Corresponding author: Alice Laciny ( email@example.com )
Academic editor: Michael Ohl
© 2016 Alice Laciny, Herbert Zettel, Irina Druzhinina.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation: Laciny A, Zettel H, Druzhinina I (2016) Workers, soldiers, and gynes – morphometric characterization and description of the female castes of Camponotus singularis (Smith, 1858) (Hymenoptera, Formicidae). Deutsche Entomologische Zeitschrift 63(2): 183-193. doi: 10.3897/dez.63.9435
Species of Camponotus Mayr, 1861 show a high variation of worker caste numbers, from a monomorphic worker caste to strong polymorphism along a continuous range of worker sizes or true worker dimorphism. Camponotus singularis (Smith, 1858) is used as a model for a Camponotus species with two very distinct worker castes (minors and majors) which are chiefly defined by morphometric data. We investigated shifts in proportions of C. singularis female castes in order to identify major allometric patters useful for characterizing caste differences in this species as well as in other Camponotini. We describe the main morphological traits which are characteristic for the respective morphs. The major worker, or “soldier”, shows many characteristics which deviate from the minor worker, but also from the alate gyne. Its traits are assumingly modified for its function in nest defence. Morphometric data clearly set Bornean specimens of Camponotus singularis (described as var. rufomaculatus Donisthorpe, 1941) apart from specimens with other proveniences, suggesting that this island population is a distinct species.
allometry, Camponotini, Camponotus, castes, dimorphism, morphometry, soldier
Within the eight recent genera of the tribe Camponotini (
During our ongoing revision of the Colobopsis cylindrica group, which includes a morphometric analysis of species, a review of previous publications revealed multiple mentions of poly- or dimorphism in workers of Camponotini (e.g.,
We herein present morphometric data on the Asian species Camponotus singularis (Smith, 1858), a large-bodied species with a wide distribution from Nepal to Borneo and Java (e.g.,
Camponotus singularis, head, frontal view, of (1) minor worker, (2) major worker, and (3) gyne. Notably different are head shape, development of posterior margin, and position of eyes. The minor has much longer scapes and maxillary palpi than the major.
CZW Coll. H. Zettel, Vienna, Austria
SKYC Sk. Yamane Collection, Korimoto, Japan
We measured specimens from most parts of the distribution area of C. singularis, except from Nepal and India. In total five gynes, ten major workers and 85 minor workers of C. singularis, as well as five minor workers and one gyne of C. singularis var. rufomaculatus Donisthorpe, 1941 were examined. During data processing we noticed that specimens from Borneo differed clearly from specimens collected in other localities. Consequently, we removed Bornean specimens from our in-depth analysis of morphs, but present some data on how to differentiate this population.
Camponotus singularis, typical form: Myanmar: 1 minor worker (BMNH), Mandalay Region, Pyin U Lwin, ca. 900 m a.s.l., V.1899, coll. Bingham [Camponotus camelinus Smith, Upper Burma Maymya, 3000 ft, 5.1899, Bingham coll.]; 1 major worker, 1 minor worker (
Camponotus singularis var. rufomaculatus: East Malaysia (Borneo): 2 minor workers (
Examined specimens were either pinned or dry mounted on card squares or triangles. Examination and measurements of specimens were carried out with a Nikon SMZ1500 binocular microscope at magnifications of up to 256×.
TL Total length. The added lengths of head (including mandibles), mesosoma, petiole, and gaster.
HW Head width. Maximum width of head in full-face view, excluding eyes if laterally protruding (few minor workers).
HL Head length. Maximum length of head in full-face view, excluding mandibles, measured parallel to midline from anterior-most point of clypeus to midpoint of occipital margin (in minor workers) or to midpoint of an imaginary line connecting the apices of posterior projections (major workers and some gynes).
HS Head size. (HW+HL) / 2.
EL Eye length. Maximum diameter of compound eye.
SL Scape length. Maximum length of antennal scape in dorsal view excluding basal neck and condyle.
ML Mesosoma length. Measured laterally from anterior surface of pronotum proper (excluding collar) to posterior extension of propodeal lobes.
PH Petiole height. Maximum height of the petiole in lateral view, measured from ventral-most point of petiolar sternum to dorsal apex.
PL Petiole length. Maximum length of petiole in lateral view, measured from inflexion point of anterior constriction to posterior margin.
NH Node height. Height of petiolar node, measured laterally, from the intersection point of the axes of maximum height and length to dorsal apex.
FeL Femur length. Maximum length of hind femur, measured from base to apex.
PS5 Length of maxillary palp segment 5, measured from base to apex.
PS6 Length of maxillary palp segment 6, measured from base to apex.
CI Cephalic index. HW / HL × 100.
SI Scape index. SL / HW × 100.
EI Eye Index. EL / HW × 100.
PI Petiole Index. PH / PL × 100.
FeI Femur Index. FeL / HW × 100.
PSI Palp Segment Index. (PS5+PS6) / HS × 100
All measurements are in millimetres and separated by caste. Due to the condition of some specimens, not all measurements were taken from all animals. Measurements of minor workers include the holotype (plotted separately in Figs
Digital photos were taken with a Leica DFC camera attached to a Leica MZ16 binocular microscope with the help of Leica Application Suite V3, and stacked with ZereneStacker 64-bit. Processing of images was performed with Adobe Photoshop 7.0.
Measurements: alate gynes (n = 5): TL 18.42–20.48 (4); ML 6.52–6.78; major workers (n = 10): TL 16.76–19.57; ML 5.22–6.07; minor workers (n = 85): TL 9.26–15.39; ML 3.85–5.28 (84).
We used two measurements to describe body size, total length (TL) and mesosoma length (ML). Although traditionally used in myrmecology to facilitate rough comparisons of castes and species, total length strongly depends on the condition of the specimen (e.g., dilation or shrinking of the gaster from storage in alcohol or the drying process) so that TL can be relatively inaccurate. In C. singularis TL and ML are directly proportional in all female castes (Fig.
Distribution of mesosoma length (ML) in relation to total length (TL) in gynes, majors and minors of Camponotus singularis.
Head size and head shape:
Measurements: alate gynes (n = 5): HW 4.17–4.37; HL 4.30–4.63; HS 4.26–4.50; CI 94–98; major workers (n = 10): HW 4.37–5.28; HL 4.63–5.54; HS 4.50–5.41; CI 91–100; minor workers (n = 85): HW 1.66–2.77; HL 2.15–3.49; HS 1.91–3.13; CI 72–83.
HW and HL were measured. In relation to body size, head size (HS) is strongly disproportional in the two worker castes (Fig.
Distribution of mesosoma length (ML) in relation to head size (HS) in majors and minors of Camponotus singularis.
Distribution cephalic index (CI) in gynes, majors and minors of Camponotus singularis, holotype minor worker plotted separately.
The three castes differ considerably in head shape (Figs
Eye size and eye position:
Measurements: alate gynes (n = 5): EL 0.79–0.82; EI 27–28; major workers (n = 10): EL 0.76–0.83; EI 23–27; minor workers (n = 85): EL 0.51–0.69; EI 36–46.
Eye size in minor workers is strongly correlated with body size (ML) (Fig.
Distribution of eye length (EL) in relation to mesosoma length (ML) in gynes, majors and minors of Camponotus singularis.
In minor workers the eyes are dorsolaterally located, close to the lateral outline of the head in frontal view (Fig.
Minor workers do not possess ocelli or other structures in their place. In one major worker from Myanmar we observed three reduced ocelli, whereas many other majors have small depressions or scars at these positions.
The clypeus is similarly shaped in major and minor workers, whereas the medial protrusion is slightly longer in gynes (Figs
The mandibles of majors are much stouter than those of minors, especially in the basal half. The mandibles of gynes are of intermediate shape.
Measurements: alate gynes (n = 3): PS5 0.50–0.52; PS6 0.46–0.47; PSI 22–23; major workers (n = 5): PS5 0.49–0.54; PS6 0.42–0.48; PSI 18–21; minor workers (n = 32): PS5 0.52–0.60; PS6 0.42–0.50; PSI 32–47.
The maxillary palpi of minors are much longer in relation to HS than those of majors and gynes (PSI 18–23 in majors and gynes vs. 32–47 in minors; Fig.
Distribution of maxillary palp segment index (PSI) in gynes, majors and minors of Camponotus singularis, as well as minors and gyne of var. rufomaculatus.
Measurements: alate gynes (n = 4): SL 3.78–3.85; SI 87–92; major workers (n = 10): SL 3.46–3.78; SI 71–80; minor workers (n = 83): SL 2.93–3.98; SI 135–189.
The antennae of majors and gynes are relatively shorter and thicker than those of minors. The scape index (SI) differs considerably (Fig.
Distribution of scape index (SI) in gynes, majors and minors of Camponotus singularis (holotype minor worker plotted separately), as well as minors and gyne of var. rufomaculatus.
Distribution of scape length (SL) in relation to mesosoma length (ML) in gynes, majors and minors of Camponotus singularis, as well as minors and gyne of var. rufomaculatus.
The mesosoma of majors is slightly more robust (wider and higher) than that of minors, though no morphometric data were recorded except ML. Gynes always possess a well-developed flight apparatus, and their mesosoma morphology is strongly modified accordingly.
Measurements: alate gynes (n = 5): FeL 5.09–5.22; FeI 116–123; major workers (n = 10): FeL 4.83–5.35; FeI 101–114; minor workers (n = 85): FeL 4.46–5.45; FeI 169–238.
We measured the length of the hind femur (FeL) as an indicator for leg length. Whereas the femur index (FeI) is highly dissimilar in minors and majors (Fig.
Distribution of femur index (FeI) in gynes, majors and minors of Camponotus singularis (holotype minor worker plotted separately), as well as minors and gyne of var. rufomaculatus.
Petiole shape and petiole index:
Measurements: alate gynes (n = 5): PH 1.48–1.63; PL 1.04–1.17; NH 1.00–1.17; PI 134–144; major workers (n = 5): PH 1.13–1.48; PL 1.04–1.09 (4); NH 0.76–0.89 (4); PI 104–136 (4); minor workers (n = 49): PH 0.59–0.87 (38); PL 0.67–1.02 (35); NH 0.30–0.61 (46); PI 66–95 (31).
PI is highest in gynes, intermediate in majors and lowest in minors, reflecting the differences in dorsoventral height of the petiole and development of the petiolar node. While gynes and majors possess a petiole that is higher than long (PI > 100), with a well-developed node, the petiole of minors is dorsally rounded and longer than high (PI < 100). Due to the mounting method applied in some specimens, petiolar characters were partially obscured and therefore could not be measured in part of the material.
No evident differences were observed when comparing the gasters of minor and major workers. Due to the presence of reproductive organs, gynes possess a larger gaster relative to the rest of the body, but without apparent structural differences compared to the other castes.
The specimens from Borneo consistently differ from the rest of the examined material by the length of their appendages (maxillary palpi, antennae, and legs; Figs
Distribution of femur length (FeL) in relation to mesosoma length (ML) in gynes, majors and minors of Camponotus singularis, as well as minors and gyne of var. rufomaculatus.
The examined gyne and minors of C. s. var. rufomaculatus possess relatively longer palpi than all the other examined specimens (PSI 43–50 in minors, 25 in gyne, see Fig.
All examined C. s. var. rufomaculatus specimens differ from the rest of the material by considerably longer scapes relative to HW and ML (SI 143–194 in minors, 97 in gyne, see Figs
Camponotus singularis var. rufomaculatus differs from the typical form by relatively longer legs (FeI 197–253 in minors, 128 in gyne, see Figs
The results of our morphometric analyses show that C. singularis possesses a true worker dimorphism sensu
Perhaps the most striking difference between the two worker subcastes is in the shape of the head, which is narrow and ovate with a well-developed collar in minors, but greatly enlarged, heart-shaped and without collar or margin in majors (Figs
When comparing mesosomal architecture between castes, both major and minor workers possess a well- developed prothorax which is slightly enlarged in majors, whereas the mesothorax is greatly hypertrophied in gynes (Fig.
A recent study focusing on morphological variation in a species of the Colobopsis cylindrica (COCY) group (Laciny et al., in prep.) found considerable size-variation within the examined minor workers and relatively uniform sizes of major workers (phragmotic in Colobopsis) and gynes. This trend is in accordance with studies on other camponotines (e.g.,
The results of this study show a clear worker dimorphism and a conspicuously broad size range of minors, which is apparent in intraspecific as well as intracolonial comparison. It is therefore a valid assumption that C. singularis colonies have evolved some form of polyethism or division of labour amongst their members. However, nothing is known about colony composition and task allocation in this species as of now. Studies on other formicine ants have yielded results that suggest a tendency towards division of labour even within the (minor) worker caste:
The high nutritional investment necessary for producing a distinct soldier subcaste suggests that these animals must serve a function greatly beneficial to colony survival and fitness (
Altogether the results obtained within this study and their comparison to trends observed in the COCY clade (Laciny et al., in prep.) and other previously studied species (e.g.,
Examined specimens from Borneo clearly differ from specimens from other localities by proportions of appendages, colour pattern and setae.
We thank the following collection curators for loans of specimens or access to collections under their care: Bui Tuan Viet (