Probolomyrmex angusticeps M. R. Smith

Taylor, R. W., 1965, A monographic revision of the rare tropicopolitan ant genus Probolomyrmex Mayr (Hymenoptera: Formicidae)., Transactions of the Royal Entomological Society of London 117, pp. 345-365 : 360-361

Probolomyrmex angusticeps M. R. Smith (figs. 1, 3 - 7, 28 - 31)

Probolomyrmex angusticeps M. R. Smith , 1949, Proc. ent. Soc. Wash. 51: 39, Type locality: Barro Colorado Island, Panama Canal Zone.

The following notes are based on a series of ten workers and five queens from a colony collected by the author on June 21 st, 1961, at the type locality, Barro Colorado Island. I have not seen the holotype or paratype of P. angusticeps , but workers from the material discussed here have been determined by comparison with them, by Dr. M. R. Smith.

Worker (figs. 28, 29)

The following characters are given additional to those stated in Smith's original description.

(1) Dimensions (N = 10): HL, 0.76 - 0.80 mm.; HW, 0.45 - 0.47 mm.; SL, 0.61 - 0.64 mm.; CI, 58 - 59; SI, 135 - 140; WL, 1.12 - 1.17 mm.; PW, 0.39 - 0.41 mm.; dorsal petiole width, 0.22 - 0.24 mm.; petiolar node index, 56 - 62; petiole height, 0.31 - 0.34 mm.; petiolar node length, 0.44 - 0.46 mm.; lateral petiolar index, 132 - 142.

(2) Palpal formula, maxillary 4: labial 2 (4 specimens dissected). Proportions of the palp segments vary slightly; typically the three basalmost approximately subequal in length, and about 1.5 times as long as broad; apical segment slightly narrower and about 3 times as long as broad; in some instances second segment elongated to almost 2.5 times its breadth; labial palp segments narrow and relatively elongate, basal having about same proportions as apical maxillary segment, and apical about 4 times as long as broad, and 1 - 25 times as long as basal segment.

Workers very uniform in structure, but showing slight variation in shape of subpetiolar process.

Queen (figs. 1, 30, 31)

The five queens mentioned above have the following dimensions: HL, 0.74 - 0.76 mm.; HW (behind eyes), 0.46 - 0.47 mm.; SL, 0.60 - 0.62 mm.; CI, 61 - 63; SI, 130 - 132; WL, 1.13 - 1.18 mm.; mesonotal width, 0.40 - 0.42 mm.; dorsal petiole width, 0.23 - 0.24 mm.; petiole height, 0.33 - 0.35 mm.; petiolar node length, 0.43 - 0.45 mm.; lateral petiolar index, 128 - 130; palpal formula, maxillary 4: labial 2 (2 specimens dissected). These specimens differ from the workers in the usual characters of full sexuality discussed in the generic diagnosis above. Colour generally as in worker (medium reddish-brown), eyes blackish-brown, each ocellus with an associated dark brown spot. Wing venation (fig. 1) similar to that of P. greavesi . Palpal structure identical to that of workers. Characters differentiating these specimens from the queen of the related P. boliviensis are given below.

The larval characters of P. angusticeps are discussed above (p. 348, figs. 3 - 7). The pupae are peculiar for ponerines in lacking cocoons.

Distribution. - Known only from the type locality, Panama: Barro Colorado Island.

Biology. - My Barro Colorado accession originally consisted of 22 workers, 7 queens (4 alate), 11 pupae (9 worker, 2 male), 2 pharate pupae and 6 larvae of various instars. These were collected from an old beetle gallery in a fairly sound, dry portion of a rotting branch about 30 cm. long and 8 - 10 cm. in diameter, lying on the floor of primary rainforest. The ants were in a single group occupying almost 3 cm. of the gallery, which was about 6 - 10 mm. in diameter. An aspirator device was used for collection, and it is presumed that the whole colony, excluding foragers, was taken. One of the queens was more worn than the others and appeared to be the " motherqueen " of the colony. The specimens were immediately placed in a glass-topped plaster-of-Paris observation cell, in which they survived for six days until the death of the queen and the larvae disrupted their behaviour. A number of eggs were accumulated during this period, at least two of them being laid by the queen. One worker completed its development while under observation.

Various small soil arthropods including assorted Collembola, Symphyla, small immature spiders, larval and adult ants, Diptera, Coleoptera and termites at all stages of development, and various arthropod and other eggs collected from leafmould, were placed in the nest from time to time. None of these organisms were attacked or accepted as food; indeed, the ants usually retreated hurriedly with their brood when confronted by other animals.

The larvae and pharate pupae, still enclosed in larval cuticles, were not placed by the nurse workers on the nest floor, but were attached to the plaster walls or glass ceiling of the brood chamber by the peculiar terminal abdominal suspensory tubercles described above (p. 348), so as to hang head downwards. Eggs and pupae were normally placed on the floor of the brood chamber, but under moist conditions they too were attached to the nest ceiling, presumably being held there by the surface tension forces of the moisture film on the glass. Pupae were invariably placed with the frontal region of the head adherent to the ceiling, and the eggs were attached either directly to the ceiling, or to the bodies of larvae or pupae.

The workers were very active and " excitable ", being reminiscent of Leptogenys or Platythyrea in this regard. They ran quite rapidly and " nervously " when disturbed and, even when settled, were constantly active, grooming themselves and their partners. The larvae were assiduously attended and were almost constantly being licked by one or more of the workers. The suspended brood was usually placed immediately above the main adult cluster, the nurse workers moving on to the ceiling to attend to the larvae.

Workers were not observed transporting their fellows, but the queen was carried about 5 cm. in the jaws of a worker on one occasion. She lay in a pupal posture and was held by the frontal part of the head, lying ventral side uppermost along the underside of the transporting worker. Pupae were always normally carried in this way, although they were sometimes dragged by the legs or antennae when being positioned by nurse workers. The larvae were always carried along the underside of transporting workers and were invariably gripped about the neck of the terminal abdominal suspensory tubercle. This mode of grasping the larvae appeared to facilitate their placement on the nest ceiling by the workers. The newly emerged workers are highly callow, and apparently take about 5 days to attain full coloration.