Revision Of The Thief Ants of North America

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Revision Of The Thief Ants

of North America

(Hymenoptera: Formicidae: Solenopsis)


Isidra Moreno Gonzalez1, and William P. Mackay1

The genus Solenopsis includes some of the most common ants, most notably the notorious “fire ants”. It also includes some of the smallest ants, which are commonly known as thief ants. Many species of this group live in nests of other ants, where they steal food and brood of their hosts. They also appear to be effective predators, and may be natural enemies of the founding queens of the imported red fire ants.

This group of ants was originally proposed as the subgenus Diplorhoptrum of Solenopsis by Mayr, in 1855. In 1862, Mayr synonymized the subgenus with Solenopsis, where it remained until 1930. Creighton (1930) revived it from synonymy with Solenopsis, where it remained as a subgenus until 1966, when Ettershank synonymized it again with Solenopsis. In 1968, Baroni-Urbani raised it to the status of a genus, later Kempf (1972) again synonymized it with Solenopsis, where it has remained (Bolton, 1987). This group of ants has been ignored, due to their small size, and the lack of good morphological characters which can be used to separate species.

A number of species and subspecies occur in North America. All of these species would be expected to be predators on the red imported fire ant, and I intend to include species from North America and Central America (total of 28) in my thesis. The following species complexes and species are defined: 1) azteca complex: S. azteca Forel and S. terricola Menozzi; 2) fujax complex: S. krockowi Wheeler, S. pergandei Forel, and S. pilosula Wheeler; 3) minutissima complex: S. depilis new species, S. ocellata new species missing info here???

Even though ants are found everywhere, they are seldom noticed unless they disturb us in some way. This group of insects is one of the most successful terrestrial arthropods, and their presence is essential in controlling the insect fauna, turning the soil and channeling energy. Ants are among the “little things that run the world” (Kremen et al., 1993).

Ants are classified in the family Formicidae, and order Hymenoptera. The family Formicidae has been divided into 11 subfamilies, over 297 genera, and more than 8,800 species. It has been estimated that over 20,000 species of ants still remain undescribed (Hölldobler & Wilson, 1990). The classification of ants is a very difficult task because they are small and in most cases it is impossible to differentiate between species without the help of a high quality microscope. Therefore, the classification of very tiny ants can be challenging and can lead to taxonomical misclassification. A good example is the group of thief ants.

The thief ants (fugax group of the genus Solenopsis, was previously considered to be members of the subgenus Diplorhoptrum) are predators on the founding queens of the red imported fire ant Solenopsis invicta, as well as, predators on the brood of this pest species. Unfortunately, very little ecological work has been done on this potentially important group of predators, due to the difficulty in identification and separation of species. I am especially interested in Solenopsis salina, due to its potential as a predator on the founding queens and brood of the red imported fire ant Solenopsis invicta in the El Paso area. The systematics of this group must be done before any serious biological work can be done with these ants. Since thief ants have been found to be effective predators of S. invicta, it is possible that they can be used as biocontrol of this pest. The objective of this research is to provide a revision of those species that are found in North America. This will serve as a basis for future work on the biology of these ants.

Definitions of Abbreviations

Specimens were measured using a micrometer in the ocular of a Wild dissection microscope. The following abbreviations are used (all measurements in mm.).

TL Total length from head to gaster.

HL Head length, anterior margin of medial lobe of clypeus to posterior border.

HW Head width, maximum width excluding eyes (Measured immediately posterior to eyes).

SL Scape length, excluding basal condyle.

CI Cephalic index, HW/HL x 100

SI Scape index, SL/HL x 100

PI Petiolar index, PL/PW x 100
The genus Solenopsis includes ants in the subfamily Myrmicinae, in which the antenna of the worker has 10 segments, with a 2-segmented club, which is nearly as large as the remainder of the funiculus. There are generally 2 clypeal carinae, and normally at least 2 teeth on the anterior margin of the clypeus. The medial point of the clypeus has single, elongate hair. The mandibles have 4 teeth. The scapes rarely reach the posterior lateral corners. The metanotal suture is impressed, often deeply impressed. The propodeum is rounded between the faces, and is without teeth. The petiole and petiole are well developed, and are usually about equal in size (in profile). Most surfaces are covered with long, erect hairs, including the head, scapes, mesosoma, legs, petiole, postpetiole and gaster. Most surfaces are smooth and polished. These ants are usually golden brown, but color ranges from pale yellow (nearly white), bicolored (head and mesosoma red, gaster black) to concolorous black.

Thief ants possess the above characteristics, but are tiny ants with very small eyes (usually 3 - 4 facets or less, never more than 18). The second and third funicular segments are as broad as long or broader than long. Most species are monomorphic or weakly dimorphic, a few of the South American species (wasmanni species complex) are strongly dimorphic with well-developed soldiers.

Various subgenera have been named, and most are of questionable validity, and several of these have been synonymized (Ettershank, 1966; Trager, 1991). We will only discuss those which occur in North America.

The subgenus Solenopsis currently contains the “fire ants” (geminata complex), as well as a mixture of other ants, including nest parasites (S. daguerrei). The group may be paraphyletic or even polyphyletic (Trager, 1991).

The subgenus Diagyne contains a single species: S. succinea. This is a very unusual species, as the workers completely lack the clypeal carinae, as well as the clypeal teeth. Although other species lack the clypeal teeth (bicolor complex, others have very reduced teeth), the female has a 10-segmented antenna (the females of all other species have an 11-segmented antenna), and the mandibular teeth of the female are very unusual, appearing to be broken (Fig. 8a). Although this species is clearly related to the others in the genus Solenopsis, it is clearly an aberrant form. As the worker could be confused with thief ants, it is included in this revision, although it is probably not closely related. It is not my intention to synonymize Diagyne. Trager (1991) suggested that the workers are not distinguishable at the species-group level from the globularia complex of the subgenus Euophthalma. This seems unlikely, but will have to be reevaluated with a thorough revision of the globularia species complex.

The subgenus Euophthalma includes 2 possibly unrelated groups: the globularia species complex and the nigella species complex. Workers of both of these groups have large eyes (more than 10, usually more than 20 ommatidia), and the second and third antennal segments of the funiculus are usually broader than long, or at most slightly longer than broad. They are monomorphic or feebly polymorphic species. The globularia complex is easily recognized as the postpetiole is greatly swollen (Fig. 12b), and appears to represent a monophyletic taxon. The nigella complex, on the other hand, is difficult to separate from members of the subgenus Solenopsis, as the dimensions of the funicular segments are variable (especially the third segment). Even if this characteristic was consistent, it (and the nearly monomorphic workers) would be little basis for considering them members of a separate subgenus. One of the members, S. huachucana, is actually a synonym of S. aurea (Trager, 1991). Trager (1991) stated that the species outside the globularia (-tenuis) group [the nigella complex] are apparently not differentiable from Bisolenopsis and Synsolenopsis, which he confirms are synonyms of Solenopsis, further supporting the hypothesis that the nigella complex is a synonym of Solenopsis. Except for the Central American species S. picquarti, the remainder of the species in the nigella group are South American. Trager (1991) suggests that the globularia species complex is related to the tenuis group of the thief ants, as they share several synapomorphic features (unspecified by Trager).

The thief ants present an interesting history. The group was originally proposed as the subgenus Diplorhoptrum of Solenopsis by Mayr, in 1855. In 1862, Mayr synonymized the subgenus with Solenopsis, where it remained until 1930. Creighton (1930) revived it from synonymy with Solenopsis, where it remained as a subgenus until 1966, when Ettershank synonymized it again with Solenopsis. In 1968, Baroni-Urbani raised it to the status of a genus, later Kempf (1972) again synonymized it with Solenopsis, where it has remained (Bolton, 1987). This group of ants is very closely related to the fire ants of the genus Solenopsis, and probably cannot be morphologically distinguished from them, but ecologically they are very different. These ants are almost exclusively subterranean, where they specialize in attacking the brood and stored food of other species of ants. Although I use Diplorhoptrum throughout this thesis, it is not my intention to resurrect this subgenus, but only as a matter of convenience. It is beyond the scope of this thesis to analyze the subgeneric structure of the entire genus Solenopsis.

Creighton (1930), first attempted to delineate species complexes in the thief ants, based on “characteristics that are fairly consistent”. Unfortunately he did not include any of the characteristics, stating that they would be presented in a future publication. Unfortunately the publication was never finished, and I have not been able to locate even an unpublished manuscript. I attempted to use his classification of 5 species complexes, but early in the revision it became clear that this was unattainable. Some of his complexes are apparently monophyletic (westwoodi group, actually the fugax complex), the molesta complex, and the azteca complex. The other groups listed (laeviceps group and the basalis-tenuis group) appear to be members of the molesta complex. It has been necessary to recognize 2 additional complexes: the succinea complex and the pygmaea complex. All of the species complexes of thief ants in North America will be defined, and distinguished by keys in this study.

Identification Keys
Key to the subgenera and species complexes of the genus Solenopsis of North America (north of Panamá)
1. Second and usually third segment of funiculus at least 1 ½ times as long as broad (Fig. 1, upper) ...…. geminata species complex (subgenus Solenopsis)
- Second and at least third segment of funiculus only slightly longer than broad, often broader than long (Fig. 1, lower) ..…………….. 2

Fig. 1. Antennae of S. geminata (upper) and S. franki idea (lower) showing the relative lengths of the second and third funicular segments.

2(1). Eyes of worker with at least 12 ommatidia (Fig. 2 -13d) .. 3
- Eyes of worker with fewer than 12 ommatidia, usually fewer than 6 (Fig. 3 -13b) ……………… 4

Fig. 2. Heads of workers of S. altinodis (left) and S. krockowi (right).

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