The authors have declared that no competing interests exist.
Conceived and designed the experiments: CP RAJ. Performed the experiments: CP RAK RAJ. Analyzed the data: CP RAJ. Wrote the paper: CP RAJ.
The South American seed-harvester ant
The sporadic but recurrent loss of flight in insects is a tantalizing evolutionary problem. Ability to fly is a tremendous adaptation that enhances dispersal, location of scattered resources (food and mates), and avoidance of predators. In species such as bees and dragonflies, flight is an integral part of daily life, while in other insects it only serves as a mechanism for dispersal. In the latter case, flight can be lost or it can occur as a polymorphism, with many examples in the orders Orthoptera, Hemiptera, Hymenoptera, and Coleoptera
Ants are an example of insects in which flight only functions for dispersal. Young queens and males fly from their natal nests to mate; queens then locate a new nesting site and shed their wings. Unlike other eusocial Hymenoptera, ants also have a permanently wingless worker caste, such that both winged and wingless females are produced in the same colony. Nevertheless, even this highly reduced function of flight has been lost numerous times and ergatoid (permanently wingless) queens occur in species belonging to most subfamilies of ants
The seed-harvester ant genus
This study expands on the observations of Kusnezov
The two study sites are shown by black symbols,
We studied
(A) dorsal view of body, (B) close-up dorsal view of thorax, (C) lateral view of body, (D) close-up lateral view of thorax, (E) frontal view of head. High resolution photos for all castes (
(A) dorsal view of body, (B) close-up dorsal view of thorax, (C) lateral view of body, (D) close-up lateral view of thorax, (E) frontal view of head.
We excavated 16 nests of
(A) dorsal view of body, (B) close-up dorsal view of thorax, (C) lateral view of body, (D) close-up lateral view of thorax, (E) frontal view of head.
|
Ergatoid | Brachypterous | |||||
Character |
Chilecito | Punta Balasto | queen | queen | |||
|
1.85 ± 0.02d | 2.07 ± 0.03c | 2.18 ± 0.02b | 2.39 ± 0.02a | |||
|
1.74 ± 0.02d | 1.91 ± 0.03c | 2.01 ± 0.02b | 2.14 ± 0.02a | |||
|
0.35 + 0.00c | 0.37 + 0.01b | 0.39 ± 0.00b | 0.43 ± 0.00a | |||
|
1.34 ± 0.02c | 1.50 ± 0.02b | 1.47 ± 0.02b | 1.56 ± 0.02a | |||
Ocellus diameter (OD) | Absentb | Absentb | 0.05 ± 0.00a | 0.05 ± 0.00a | |||
|
1.16 ± 0.01d | 1.30 ± 0.02c | 1.39 ± 0.01b | 1.49 ± 0.01a | |||
|
0.47 ± 0.01d | 0.52 ± 0.01c | 0.56 ± 0.01b | 0.59 ± 0.00a | |||
|
0.68 ± 0.01d | 0.78 ± 0.01c | 0.88 ± 0.01b | 0.93 ± 0.01a | |||
|
2.13 ± 0.03d | 2.37 ± 0.03c | 2.55 ± 0.03b | 2.69 ± 0.02a | |||
|
1.80 + 0.02c | 2.09 + 0.03b | 2.06 + 0.02b | 2.26 + 0.02a | |||
Gastral t |
1.50 ± 0.02d | 1.74 ± 0.03c | 2.22 ± 0.02b | 2.35 ± 0.02a | |||
Number of ovarioles | 2.00 ± 0.00b | 2.00 ± 0.00b | 13.20 ± 0.37a | 13.00 ± 0.58a | |||
Spermatheca | No | No | Yes | Yes |
We quantified 14 morphological characters (11 external, three internal) for workers, ergatoid queens, and brachypterous queens. External characters were head width, head length, scape length, maximum eye diameter, diameter of the anterior ocellus, pronotal width, mesosomal length, hind femur length, petiole width, post-petiole width, and width of the first gastral tergite [see also 7]; internal characters were number of ovarioles, presence/absence of a spermatheca, and presence of the flight muscles. The 11 external characters were measured by projecting an image from a dissecting microscope to a video monitor; this image was measured to 0.01 mm using NIH Image (available at
Scores of all individuals (
Predictor variable | Function 1 | Function 2 |
Head width | 0.008 | 1.368 |
Head length | −0.286 | −0.613 |
Maximum ocular diameter | 0.076 | 0.412 |
Scape length | −0.058 | 0.304 |
Pronotal width | −0.031 | 0.276 |
Petiole width | −0.218 | −0.289 |
Post-petiole width | −0.003 | −0.066 |
Thorax length | −0.266 | −0.554 |
Hind femur length | −0.022 | 0.532 |
Gastral tergite width | 1.438 | −0.664 |
See also
We performed a morphometric analysis of the female castes using multivariate analysis-of-variance (MANOVA). The data set included 10 continuous external characters; ocellus diameter was treated as a discrete variable because workers lack ocelli (see below). An
Colony code | Queen pupae | Callow queens | Fullypigmentedqueens | Males | Workers | Mating status & ovary data |
RAJ 4363 | 13 BQ | 0 | 1 BQ (d) | 13 | >111 | Mated, 12 ovarioles, many yellow bodies |
RAJ 4366 | 0 | 6 BQ | 0 | 1 | >196 | Virgin, 15 ovarioles ( |
RAJ 4370 | 4 | 0 | 0 | 0 | >87 | ND |
RAJ 4383 | 0 | 14 BQ | 0 | 0 | >179 | ND |
RAJ 4385 | 0 | 5 BQ | 1 BQ (d) | 0 | >186 | Not dissected |
RAJ 4386 | 0 | 4 BQ | 0 | 0 | >44 | ND |
RAJ 4388 | 1 BQ | 52 BQ | 2 BQ (1d) | 30 | ND |
ND |
RAJ 4404 | 0 | 0 | >20 EQ | 15 | ND | Virgin, 12–14 ovarioles ( |
RAJ 4412 | 0 | 0 | 10 EQ | 9 | ND | Virgin ( |
RAJ 4413 | 0 | 0 | >50 EQ | 14 | ND | Virgin, 14 ovarioles ( |
Another six colonies from Punta Balasto yielded no sexuals. Colony codes refer to R.A. Johnson collection numbers.
ND = No data.
We estimated relative wing size for two species with brachypterous queens (
(A) normal winged queen of
(a) = wing deflector (basalare muscle), (b) = wing flexors (third axillary muscles), (c) = position of the phragmata (yellow) and indirect flight muscles (grey) in the thoracic cavity as they would occur in fully-winged queens.
We dissected the thorax of 3–4 individuals per caste to determine presence/absence of flight muscles and to further assess the degree of fusion/articulation between the thoracic sclerites. Specimens that had been stored in 70% ethanol were disarticulated with microforceps, stained for 1 h in a 0.2% aqueous solution of methylene blue, returned to 70% ethanol, then examined under a dissecting microscope. We considered two sclerites to be articulated (mobile) if we could cleanly isolate them from each other by gently pulling them apart, and sutured (immobile) if pulling resulted in an uneven break of their margins.
We also took scanning electron microscope (SEM) micrographs of the thorax ( = mesosoma) of workers, ergatoid queens, and brachypterous queens of
Dry mass was measured by collecting five ergatoid queens and five workers from each of three Chilecito colonies, as well as six brachypterous queens from Punta Balasto; brachypterous queens had been previously stored in 95% ethanol. Individuals were placed in an oven at 50–55° C for >72 h, then weighed. Dry mass of queens and workers was averaged within each colony, and these mean colony values were used to calculate the grand mean. Likewise, the queen to worker mass ratio was calculated for each colony, then averaged across colonies.
Total fat content was determined for six fully-pigmented ergatoid gynes. Individuals were dried at 55° C for >72 h, weighed, and then each individual was placed in a vial of petroleum ether (boiling point 30–60° C). Individuals were crushed to expose fat bodies and remained in ether for >24 h. The contents of each vial were rinsed through filter paper, dried, and weighed to 0.01 mg. Percent total fat content was calculated using the formula: 100×(DM – FFDM)/DM, where DM is dry mass and FFDM is fat free dry mass. All brachypterous gynes that we collected were callows (i.e. recently emerged), such that total fat content was not determined.
Voucher specimens are deposited in the R.A. Johnson collection, Tempe, AZ, USA [RAJC], and the Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”, Buenos Aires, Argentina [MACN].
Sexual castes were conspicuous during excavations at both sites: queens at Punta Balasto were callows (i.e., pale coloration) with tiny wings (
Discriminant analysis correctly classified 94.7% (90 of 95) of all individuals. One worker from each site was misclassified as a worker from the other site, one ergatoid queen was misclassified as brachypterous, and two brachypterous queens were misclassified as ergatoid queens. The standardized coefficients of the canonical functions indicated that tergite width was the primary contributor to discriminant function 1, accounting for 90.5% of the variance. Head width was the primary contributor to discriminant function 2, accounting for 8.0% of the remaining variance (
Both ergatoid and brachypterous queens had a spermatheca and 12–15 ovarioles, whereas workers lacked a spermatheca and had two ovarioles (Kruskal-Wallis test,
The dorsal thoracic sclerites were completely fused in workers, while there was a clear boundary (suture) between the pronotum, mesonotum, and metanotum in ergatoid and brachypterous queens (
Both queen phenotypes lacked indirect wing muscles and any trace of the internal phragmata; the latter are cuticular projections present in all flying insects and function to support the longitudinal wing muscles. Unlike ergatoid queens, brachypterous queens had a fully developed set of direct flight muscles, which were attached to the base of the tiny forewings (
Gynes and males were found in most colonies at this time of year. Punta Balasto colonies produced 1–52 brachypterous queens (either as pupae or callows); all callow queens had undeveloped ovarioles (
Dry mass averaged 7.57 ± 0.10 mg and 2.99 ± 0.20 mg for ergatoid queens and workers, respectively, yielding a queen to worker dry mass ratio of 2.55 ± 0.14. These ergatoid queens had 25.78 ± 2.22% total fat content (
Many ants have evolved non-flying queens as a replacement for the ancestral winged queens
Although they cannot fly, brachypterous queens in
The absence of the indirect flight muscles in brachypterous queens is also reflected in the shape and size of the dorsal thoracic plates. The mesonotum is flattened in profile while the thoracic box is strikingly reduced in volume when compared with normal winged queens of
Ocelli are involved in stabilization reflexes during flight
Although workers from our two sites differed in size (
In sharp contrast to
Typically, ant species with ergatoid queens reproduce using DCF, i.e., queens cannot start a new colony alone
If
The existence of different non-flying phenotypes in two populations of
We thank Mathieu Molet for valuable comments on the manuscript.