Strumigenys pergandei

The first observations of Strumigenys feeding on collembella were made by Wesson (1935) in a study of S. pergandei.

Identification
Bolton (2000) - A member of the Strumigenys pergandei-group. Closely related to Strumigenys angulata but more widely distributed. See the identification section of angulata for a discussion of their differences.

Distribution based on Regional Taxon Lists
Indo-Australian Region: Tokelau. Nearctic Region: United States.



Biology
Wesson (1935) made, for the time, some remarkable observations of this species. "During the summer of 1935 I was able to find over 20 colonies of S. pergandeiin the sandstone hills around Jackson, Ohio. These colonies were found under a wide range of conditions, but since eight were taken on a sandstone bluff only 30 yards long, and since most of the other colonies were found under similar conditions, this locality is described as typical. The soil consisted of two or three inches of pine duff on a shallow layer of clay resting on bed rock. The vegetation consisted of large patches of reindeer moss, Cladonia rangiferina, a few scattered weeds and grass tufts, and numerous small pines, Pinus echinata. The nature of the bed rock, and the insulating character of the moss assured constant moisture.

The Strumigenys were to be found by turning over the reindeer moss and carefully examining the pine needles beneath; but no colony was found which was not in, or in very close proximity to, the nest of another larger ant. Specifically, one nest was found in a dead root in clay near a colony of Lasius brevicornis; another was found in clay in the midst of a nest of Formica subsericea under a large flat rock; a third nest was found in the pine duff close to a colony of Aphaenogaster fulva; a fourth was found in a small dead pine stump together with a colony of Aphaenogaster fulva. All the other colonies were found very near nests of A. fulva.

In order to study S. pergandei, an artificial nest was constructed attempting to duplicate the natural conditions as nearly as possible. A modified Fielde nest was divided into three chambers such that a colony of Strumigenys, consisting of about 60 workers, lived in one chamber, a colony of Aphaenogaster lived in another chamber, and both species had access to the third, the forage chamber. The Strumigenys chamber was separated from the rest of the nest by a slit through which the little ants could pass but the Aphaenogaster could not. The Strumigenys were given clay and dead wood in which to excavate and the other two chambers were filled with dirt and pine needles.

From the beginning the Strumigenys showed no fear of the Aphaenogaster, but wandered freely among them. The Aphaenogaster were aware of their presence and smelled them cautiously, but on only a few occasions did they show any resentment: a worker would pick up a Strumigenys. which feigned death, then would carry her to the edge of the chamber and drop her; whereupon the little ant would uncurl unharmed and reenter the chamber. Over a period of ten months not a single Strumigenys was injured by an Aphaenogaster. The Strumigenys as a rule paid no attention to the other ants, but when an Aphaenogaster worker once broke into their nest they pulled fiercely at her legs.

At first the Strumigenys did not thrive. They occasionally nibbled at the food given to the Aphaenogaster, and drank some honey which had been spilled on the dirt. But their restlessness showed that they were not doing well. Then, on examining a wild colony, I captured a worker carrying in her mandibles a small white object, which, on closer examination was found to be a springtail, Podurid. With this hint, I proceeded to examine more closely the wild colonies of Aphaenogaster, and found that in and around most of them were swarms of springtails. A great many springtails were then collected and introduced in the artificial nest. To my surprise and gratification, the Strumigenys captured over a dozen springtails in the nest in two hours. They appeared perfectly content thereafter as long as an ample supply of springtails was present, but when during the winter the supply became small, they showed their restlessness again, and began to devour their brood. I have not been able to make them take any other kind of small insect.

To hunt the springtails, the Strumigenys either lie in wait in some nook, or they explore in crannies and crevices in search of their prey. In either case the method of capture is the same. The moment the worker scents the springtail, which is one to four mm. away, depending on its size, she stops suddenly, slowly exploring with her antennae in its direction. Having waited for a few minutes, she moves by slow advances to within 1 mm of it. Then she folds her antennae, lowers her head to the ground, and moves imperceptibly in the direction of the springtail until her mandibles almost touch it. She then waits until the springtail moves against her mandibles. When this happens, she strikes, seizing the springtail in her mandibles, piercing it with her sharp maxillary lobes; then drawing it back and stinging it. If, on the other hand, the springtail fails to move, she arouses it by vibrating her antennae around it. If the springtail moves away without touching her mandibles, she again repeats her approach. The source and species of the springtail make no apparent difference to the Strumigenys.

Concerning the nest life of Strumigenys, a number of interesting observations were made. The Strumigenys use their mandibles to excavate the nest, tend the brood and carry other workers to a new nesting site. The larvae are fed by being placed on top of whole springtails. The workers regurgitate to each other by approximating the under sides of their heads. The largest nest found contained 120 workers. All the nests consisted of a large irregular cavity, 2 to 10 cc. in volume, entered by a narrow tunnel.

The basic biology of Strumigenys may then be outlined. Strumigenys pergandeiis an independent ant feeding almost exclusively on springtails. Springtails occur in large numbers in and around the nests of many species of ants. Consequently the Strumigenys gain access to these nests in order to reach their prey. Thus a loose form of symbiosis has developed, further evidenced by the nonhostile attitude of the host ant toward the Strumigenys. The Strumigenys are not restricted to any definite type of springtail, nesting site, or species of host ant. They may even be found living unassociated with any other species of ant in a locality naturally abounding in springtails. Almost all the morphological peculiarities noted are adaptations to this diet: the elongate mandibles and the produced maxillary lobes provide a suitable apparatus to capture and hold the slippery springtails; the extraordinary development of the pilosity on scape and funiculus probably indicates a corresponding development of the sense of smell. The small size of the Strumigenys colonies may be due to the fact that springtails do not usually occur in numbers sufficient to support a large colony; no instance was recorded where there was more than one colony in the same host nest. The petiolar pads and the large size of the nest cavity excavated by these little ants are also probably adaptations to this specialized diet. "

Wesson and Wesson (1939) made the following behavioral observations: "On three occasions workers were seen to bring alternately left and right forelegs to the vertex of the head, rubbing the tarsi forward and placing them on the ground. Whether this was cleaning operation or a means of transfering some substance to the substratum, or has some other significance is not known. The ant did not clean the tarsus after rubbing the head, nor did it rub any other part of its body ". Similar behavior was subsequently found in other Strumigenys by Masuko (1984) (see Strumigenys behavior).

Wesson and Wesson (1939) also noted these biological details: In a previous paper it was shown that S. pergandei lives, in southern Ohio at least, near the colonies of various other species of ants, hunting the Collembola which often abound in the nests of these other species. Since the publication of that paper, we have found S. pergandei on more than 30 occasions in this region, and only once was it not obviously associated with another ant. Stray workers have been found in nest galleries of Camponotus pennsylvanicus, Formica fusca, and Formica integra. Three workers were found in an outlying gallery of a F. fusca mound. The gallery led from the mound to a small kitchen midden to which the pergandei seemed to be going, not, as experiments, have indicated, for the debris there, but for the Collembola which lived about the refuse. Under a large, flat stone in maple woods was found a fine colony of S. pergandei in the center of a large Formica subsericea colony the galleries of which surrounded the pergandei nest on all sides and below. Workers were observed in the galleries of about 28 additional nests of Aphaenogaster fulva under stones. Four or five pergandei workers could often be seen creeping away from the superficial galleries when a stone covering one of these nests was overturned. One such fulva-pergandei association was of special interest because of its similarity to the colony in the midst of the F. subserica nest described above. The pergandei were nesting in a shallow, nearly circular, earthen chamber about 3 cm. in diameter, immediately under a large, flat stone covering a colony of A. fulva. Large, flattened chambers and galleries of the fulva surrounded the pergandei nest for at least 8 cm. on all sides. The walls of the chamber separating the Strumigenys from the Aphaenogaster were about 1/2 cm. thick. With one exception we have always found S. pergandei in the soil, that one exception being in a log.

The developmental periods are approximately: egg, 15 to 16 days; larval stage, about 42 days; pupal stage, about 18 days.

Nomenclature

 *  pergandei. Strumigenys pergandei Emery, 1895c: 326, pl. 8, figs. 17, 18 (w.q.m.) U.S.A. Wheeler, G.C. & Wheeler, J. 1955a: 144 (l.). Combination in S. (Cephaloxys): Emery, 1924d: 325; in S. (Trichoscapa): Smith, M.R., 1943f: 307; Creighton, 1950a: 308; in Smithistruma (Wessonistruma): Brown, 1948e: 106; Brown, 1953g: 51; in Pyramica: Bolton, 1999: 1673; in Strumigenys: Baroni Urbani & De Andrade, 2007: 126. See also: Bolton, 2000: 116.

Worker
Bolton (2000) - TL 2.5-2.9, HL 0.66-0.72, HW 0.47-0.50, CI 68-74, ML 0.16-0.18, MI 25-30, SL 0.37-0.42, SI 79-84, PW 0.29-0.33, AL 0.64-0.72 (10 measured).

Basal lamella of mandible narrowly triangular, high and slightly recurved, fully exposed and with apices of lamellae meeting above the labrum when mandibles fully closed. Basal lamella very widely separated from basal tooth, distance between them about equal to the dentate length of the blade in full-face view. Basal and third tooth on mandible triangular and acute, second tooth broad and bluntly rounded; basal longer than second or third teeth.

Anterior and lateral margins of clypeus meeting through evenly rounded broad curves in full-face view. Eye small, with only about 6-8 ommatidia in total. Cephalic dorsum without standing hairs of any form but densely clothed with spoon-shaped ground- pilosity.

Apicoscrobal hair absent. Pronotal humeral hair absent. Dorsum of promesonotum with large curved spoon-shaped ground-pilosity but without erect hairs. Disc of postpetiole with posteriorly curved spatulate hairs. Middle and hind basitarsi each with 1-2 long fine flagellate hairs.

Type Material
Bolton (2000) - Syntype workers, queens and males, U.S.A. Maryland, District of Columbia, Pennsylvania (T. Pergande) [examined].