Morphological Measurements

AntWiki: The Ants --- Online

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.

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

Longino Crematogaster Measurements.jpg

Measurements used by Boudinot & Fisher (2013[1]) as part of a revision of Meranoplus.

Boudinot & Fisher 2013, Meranoplus measurements.jpg

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.

Measurements used by Branstetter (2013[2]) in his study of Stenamma species.

Branstetter 2013 Stenamma measurements.jpg

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.
Head width (full-face view): maximum width of the head, eyes excluded.
Frontal lobes distance (full-face view): the maximum distance separating the outer margins of the frontal lobes.
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.
Scape length (most suitable view): length of the first antennal segment, exclusive of the basal condyle.
Eye length (most suitable view): the maximum diameter of the compound eye, including the outer ring of ommatidia, which are often black.
Antennal club length (most suitable view): the combined length of antennal segments 9-12, each measured individually and then summed together.
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.
Pronotum width (dorsal view): maximum width of the pronotum.
Propodeal spine length (profile view): distance from the center of the propodeal spiracle to the tip of the propodeal spine.
Spiracle to declivity length (profile view): minimum distance from the center of the propodeal spiracle to the propodeal declivity.
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.
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.
Petiole width (dorsal view): maximum width of the petiole.
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.
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.
Postpetiole width (dorsal view): maximum width of the postpetiole.
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.
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.


Cephalic index: HW/HL × 100.
Scape index: SL/HW × 100.
Relative eye length: EL/HW × 100.
Frontal lobes index: FLD/HW × 100.
Propodeal spine index: PSL/SDL.
Metafemur index: HW/MFL × 100.
Antennal club index: segments 11+12/ACL × 100.
Antennal club index: ACL/SL × 100.

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

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


  1. Boudinot, B.E. & Fisher, B.L. 2013. A taxonomic revision of the Meranoplus F. Smith of Madagascar (Hymenoptera: Formicidae: Myrmicinae) with keys to species and diagnosis of the males. Zootaxa 3635, 301-339.
  2. Branstetter, M.G. 2013. Revision of the Middle American clade of the ant genus Stenamma Westwood (Hymenoptera, Formicidae, Myrmicinae). ZooKeys 295, 1–277.