[An email exchange over the course of a single day (June 21--coincidentally, the Summer Solstice) has led to something of an understanding about what this ant may be---or at least a general direction to look at. In the interest of accuracy, I am revising this post.]
Whatever it is, it’s a Ponerine. All Ponerines have painful stings – some are reluctant to use it but others are more than happy to give you a jab or 2.
Their stings bothered some members of our Fathers' Day party at Paupau Beach, while others were not bitten at all.
James Wetterer, who has taken on the task of cataloguing the ants of the Marianas, has forwarded an apparently unpublished list, with the following three members of genus Hypoponera listed:
The red imported fire ant is the most common and troublesome species of ant in the southeastern United States. The worker (wingless female) is reddish or dark brown and may be large (6 mm) or small (3 mm). Winged males are black and have colorless to pale brown wing veins. Winged females are mostly light brown. Distinguishing among species of ants in the genus Solenopsis is difficult.
- Hypoponera confinis (Roger) Tinian, from specimens.
- Hypoponera pruinosa (Emery) Tinian and Papago, Saipan, both from specimens.
- Hypoponera punctatissima (Roger) Guam and Asuncion, from published records.
The latter species has been noted as an invasive tramp species, elsewhere.
Aubrey Moore has contributed further:
Were these ants stinging you? If so, they were females. The stinger is a modified ovipositor, so only females can sting.
Leading me to be even more cautious, Google teaches that in some ants, reproductive (winged) females may be dimorphic: Solenopsis spp. have macrogyne and microgyne populations, that make nuptial flights at different times of year.
Two days later, Fe is still suffering from intense itching: when someone touches one of the swollen bites (some of which are starting to develop white heads), she reports that these bites all over her body itch intensely. Mainly, on her back, back of the neck especially, and a couple on her arms. I was also attacked, but my reptilian skin seems to protect me better: only a couple of bites have swollen up. Fe says she was wearing perfume that day: she was clearly the most seriously affected, as she sat in the water for an hour or two, writhing in discomfort. They swarmed all over the back of my (red) shirt, and some others were bitten, and in at least one case, the bites subsided within a day.
This is clearly a seasonal event, and quite probably lunar. Winged ants of another species, a carpenter ant (Camponotus cf. chloroticus) swarmed out of the sill of the front door of our apartment on the same night. They are still bothering me two days later (just like their workers do, all the time.)