Poneracantha ingeborgae

A specialized millipede feeder from Colombia.

Identification
A member of the rastrata complex (in the rastrata subgroup of the rastrata species group). Mandibular masticatory margin denticulate, promesonotal suture present, at least partially, and the longitudinal costulate on the propodeal dorsum and postpetiolar tergite is frequently ill-defined to effaced. (Lattke 1995)

Key to New World Gnamptogenys Species

Distribution based on Regional Taxon Lists
Neotropical Region: Colombia.

Biology
Some general aspects of Gnamptogenys biology are stated first, followed by details about this ant's foraging habits.

Brown (1993):

Hunting and Prey Handling

At the time of collection, the G. ingeborgae nest contained about 15 workers, one queen, larvae, a few pupae in cocoons, and a tiny midden of the whitish, separated empty body rings of two or more small, possibly immature millipedes, thought to belong to suborder Spirobolida. This colony was returned alive to the U. S. with some loss of workers and brood, and a few adult workers, a queen and males emerged during July-December 1976.

The colony was kept in a small plaster-of-Paris nest with glass top and chamber diameter of 9 cm. The ants constructed a rude nest cell at one wall of this; the cell cavity occupied about cm2, and consisted of tiny fragments of rotten wood and other debris, open to view above.

The first prospective prey, introduced to the chamber opposite the nest cell, was a local (Ithaca, New York) julid millipede found beneath a stone in my garden. It was probably of genus Orojulus, common in this area. The millipede, ca. 1.5 cm long, uncoiled and began to move around the perimeter of the chamber, where it soon encountered a foraging ant worker. The reaction was immediate and energetic. The ant attacked the millipede, grasping it with its mandibles in the anterior quarter of the body while facing in the same direction as the millipede, astride its dorsal aspect, and with the tip of its gaster applied to the victim’s integument, apparently at an intersegmental articulation. The millipede responded by coiling and uncoiling violently for a few seconds while it apparently was being stung, but within 20 seconds it was stretched out and was still, with an occasional twitch for a few seconds. The behavior of the ant appeared to be frantic, and soon gave way to obvious aversion and signs of acute distress, dismounting, rapid cleaning strokes of the forelegs over head and antennae, and body dragging on the chamber floor--actions that I had learned from watching other Ponerinae, including Gnamptogenys spp., are attempts to get rid of noxious substances produced by the arthropods attacked.

During this aversive episode, which lasted for a minute or so, the ant stood apart from the dead or paralyzed millipede and showed no interest in it, eventually leaving it alone after further bouts of self-cleaning with the legs. Similar results followed introductions of julids to the chamber on several later occasions, including some that were made after offerings of alternative millipede taxa, including Oxidus in particular. The one exception to this behavior occurred when a julid, circling the perimeter of the chamber, chanced to breach the ants’ nest cell wall. On this occasion the response of the ants was immediate and obviously one of distressed defense, with some workers moving brood, while two attacked the millipede. After one of the ants stung the millipede to immobility, its body was removed after a lapse of a few minutes by a single worker that dragged the carcass to the opposite side of the observation chamber in several stages, punctuated with grooming motions of its own body.

The impression left by the observations of julid contacts was that the ants responded defensively, and also aversively, in a relatively frenetic, generalized way. By contrast, behavior during trials with another millipede were more deliberate and stereotyped.

This millipede was Oxidus (=Fontinaria) gracilis, according to T. Eisner after William T. Keeton. This species was moderately abundant during the summer and fall of 1976 in wood-bark chip mulch put around shrubbery on parts of the Cornell University campus at Ithaca. It is a pinkish-brown polydesmid, averaging about 1.2 cm long in this population. About 20 trials were made with introductions of Oxidus into the chamber containing the G. ingeborgae (denoted G) nest.

When the Oxidus (O) made its circumambulation of the chamber, it soon met a foraging G worker coming in the opposite direction. On contact, O reversed its walk, while G showed an obvious alertness and followed the millipede closely. Its approach was not the frenzied attack it and other G workers had made on the julids, but instead was a relatively methodical overtaking of O, followed by a climb onto its back on the anterior half of the millipede’s body, which it grasped with its mandibles. With its antennae playing about the dorsum of O near O’s head, G moved back slightly and applied its gastral tip to the region of an intersegmental articulation on the dorsum of O, where it apparently inserted its sting, judging from the sudden violent reaction of O, which coiled and uncoiled two or three times rapidly for 20-25 sec, and then lay still in the stretched-out position (Fig. 5). G continued in the stinging position for a few seconds longer, and then dismounted and stood beside O as it groomed its own antennae, head and body with its forelegs.

Much to my surprise and annoyance, G briefly frisked its antennae over O, and then left O lying on the chamber floor, stretched out dorsal side up and motionless. G wandered a bit, ending by returning to its own nest cell, taking about 9 min to arrive there. After approximately the same period of time in the nest, grooming and remaining still, the same G worker left the nest cell and returned by a fairly direct route over the distance of 5-6 cm to the the millipede. After two episodes of dragging the carcass a short way toward the nest cell, with some brief excursions and grooming sessions a short distance away from O, the ant left O lying venter up. After about 10-15 min of this seemingly dilatory behavior, G went to O head first and reached over the upraised mandibles of the immobile millipede to seize it with her own jaws by the “throat,” that is, the ventral region of O just caudad of O’s mouthparts, somewhat as in Fig. 6, but without the under-curling of the ant’s gastral apex. With a heave from this position, the ant threw the body of the millipede over her own body and, facing the same way while maintaining her grip in the millipede’s postgnathal region, moved off to the nest with the carcass over her back and trailing to the floor behind. This maneuver recalled for me the figure in W. M. Mann’s National Geographic Magazine article (Mann, 1934: P1. VI) on ants, of Gnamptogenys (= Emeryella) schmitti carrying a millipede in Haiti, and scenes I myself had witnessed with other millipede-hunting ants of the same genus (unpublished notes).

Once arrived in the nest cell, G threw her burden down off her back by a forward heave, so that the carcass was again stretched out, ventral side up, with the ant still clutching its throat. The ant commenced working in this postgnathal area of O, her mandibles biting into an intersegmental area, and in a short timema matter of seconds she broke off the head-and-gnathal region of the millipede by pulling backwards. At that time, I was surprised and puzzled at the apparent ease with which this decapitation was accomplished, and it was only after repeated observations of Gnamptogenys ingeborgae operating with Oxidus prey that I saw how this was done, always in the same way. Once my attention was drawn to the whole action, and not just the biting, I saw that the ant’s gaster was curled under the head of the millipede, with the tip applied to a postcephalic tergum of O (fig. 6). In this way, the gastral apex of the ant served as a fulcrum against which the mandibular tug could break off the head and associated segments of the millipede in a neat and efficient way. Once the cephalic complex of the millipede was removed, the ant dragged the carcass to the vicinity of some half-grown larvae lying on the floor of the cell nearby. In the presence of the prey, the larvae showed some movement of the head and the “neck” area of their thoracic segments, and they were soon placed on the prey at the truncated end of its body, where they began to feed as they stretched out their long, neck-like forebodies to reach inside the open ring. They remained thus, continuously feeding, and at times the huntress and other adult nestmates joined in the feast, side by side with the larvae. As the larval heads reached deeper into the prey, the first body ring of O came loose as it was freed of its muscles and other tissues; it was removed and laid aside by a worker (fig. 7). The feeding progressed on into the carcass in the same way, with the rings being removed one at a time until the contents of the body of the millipede had in a few hours been consumed. At the same time some larvae were put on the cephalic part of the millipede, which was eaten out in the same way.

Nomenclature

 *  ingeborgae. Gnamptogenys ingeborgae Brown, 1993: 279, figs. 3-7 (w.q.) COLOMBIA.

Worker
Holotype: TL 4.8, HL 0.96, HW 0.90 (CI 94), ML 0.42 (MI 44), SL 0.88 (SI 97), EL 0.22, WL 1.52 mm. Paratypes, from type nest series (n=8): TL 4.4-4.9, HL 0.92-0.97, HW 0.86-0.90 (CI 91-94), WL 1.49-1.54 mm.

Head with broadly rounded posterior corners and weakly convex sides; vertexal outline, while weakly concave mesad, is filled in by the convex outline of the nuchal rim behind it. First segment of antennal funiculus longer than the second; first through third longer than broad; fourth through tenth about as broad as long, apical antennomere about twice as long as broad. Costulae of cephalic sculpture strong and regular but slightly wavy in a few places, with scattered coarse piligerous punctures intervening, about 20-23 costulae between the frontal carinae. Legs mostly smooth and shining. The row of stiff hairs in the basitarsal concavity opposing the strigil of the fore leg lacks a longer, specialized, stiff seta. Erect and suberect, fine, fairly abundant pilosity on most body surfaces, including underside of head.

Queen
Dealate, paratypes from type nest (n=2): TL 5.7-5.8, HL 0.98, HW 0.91-0.92 (CI 93-94), ML 0.38 (MI 39), SL 0.89 (SI 97-98), EL 0.24, WL 2.28-2.30 mm. One queen enclosed and lost her wings about a month after the original collection.

Male
(n=l of 9 ex type nest): TL 4.0, HL 0.72, HW including eyes 0.58, ML 0.24), SL 0.18, EL 0.31, WL 1.44, forewing L 3.6 mm.. Mandibles with nearly straight masticatory margins with small but distinct, separated teeth in series. Median dorsal part of head around antennal insertions and ocelli with some fine, indefinite rugulation, but remainder of body prevailingly rather smooth and fairly shining. Notauli distinct and deep, forming a complete Y. Petiole low and somewhat flattened; prora forming a distinct lip. Color piceous black, with dark brown legs and wing veins; antennae yellowish brown; however, other males reared in the type nest are dull yellow or brownish yellow, and probably teneral.