Acrobat Ants in South Austin, Texas

This article by Jerry Cates and Mitch Page, first published on 18 November 2012, was last revised on 12 November 2014. © Bugsinthenews Vol. 13:11(02).

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001. Lateroventral body

On 12 October 2012 the maintenance director of a medical facility located near the southern tip of Austin, Texas, called to say that a moderately sized ant, with a red head and midsection, and a black abdomen, had invaded one of their facility bedrooms. Since the invasion appeared to involve the much-feared red imported fire ant (RIFA), I immediately rearranged my schedule and drove to the site. By the time I arrived, the ants were no longer present, but specimens had been collected and were on hand for me to examine. One look with the unaided eye determined — to my relief — that they were harmless acrobat ants. Glancing out the bedroom’s window, it was easy to see where they had come from. Some 20 feet from the window stood a good sized island of lush, thick, well-tended botanicals, as high as it was wide. My mind’s eye immediately envisioned extensive colonies of aphids, mealybugs and scale, hidden from view within the island’s leafy boughs.

But mostly I was grateful the invaders were not fire ants. They are easily mistaken, one for the other. The average red imported fire ant worker (Solenopsis invicta, whose workers are continuously polymorphic, i.e., of widely varying sizes) is about the same size as the typical acrobat ant (Crematogaster spp., whose workers, in those species most often observed in central Texas, are mainly monomorphic); both have reddish anteriors with black posteriors; and both — being members of the same ant subfamily (Myrmecinae) — have elongated petioles divided in two nodes. But the red imported fire ant has an elliptical posterior that is widest at its middle, while that of the acrobat ant, when viewed from above, is shaped like a heart (some actually call them Valentine ants for this reason), widest at its anterior aspect, and gently tapering back to its posterior tip. Though that feature alone is diagnostic of the genus Crematogaster, its most heralded distinction is that, when disturbed, it pitches its rear end upward over its head. Among all our common ant species these alone are able to perform such acrobatic feats. If, perchance, you guessed that their common name derives from that distinctive behavioral trait, you are correct.

Acrobat ants sometimes invade human dwellings, as they did in this instance. Oi & Vail (2011) refer to these as only occasional structural pests, though in eastern Arkansas and western Tennessee they are considered important pests of structures. Isolated workers may show up inside structures from time to time, most likely while scouting for food sources or a new nesting spot. When they invade structures in large numbers, their purpose is typically not so much to find food as to find and take up residence in a cryptic retreat, where they can establish a new nest. In man-made buildings acrobat ants often nest in walls, particularly around doorways and windows where they exploit the multitude of voids those structural elements typically afford. Later they may make their presence known by ejecting trash (including bodies and body parts of deceased nest-mates and other ant species) from small cracks or holes around the door and window frames where their nests are positioned. Such trash heaps, often referred to as frass, are composed of dry, granular, loose, particulate litter (quite unlike the mud-like structures made by termites around the ejection holes they make in walls they are infesting).

The discovery of such trash heaps inside homes strikes fear in the hearts of home owners who take them as evidence that wood-destroying-insects are damaging the wood, insulation, and wallboard that holds their homes together. In a sense they are correct, but in another, their fears are misplaced. More often than not, acrobat and carpenter ant infestations in a wall are symptoms of concomitant moisture-mediated fungal infections in that same location that preceded the ants. Such infections weaken, and even rot, the moisture-laden wood inside the wall. The ants are merely attracted to the moisture, and  when they find that they can easily excavate the rotted wood to enlarge their nests, they stick around awhile. The fortuitous confluence of these two conditions make such locations imminently suitable for nesting purposes, so we can scarcely blame the ants for converting those advantages to their use. Once moisture issues around doors and windows are corrected, any ant infestations associated with them will usually resolve themselves automatically. Thus, the best defense against ants in walls is good moisture control, which usually involves the proper use of trim and cowling, judicious applications of caulk, and — in the humble opinion of this practitioner — an infusion of 10% aqueous disodium octaborate tetrahydrate (Tim-Bor®), with enough surfactant added to break the surface tension and hasten its absorption into the wood (in accordance with the specimen label).

Many species of acrobat ants move their nests several times during the year. It is not unusual to find them forming long lines one or two abreast, with many of the sojourning workers empty handed, while others are hard at work toting the community’s eggs, larvae, and pupae in their mandibles, and the jaws of still others are loaded up with mealybugs, scales, or aphids. Chances are, the ants that invaded this facility did so while searching for a new home, or while on the move from an old nest to a new one.

Their food, throughout the year, consists primarily of the honeydew secretions of homopteran insects. In fact, they are well-known for farming colonies of such insects as a means of providing their members with a ready supply of the latter’s sweet liquid exudations.

As I mention in an earlier article on acrobat ants found in Temple, Texas, most gardeners are dismayed to find evidence of homopteran incursions onto their  garden plants because, once established, the damage done by these organisms can be extensive and difficult to control. Since acrobat ants work hard to disperse scale, aphids, and mealybugs, one might think the first thing a good gardener should do is to control these ants. Again, first impressions are not always best, as the following demonstrates:

“The cultivation of Homoptera by ants is usually considered detrimental to plants, but any damage may be offset by the ants’ predation on defoliators. Another factor that may contribute to the stability of the ant-Homoptera-plant relationship is the ability of some homopterans to withdreaw large quantities of sap without seriously injuring trees, thereby allowing them to feed on the same plant year after year (Bradley and Hinks 1968). A portion of the sap sustains the aphids, but most is passed on as honeydew to the ants. In return, the ants protect the aphids and the trees from their enemies.” (Hansen and Klotz 2005).

In my work with landscapers and gardeners I encourage a balanced approach based on habitat modification that favors diversity over extermination. For homopteran control I release parasitic wasps that help keep their numbers in check. The net effect is to reduce the number of acrobat ants without chemical pesticides. The more organisms we have in our surroundings, the more likely they will all work together, in a loose symbiosis that helps maintain a healthy ecosystem favorable to all. When that happens, everybody benefits.

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General Anatomy: The taxonomical identity of this acrobat ant colony at the species level has not yet been determined. The genus Crematogaster is species rich and complex. Until recently it has been difficult for practicing field analysts such as I to proceed beyond the basic generic level, simply because the literature on these ants has been sparse and lacking in detail. Oi & Vail (2011), for example, mention only four species (Crematogaster lineolata, C. cerasti. C. laeviuscula, and C. ashmeadi), without providing details that might be used to tell them apart. Morgan (2009), in her revision of the genus in North America, explains that workers of many species vary considerably from specimen to specimen, to the point that — in the past — misidentification, and erroneous assignment of multiple species names to ants within a single species, was common. She notes that over 900 species, subspecies, varieties, races, strips, and transferred names have been recorded for the genus worldwide, making it the  7th largest ant genus known. 51 separate species were, prior to her revision, assigned to North America. She found that 13 of those were synonyms, i.e., the same species had been described under two or more names. That left 38 described species in North America, of which 24 had been observed in the United States, 15 of them in Texas at one time or another. I will be concentrating on those latter 15 species, in an effort to determine if any of them match the ants described here.

Propodeal spines

Gaster & petiole

Dorsolateral body

Dorsolateral petiole

Antenna

Spatulate stinger

Alitrunk & petiole

Although the photographs shown here provide a general view of the basic morphological structures of these ants, they do not depict them in a standard format.

Accordingly, I am mounting several specimens from this collection on dry point mounts, preparatory to taking new photographs that will depict the ants in conformity to the drawings provided in Morgan (2009) and the photos in Blaimer (2012, a & b). Precision measurements are also being made of the 13 anatomical structures described in Morgan (2009), as previously defined by Longino (2003). Once that is done, it should not be difficult to arrive at the correct species identification.

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Taxonomy:

• Kingdom Animalia (ahn-uh-MAYHL-yuh)  — first described in 1758 by the Swedish taxonomist Carolus Linnaeus [23 May 1707 – 10 January 1778], using the Latin word animal = “a living being,” from the Latin word anima = “vital breath”, to refer to multicellular, eukaryotic organisms whose body plans become fixed during development, some of which undergo additional processes of metamorphosis later in their lives; most of which are motile, and thus exhibit spontaneous and independent movements; and all of whom are heterotrophs that feed by ingesting other organisms or their products;
• Phylum Arthropoda (ahr-THROPP-uh-duh) — first described in 1829 by the French zoologist Pierre André Latreille [November 20, 1762 – February 6, 1833], using the two Greek roots αρθρον (AR-thrawn) = jointed + ποδ (pawd) = foot, in an obvious reference to animals with jointed feet, but in the more narrow context of the invertebrates, which have segmented bodies as well as jointed appendages;
• Class Insecta (Linnaeus, 1758) — named using the Latin word insectum, a calque of the Greek word ἔντομον ( EN-toh-mawn) = “(that which is) cut into sections”; comprised of arthropods with chitinous external (exo-) skeletons, a three part body composed of a distinct head, thorax, and abdomen, the midmost part having three pairs of jointed legs, and the foremost part having a pair of compound eyes and antennae;
• Order Hymenoptera (hye-muhn-OPP-turr-uh) — first described in 1758 by the Swedish taxonomist Carl Linnaeus (1707 – 1778), who combined the Greek words ὑμήν (pron. humēn) = “membrane” + πτερόν (TARE-awn) = “wing”, thus ὑμενόπτερος (ewe-mehn-OPP-turr-ose) = “membrane-winged” to refer to insects with membranous wings, specifically the sawflies, wasps, bees, and ants; this is one of the largest orders of insects, and includes over 130,000 species;
• Family Formicidae (fohr-MISS-uh-dee) — first described in 1809 by the French zoologist Pierre André Latreille (1762 – 1833), from the Latin formica = “ant” to refer to hymenopteran insects that have elbowed antennae and a narrow waist that separates the thorax from the abdomen with a node-like petiole; at present, 20 distinct subfamilies of ants are recognized:
  • Subfamily Aenictogitoninae: a subfamily comprising a single genus, Aenictogiton, with seven known species of rarely collected ants found in Central Africa with morphological and phylogenetic affinities to the army ant genus Dorylus; only males have been collected, and nothing is known about their workers, queens or behavior;
  • Subfamily Agroecomyrmecinae: characterized by the following derived traits (see Bolton 2003): mandibular masticatory margins oppose at full closure but do not overlap; eye at extreme posterior apex of deep antennal scrobe; antennal sockets and frontal lobes strongly migrated laterally, far apart and close to lateral margins of head; abdominal segment IV with complete tergosternal fusion; sternite of abdominal segment IV reduced, tergite much larger than sternite and strongly vaulted;
  • Subfamily Amblyoponinae (including the subfamily Apomyrminae): mostly specialized subterranean predators, comprised of a single genus of two species native to California; characterized by the following traits (see Bolton 2003): workers of this ant subfamily — which was formerly considered a tribe within the subfamily Ponerinae — exhibit the following characters: eyes small or absent, if present situated behind the mid-length of side of head; anterior margin of clypeus with specialized dentiform setae; the promesonotal suture is flexible; the petiole is broadly attached to abdominal segment 3 and is absent a distinct posterior face; the postpetiole is absent; a sting is present and is well developed.
  • Subfamily Aneuretinae: this subfamily is comprised of a single extant tribe containing a single extant genus and a single extant species (several extinct tribes, genera, and species have also been described), namely the Sri Lankan relict ant (Aneuretus simoni); this is one of the few ant species considered endangered;
  • Subfamily Cerapachyinae: a subfamily of 5 genera and 217 recognized species, distributed throughout the tropics; they possess spines on the pygidium; their antennae are short and thick; and they lack dorsal thoracic structures; they prey on other ant species;
  • Subfamily Dolichoderinae: presently not divided into tribes, but comprised of 24 genera, including the Argentine ant (Linepithema humile), the erratic ant (Tapinoma erraticum), the odorous house ant (Tapinoma sessile), and cone ants in the genus Dorymyrmex; these ants are distinguished by having a single petiole, absent a post-petiole, and lacking a sting but possessing an apical slit-like orifice at the posterior abdomen instead of the round acidopore encircled by hairs typical of the Formicinae subfamily;
  • Subfamily Ecitoninae (incl. “Dorylinae” and “Aenictinae”): New World and Old World army ants; in the New World, these ants are found in the tribes Cheliomyrmecini (containing the single genus Cheliomyrmex) and Ecitonini (containing the four genera Neivamyrmex, Nomamyrmex, Labidus, and Eciton); the genus Neivamyrmex — the largest of all army ant genera — contains more than 120 species, all native to the United States; the predominant species of the genus Eciton, E. burchellii, has been given the common name “army ant” and is considered the archetypal species; Old World army ants are usually divided into two tribes, Aenictini and Dorylini, but are often treated as a single tribe, Dorylini, alone; each contains a single genus; the genus Aenictus contains over 100 species, and the genus Doryus contains the aggressive “driver ants”, of which 70 species are known;
  • Subfamily Ectatomminae: In North America a single genus, Gnamptogenys, is represented; that genus is not native to North America but has been introduced;
  • Subfamily Formicinae: First described in 1836 by the French entomologist Amédée Louis Michel le Peletier, comte de Saint-Fargeau (1770 – 1845), usu. referred to as Lepeletier, from the Latin formica = “ant” to refer to a subfamily of ants whose evolutionary development is not as robust as most other subfamilies, e.g., they generally retain such primitive features as pupal cocoons, ocelli in workers, and a lesser tendency toward reduced palpal or antennal segmentation; all formicines have reduced stings and enlarged venom reservoirs, with a venom gland that is uniquely specialized to produce formic acid, and a one-segmented petiole having the form of a vertical scale;;
  • Subfamily Heteroponerinae:
  • Subfamily Leptanillinae: comprised of two tribes, the Anomalomyrmini (two genera, seven species) and Leptanillini (three genera, 41 species); within the tribe Leptanillini the larva provide their hemolymph as food to the queen through specialized processes on their prothorax and third abdominal segment; this behavior resembles that of the unrelated Adetomyrma, also called Dracula ants, which actually pierce their larvae to get at the body fluids; ants in the genera Leptanilla and Phaulomyrma are minute, yellow, blind, and subterranean;
  • Subfamily Leptanilloidinae: 1 tribe, 3 genera, 15 species;
  • Subfamily Martialinae: 1 genus containing a single species, Martialis heureka, discovered in 2000 from the Amazon rainforest near Manaus, Brazil, and placed as the sole member of a new subfamily (Martialinae); the generic name, which means “from Mars,” refers to its unusual “out-of-this-world” morphology; the specific epithet heureka honors the surprise that accompanied its discovery; it is the oldest known extant species of ants;
  • Subfamily Myrmeciinae (incl. “Nothomyrmeciinae”): once distributed worldwide but now restricted to Australia and New Caledonia; one of several ant subfamilies which possess gamergates, i.e., female worker ants which are able to mate and reproduce, thus sustaining the colony after the loss of the queen; formerly composed of a single genus, Myrmecia, but revised (Ward & Brady 2003) to include two tribes and four genera; three additional genera, one form genus, and nine species were later described (Archibald, Cover and Moreau 2006) from the Early Eocene of Denmark, Canada, and Washington;
  • Subfamily Myrmicinae: approximately 130 genera in 23 tribes, and 10 additional genera not assigned to specific tribes, all cosmopolitan; the pupae lack cocoons; some species retain a functional sting; the petioles have two nodes; nests are permanent, in soil, rotting wood, under stones or in trees; the subfamily includes leaf cutters (tribe Attini), acrobat (tribe Crematogasterini), harvester (tribe Myrmicini), big-headed (tribe Pheidolini), and fire (tribe Solenopsidini) ants;
  • Subfamily Paraponerinae: comprised of a single genus, Paraponera, containing a single species (Paraponera clavata), known as the lesser giant hunting ant, the conga ant, or the bullet ant (so named for its powerful sting); this ant inhabits lowland rainforest, from Nicaragua and eastern Honduras, and south to Paraguay; the ant is called “hormiga veinticuatro” by locals to refer to the 24 hours of pain following each sting;
  • Subfamily Ponerinae: about 1,600 species in 28 extant genera, including Dinoponera gigantea, which is one of the largest species of ant found in the world; distinguished from other formicine subfamilies by their constricted abdomens;
  • Subfamily Proceratiinae: similar to Ponerinae but the promesonotal suture is fused and the frontal lobes, elevated rather than transverse, are frequently reduced; antennal sockets are exposed in frontal view; in most species abdominal tergite 4 is much enlarged and vaulted, while abdominal sternite 4 is reduced; these are specialized predatory ants that are represented in California by a single species;
  • Subfamily Pseudomyrmecinae: three genera of slender, wasp-like ants that forage alone and readily sting when molested;
• Subfamily Myrmicinae (murr-mih-SEE-nee) — approximately 130 genera in 23 tribes, and 10 additional genera not assigned to specific tribes, all cosmopolitan; the pupae lack cocoons; some species retain a functional sting; the petioles have two nodes; nests are permanent, in soil, rotting wood, under stones or in trees; the subfamily includes leaf cutters (tribe Attini), acrobat (tribe Crematogasterini), harvester (tribe Myrmicini), big-headed (tribe Pheidolini), and fire (tribe Solenopsidini) ants;
• Tribe Crematogastrini (crimm-ATT-oh-gass-TREE-nee);
• Genus Crematogaster (crimm-att-oh-GASS-tur) — first described in 1831 by the Danish paleontologist, zoologist, and archeologist Peter Wilhelm Lund (1801 – 1880), whose scientific work was concentrated in Brazil; he is considered the father of Brazilian paleontology and archeology; Lund apparently crafted the generic name using the Latin word cremare = to burn, consume, conjoined with the Greek root γαστηρ (GAS-turr) = stomach, belly, to refer to ants that are capable of producing burns or a burning sensation using noxious excretions applied by spatulate “stingers” positioned at the distal posteriors of their abdomens;
• Species presently in conjecture…

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References:

• Blaimer, Bonnie B. 2012a. A Subgeneric Revision of Crematogaster and Discussion of Regional Species-groups (Hymenoptera: Formicidae). Zootaxa 3482:47-67
• Blaimer, Bonnie B. 2012b. Taxonomy and Species-groups of the Subgenus Crematogaster (Orthocrema) in the Malagasy region (Hymenoptera: Formicidae). Zookeys 199:23-70
• Bolton, Barry. 1994. Identification Guide to the Ant Genera of the World. Harvard University Press.
• Bolton, Barry. 2003. Synopsis and Classification of Formicidae. Memoirs of the American Entomological Institute, Vol. 71.
• Klotz, John H. and Laurel D. Hansen. 2008. Urban Ants of North America and Europe: Identification, Biology, and Management. Cornell University Press.
• Mallis, Arnold, Stoy Hedges (Ed.) et al. 2011. The Mallis Handbook of Pest Control, 10th Edition. The Mallis Handbook Company.