Pseudomyrmex flavicornis

An obligate acacia mutualist that can inhabit a few different species of Vachellia. This is the Pseudomyrmex species noted in Belt's 1874 account of his travels in Nicaragua.

Identification
Ward (1993) - Key traits of this species are the laterally rounded median clypeal lobe, large size (worker HW > 0.98; queen HW 1.12-1.19, n=13), broad opaque head (worker CI > 0.82; queen CI 0.73-0.76) and dark color. P. flavicornis is one of three species in the P. ferrugineus group whose workers and queens have a black or very dark brown body (mesosoma sometimes contrastingly lighter). The other two, Pseudomyrmex mixtecus and Pseudomyrmex veneficus, are allopatric to P. flavicornis and smaller in size; other distinguishing features are mentioned in the key and discussed under those species. A tendency towards lighter coloration of the mesosoma in northern populations of P. flavicornis sometimes makes it difficult to distinguish this species from sympatric dark-colored Pseudomyrmex ferrugineus. Even the darkest workers and queens of the latter species are, however, smaller (an average difference in workers, almost absolute in P. ferrugineus queens which have HW 0.96-1.12 (n = 37)) with longer eyes and shorter scapes for a given relative head breadth; and they show some reflectance of light on the upper third of the head between the ocelli and compound eye in contrast to the more or less opaque-headed P. flavicornis. In addition, P. ferrugineus queens have more elongate heads (CI 0.68-0.73) than those of P. flavicornis.

Distribution
Guatemala to Costa Rica

Distribution based on Regional Taxon Lists
Neotropical Region: Belize, Costa Rica, El Salvador, Guatemala, Honduras, Mexico, Nicaragua.

Biology
This is a monogynous species ranging from which inhabits Acacia collinsii and, less frequently, A. cornigera and A. hindsii. It was one of the first acacia-ants to be brought to the attention of naturalists, thanks to an early account of its biology by Thomas Belt (1874) (under the name Pseudomyrma bicolor). In the more recent literature P. flavicornis has usually been referred to as "P. belti", but note that there is not a perfect correspondence in name usage. (Ward 1993)

Belt (1874):

Clambering down the rocks, we reached our horse and mule, and started off again, passing over dry weedy hills. One low tree, very characteristic of the dry savannahs, I have only incidentally mentioned before. It is a species of acacia, belonging to the section Gummiferae, with bi-pinnate leaves, growing to a height of fifteen or twenty feet. The branches and trunk are covered with strong curved spines, set in pairs, from which it receives the name of the bull's-horn thorn, they having a very strong resemblance to the horns of that quadruped. These thorns are hollow, and are tenanted by ants, that make a small hole for their entrance and exit near one end of the thorn, and also burrow through the partition that separates the two horns; so that the one entrance serves for both. Here they rear their young, and in the wet season every one of the thorns is tenanted; and hundreds of ants are to be seen running about, especially over the young leaves. If one of these be touched, or a branch shaken, the little ants (Pseudomyrma bicolor, Guer.) swarm out from the hollow thorns, and attack the aggressor with jaws and sting. They sting severely, raising a little white lump that does not disappear in less than twenty-four hours.

These ants form a most efficient standing army for the plant, which prevents not only the mammalia from browsing on the leaves, but delivers it from the attacks of a much more dangerous enemy—the leaf-cutting ants. For these services the ants are not only securely housed by the plant, but are provided with a bountiful supply of food, and to secure their attendance at the right time and place, the food is so arranged and distributed as to effect that object with wonderful perfection. The leaves are bi-pinnate. At the base of each pair of leaflets, on the mid-rib, is a crater-formed gland, which, when the leaves are young, secretes a honey-like liquid. Of this the ants are very fond; and they are constantly running about from one gland to another to sip up the honey as it is secreted. But this is not all; there is a still more wonderful provision of more solid food. At the end of each of the small divisions of the compound leaflet there is, when the leaf first unfolds, a little yellow fruit-like body united by a point at its base to the end of the pinnule. Examined through a microscope, this little appendage looks like a golden pear. When the leaf first unfolds, the little pears are not quite ripe, and the ants are continually employed going from one to another, examining them. When an ant finds one sufficiently advanced, it bites the small point of attachment; then, bending down the fruit-like body, it breaks it off and bears it away in triumph to the nest. All the fruit-like bodies do not ripen at once, but successively, so that the ants are kept about the young leaf for some time after it unfolds. Thus the young leaf is always guarded by the ants; and no caterpillar or larger animal could attempt to injure them without being attacked by the little warriors. The fruit-like bodies are about one-twelfth of an inch long, and are about one-third of the size of the ants; so that an ant carrying one away is as heavily laden as a man bearing a large bunch of plantains. I think these facts show that the ants are really kept by the acacia as a standing army, to protect its leaves from the attacks of herbivorous mammals and insects.

The bull's-horn thorn does not grow at the mines in the forest, nor are the small ants attending on them found there. They seem specially adapted for the tree, and I have seen them nowhere else. Besides the Pseudomyrma, I found another ant that lives on these acacias; it is a small black species of Crematogaster, whose habits appear to be rather different from those of Pseudomyrma. It makes the holes of entrance to the thorns near the centre of one of each pair, and not near the end, like the Pseudomyrma; and it is not so active as that species. It is also rather scarce; but when it does occur, it occupies the whole tree, to the exclusion of the other. The glands on the acacia are also frequented by a small species of wasp (Polybia occidentalis). I sowed the seeds of the acacia in my garden, and reared some young plants. Ants of many kinds were numerous; but none of them took to the thorns for shelter, nor the glands and fruit-like bodies for food; for, as I have already mentioned, the species that attend on the thorns are not found in the forest. The leaf-cutting ants attacked the young plants, and defoliated them, but I have never seen any of the trees out on the savannahs that are guarded by the Pseudomyrma touched by them, and have no doubt the acacia is protected from them by its little warriors. The thorns, when they are first developed, are soft, and filled with a sweetish, pulpy substance; so that the ant, when it makes an entrance into them, finds its new house full of food. It hollows this out, leaving only the hardened shell of the thorn. Strange to say, this treatment seems to favour the development of the thorn, as it increases in size, bulging out towards the base; whilst in my plants that were not touched by the ants, the thorns turned yellow and dried up into dead but persistent prickles. I am not sure, however, that this may not have been due to the habitat of the plant not suiting it.

These ants seem at first sight to lead the happiest of existences. Protected by their stings, they fear no foe. Habitations full of food are provided for them to commence housekeeping with, and cups of nectar and luscious fruits await them every day. But there is a reverse to the picture. In the dry season on the plains, the acacias cease to grow. No young leaves are produced, and the old glands do not secrete honey. Then want and hunger overtake the ants that have revelled in luxury all the wet season; many of the thorns are depopulated, and only a few ants live through the season of scarcity. As soon, however, as the first rains set in, the trees throw out numerous vigorous shoots, and the ants multiply again with astonishing rapidity.

Amador-Vargas (2019) found that Pseudomyrmex spinicola made larger clearings around the trees they occupied than Pseudomyrmex flavicornis and Pseudomyrmex nigrocinctus. Pseudomyrmex spinicola workers also pruned thicker vegetative tissue, and their workers had broader heads - presumed to allow more robust pruning due to larger muscles in the head providing greater mandibular force. More generally, this study and its findings are unusual in examining plant pruning differences provided by ants involved in acacia mutalisms. Most studies of ant-provided benefits to plants in these ant-plant mutualisms focus on herbivory.

Nomenclature

 *  flavicornis. Pseudomyrma flavicornis Smith, F. 1877b: 67 (w.) NICARAGUA. Combination in Pseudomyrmex: Kempf, 1972a: 218. Senior synonym of belti, fellosa, obnubila: Ward, 1989: 438. See also: Ward, 1993: 141.
 * belti. Pseudomyrma belti Emery, 1890b: 63, pl. 7, fig. 1 (w.q.) COSTA RICA. [Also described as new by Emery, 1894k: 52.] Wheeler, G.C. & Wheeler, J. 1956: 380 (l.). Combination in Pseudomyrmex: Wheeler, G.C. & Wheeler, J. 1956: 380. Junior synonym of flavicornis: Ward, 1989: 438. See also: Wheeler, W.M. 1942: 159.
 * obnubila. Pseudomyrma belti var. obnubila Menozzi, 1927c: 273 (w.q.) COSTA RICA. Combination in Pseudomyrmex: Kempf, 1972a: 216. Junior synonym of flavicornis: Ward, 1989: 438.
 * fellosa. Pseudomyrma belti subsp. fellosa Wheeler, W.M. 1942: 160 (w.) NICARAGUA. Combination in Pseudomyrmex: Kempf, 1972a: 216. Junior synonym of flavicornis: Ward, 1989: 438.

Worker
Ward (1993) - Worker measurements (n = 29). —HL 1.06-1.42, HW 0.99-1.21, MFC 0.075-0.114, CI 0.83-0.94, REL 0.39-0.45, REL2 0.45-0.51, OOI 1.28-2.71, VI 0.59-0.73, FCI 0.068-0.098, SI 0.40-0.46, SI2 0.82-0.97, NI 0.61 -0.68, PLI 0.57-0.67, PWI 0.60-0.72, PPWI 1.36-1.80.

Similar to P. ferrugineus (q.v.) except as follows. Larger, with shorter eyes, on average. Head densely punctulate, opaque; overlying rugulo-punctate sculpture on propodeum tending to be better developed than in P. ferrugineus. Pilosity and pubescence denser on average. Head black, gaster and postpetiole dark brown to black, mesosoma and petiole varying from black to a contrasting lighter brown or reddish-brown; mandibles, scapes, fronto-clypeal complex, and apices of legs brown.

Type Material
Ward (1993):

Lectotype worker, Nicaragua [Examined].

Pseudomyrma belti Syntype workers, Alajuela, Costa Rica (BMNH, ) [Examined].

Pseudomyrma belti var. obnubila Syntype worker, San Jose, Costa Rica (H. Schmidt) [Examined].

Pseudomyrma belti subsp. fellosa Syntype workers, Nicaragua (W. Fluck); Granada, Nicaragua (C. F. Baker) [Examined].