal shield between anterolateral denticle apices in dorsal view. (= PW, Schödl 2007)
• PWP: Promesonotal Width, Posterior. Distance between posterior-most promesonotal spine or denticle apices.
• PTH: Petiole Height. Measured from petiole sternum to apex in lateral view.
• PTL: Petiole Length. Measured from anterior to posterior inflections of petiole node.
• SPL: Propodeal Spine Length. Workers: distance from inner posterior margin of propodeal spiracle to propodeal spine apex. Gynes: maximum propodeal spine length from basal inflection of spine, to spine apex.
• TL: The total outstretched length of the ant from the mandibular apex to the gastral apex; when measured in profile the sum of Mandibular length + head length + mesosomal length + lengths of waist segments + length of gaster.

Specialised Indices

Note that in about 70% of cases indices are multiplied by 100 (giving a range of 0-100) rather than being expressed as simple ratios (with a range of 0-1).

• ACI: antennal club index. This index attempts to quantify the degree of distinctness of the club. The product of segment length and width is a proxy of overall segment size, and the index measures relative change in size from segment 10 to 9 compared to relative change from 9 to 8. A sharply differentiated two-segmented club will have segment 10 much larger than 9, and segment 9 about the same size as 8, and the index will be greater than zero. A club in which segments 10 to 8 gradually decrease in size will have similar first and second terms in the formula, and the index value will be near zero. A distinctly 3-segmented club will have a value less than zero.
• MTI (Mandibular Torular Index). In full face view the distance between points where outer margins of fully closed mandibles intersect anterior clypeal margin, divided by distance between outer-most points of lower margins of toruli.
• OI (ocular index): EL/HL x 100.
• PetI (petiole index): PW/PL x 100.
• PetHI (petiole height index): PTH/PTL x 100.
• PetWI (petiole width index): PTW/PTL x 100.
• PpWI (postpetiole width index): PPW/PPL x 100.
• SPI (propodeal spine index): SPI/WL X 100.

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The following measurements were used by Jack Longino as part of his work on Crematogaster.

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Measurements used by Boudinot & Fisher (2013) as part of a revision of Meranoplus.

FIGURES 1–7. Measurements of Meranoplus species. 1. Measurements with head in full face view. 2. Measurements of head in profile; scrobal carina indicated by “sc”. 3. Scape length. 4. Mesosoma profile measurements. 5. Measurements of the petiole and postpetiole in profile. 6. Promesonotal shield measurements, dorsal view. 7. Abdominal tergum IV measurements, dorsal view.

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Measurements used by Branstetter (2013) in his study of Stenamma species.

HL

Head length (full-face view): maximum length of the head, measured from the posterior margin of the head to the anterior-most extremity of the clypeus.

HW

Head width (full-face view): maximum width of the head, eyes excluded.

FLD

Frontal lobes distance (full-face view): the maximum distance separating the outer margins of the frontal lobes.

PCW

Posterior clypeus width (full-face view): width of the posterior extension of the clypeus (depressed area between the frontal lobes), measured at the midpoint of the antennal sockets.

SL

Scape length (most suitable view): length of the first antennal segment, exclusive of the basal condyle.

EL

Eye length (most suitable view): the maximum diameter of the compound eye, including the outer ring of ommatidia, which are often black.

ACL

Antennal club length (most suitable view): the combined length of antennal segments 9-12, each measured individually and then summed together.

ML

Mesosoma length (profile view): greatest distance from the approximate inflection point, where the pronotum curves into the cervical shield, to the posterior apex of the propodeal lobes.

PrW

Pronotum width (dorsal view): maximum width of the pronotum.

PSL

Propodeal spine length (profile view): distance from the center of the propodeal spiracle to the tip of the propodeal spine.

SDL

Spiracle to declivity length (profile view): minimum distance from the center of the propodeal spiracle to the propodeal declivity.

PL

Petiole length (profile view): maximum length of the petiole, measured from the narrowest point of the anterior constriction, to the posterior margin; because the propodeal lobes usually obscure the anterior constriction in lateral view, this point must be approximated.

PH

Petiole height (profile view): maximum height of the petiole, measured at a right angle to PL and taken from the dorsal surface of the petiolar node to the ventral surface of the postpetiolar helcium, where it inserts into the petiole.

PW

Petiole width (dorsal view): maximum width of the petiole.

PPL

Postpetiole length (profile view): maximum length of the postpetiole, measured from the posterior margin of the enlarged portion of the helcium, to the posterior margin of the postpetiole.

PPH

Postpetiole height (profile view): height of the postpetiole, measured in a line perpendicular to PPL. If the ventral surface is concave upward, the measurement should be taken from the uppermost portion of the curve.

PPW

Postpetiole width (dorsal view): maximum width of the postpetiole.

MFL

Metafemur length (most suitable view): maximum length of the metafemur, measured from the distal margin of the trochanter to the metafemur apex. In most cases this measurement was taken from the anterior side of the metafemur.

MTL

Metatibia length (most suitable view): maximum distance of the metatibia, measured from the proximal constriction, just before the inserting condyle, to the apex. This measurement is always performed on the dorsal surface of the metatibia so that the proximal condyle is not obscured by the metafemur.

Indices

CI

Cephalic index: HW/HL × 100.

SI

Scape index: SL/HW × 100.

REL

Relative eye length: EL/HW × 100.

FLI

Frontal lobes index: FLD/HW × 100.

PSI

Propodeal spine index: PSL/SDL.

MFI

Metafemur index: HW/MFL × 100.

ACI1

Antennal club index: segments 11+12/ACL × 100.

ACI2

Antennal club index: ACL/SL × 100.

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Measurements used by Guerrero, R.J. et al. (2024) while studying Rhopalothrix species.

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  Guerrero et al. (2024), Figure 1. Measurements recorded in the habitus of Rhopalothrix worker. Definitions of the acronyms are described in Material and methods.

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Zhong et al (2024) included these measurements for Myrmica species.

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  Zhong et al. (2024), Fig. 1. Illustrations of measurements of Myrmica nr. urbanii Radchenko & Elmes, 1998. A, head in full-face view; B, body in lateral view; C, propodeal spines in dorsal view; D, body in dorsal view.

Temnothorax

The following morphometric characters were used for Temnothorax species by Seifert & Csősz (2015) and Csösz et al. (2015).

Fig. 1. Definition of morphological characters of workers of Temnothorax nylanderi species-complex. Head in dorsal view (1) with measurement lines for CL, CWb, PoOC and SL; frontal region of the head dorsum (2) with measurement lines for FRS; dorsal view of mesosoma (3) with measurement lines for MW, PEW, PPW, SPBA, SPTI and SPWI; lateral view of mesosoma (4) with measurement lines for ML, PEH and PPH; lateral view of propodeum, petiole and postpetiole (5) with measurement lines for HOH, NOL, PPL, SPL and SPST; lateral view of propodeum, petiole and postpetiole (6) with measurement lines for NoDL and PPL.

• CL: maximum cephalic length in median line; the head must be carefully tilted to the position with the true maximum. Excavations of hind vertex and/or clypeus reduce CL (Fig 1A).
• CS: cephalic size; the arithmetic mean of CL and CWb.
• CWb: maximum width of head capsule, measured just posterior to the eyes (Fig 1A).
• EL: maximum diameter of the eye.
• FRS: distance of the frontal carinae immediately caudal of the posterior intersection points between frontal carinae and the lamellae dorsal to the torulus. If these dorsal lamellae do not laterally surpass the frontal carinae, the deepest point of scape corner pits may be taken as reference line. These pits take up the inner corner of scape base when the scape is fully switched caudad and produce a dark triangular shadow in the lateral frontal lobes immediately posterior to the dorsal lamellae of scape joint capsule (Fig 1B).
• ML: mesosoma length from caudalmost point of propodeal lobe to transition point between anterior pronotal slope and anterior propodeal shield (preferentially measured in lateral view; if the transition point is not well defined, use dorsal view and take the center of the dark-shaded borderline between pronotal slope and pronotal shield as anterior reference point) (Fig 1D).
• MW: maximum mesosoma width; this is in workers pronotal width (Fig 1C).
• NOdL: anterior length of petiole measured in dorsal view. Distance from the centre of anteriormost seta pit on the petiolar node to the level of the constriction of articulation condyle with propodeum. Measuring requires a change of focus from above (seta pit) to below (constriction). Dorsal view is achieved when the dorsalmost point of anterior petiolar peduncle at the level of its strongest constriction and the dorsalmost point of caudal petiolar margin are in the same focal level (Fig 1F).
• NOH: maximum height of the petiolar node, measured in lateral view from the uppermost point of the petiolar node perpendicular to a reference line set from the petiolar spiracle to the imaginary midpoint of the transition between dorso-caudal slope and dorsal profile of caudal cylinder of the petiole (Fig 1E).
• NOL: length of the petiolar node, measured in lateral view from petiolar spiracle to dorsocaudal corner of caudal cylinder. Do not erroneously take as reference point the dorso-caudal corner of the helcium, which is sometimes visible (Fig 1E).
• PEH: maximum petiole height. The chord of ventral petiolar profile at node level is the reference line perpendicular to which the maximum height of petiole is measured (Fig 1D).
• PEW: maximum width of petiole (Fig 1C).
• PL: total petiole length measured in dorsal view; distance between the dorsalmost point of caudal petiolar margin and the dorsalmost point of anterior petiolar peduncle at the transversal level of its strongest constriction. Positioning of petiole as in NOdL (Fig 1F).
• PoOC: postocular distance. Use a cross-scaled ocular micrometer and adjust the head to the measuring position of CL. Caudal measuring point: median occipital margin; frontal measuring point: median head at the level of the posterior eye margin (Fig 1A).
• PPH: maximum height of the postpetiole in lateral view measured perpendicularly to a line defined by the linear section of the segment border between dorsal and ventral petiolar sclerite (Fig 1D).
• PPL: maximum length of the postpetiole measured in lateral view perpendicular to the straight section of lateral postpetiolar margin (Fig 1E).
• PPW: maximum width of postpetiole (Fig 1C).
• SL: maximum straight line scape length excluding the articular condyle (Fig 1A).
• SPL: minimum distance between the center of propodeal spiracle and the subspinal excavation measured in lateral view (i.e. the same view that is applied to measure ML). Note: in lateral view propodeal spiracle and the caudal margin of propodeal declivity might not be in the same focal level, hence slight adjust might be necessary while measuring SPL between the two endpoints (Fig 1E).
• SPBA: the smallest distance of the lateral margins of the spines at their base. This should be measured in dorsofrontal view, since the wider parts of the ventral propodeum do not interfere with the measurement in this position. If the lateral margins of spines diverge continuously from the tip to the base, a smallest distance at base is not defined. In this case, SPBA is measured at the level of the bottom of the interspinal meniscus (Fig 1C).
• SPST: distance between the center of propodeal stigma and spine tip. The stigma centre refers to the midpoint defined by the outer cuticular ring but not to the centre of real stigma opening that may be positioned eccentrically (Fig 1E).
• SPTI: the distance of spine tips in dorsal view; if spine tips are rounded or truncated, the centers of spine tips are taken as reference points (Fig 1C).
• SPWI: maximum distance between outer margins of spines; measured in same position as SPBA (Fig 1C).

The deviation of propodeal spines from longitudinal mesosomal axis (i.e. angle between the spines and the mesosomal axis) is often of high diagnostic value within the Temnothorax nylanderi species-group. Longitudinal mesosomal axis is defined by Seifert et al. (2014) as follows: in lateral view as straight line from the center of propodeal lobe to the border point between anterior pronotal shield and propleuron.

References

• Csösz, S., Heinze, J., Mikó, I. 2015. Taxonomic synopsis of the Ponto-Mediterranean ants of Temnothorax nylanderi species-group. PLoS ONE 10(11): e0140000 (doi:10.1371/journal.pone.0140000).
• Seifert, B. & Csősz, S. 2015. Temnothorax crasecundus sp. n. – a cryptic Eurocaucasian ant species (Hymenoptera, Formicidae) discovered by Nest Centroid Clustering. ZooKeys 479, 37-64 (doi:10.3897/ zookeys.479.8510).
• Seifert, B., Csősz, S., Schulz, A. 2014. NC-Clustering demonstrates heterospecificity of the cryptic ant species Temnothorax luteus (Forel, 1874) and (Bondroit, 1918) (Hymenoptera: Formicidae). Contrib Entomol. 61: 47–57.
• Zhong, Y., Huang, Y., Wu, T., Liu, Z. 2024. A new spineless Myrmica species from China, with the reinstatement of M. zhengi Ma & Xu (Hymenoptera: Formicidae). Journal of Asia-Pacific Entomology 27, 102261 (doi:10.1016/j.aspen.2024.102261).