Proceratium avium, Brown, W. L., 1974

Brown, W. L., 1974, A remarkable new island isolate in the ant genus Proceratium (Hymenoptera: Formicidae)., Psyche 81, pp. 70-83 : 72-81

Proceratium avium new species Figs 1, 2

Holotype worker TL 48 HL 096, HW 094 (CI 98) MI 0.32 scape L 0.93, greatest diameter of eye 0.09, WL 1.43, I petiole in side view 0.60, L petiolar node as seen from above 0.47 W petiolar node 0. 47, L hind tibia 0. 97, L hind metatarsus 0. 81 mm For head length (HL), measurement is taken from the anterior lateral corners of the head (clypeus), head width (HW) excludes the eves, and is taken just behind them The petiolar node length excludes the brief anterior peduncle and is taken from the approxi-mate base of the anterior nodal slope The length of the gaster is taken in side view from the dorsal side of the juncture with the postpetiole straight to the most posterior part of the curve of the second (downcurved) gastric segment (true abdominal segment IV).

Paratype workers (19 measured from 3 colonies at the type locality): TL 4.7 - 5.0, HL 0.92 - 0.98, HW 0.91 - 0.98 (CI 96 - 101). ML 0.31 - 0.34, scape L 0.90 - 0.99, greatest diameter of eye close to 0.09, WL 1.38 - 1.49, L petiole in side view 0.59 - 0.61, L petiolar node 0.42 - 0.47. W petiolar node 0.41 - 0.47, L hind tibia 0.96 - 1.02, L hind metatarsus 0.80 - 0.87 mm.

Composite description: Form of head and body more or less as shown in Figs. 1 and 2. Variation occurs in the following traits: Posterior border of head in full-face view varying from transverse. nearly straight (holotype) to broadly rounded with or without a narrow flattened or even feebly concave median portion, as in Fig. 2. Nuchal carina (on cervical face of head) continuing as a ventrolateral margin halfway down each side of head. Sides of head varying from approximately straight and parallel (Fig. 2) to gently convex and slightly converging anteriad. Lyes each composed of a single clear, convex facet. Median lobe of clypeus with sides feebly sinuous, as in I' ig. 2, or merely weakly convex. Mandibles with 4 strong teeth, but one of these is sometimes double; in addition a small offset tooth is sometimes developed at the basal angle, normally hidden when closure is complete. The inner margin of the peduncle of the mandible has a low, suboblong, sinuous or even bidentate ridge or lamella. visible only when the mandible is open. The scapes vary in length and slightly in apical thickness, so that when laid straight back they surpass the posterior border of the head by amounts ranging from less than half their apical thickness to more than their apical thickness.

Labral shield bilobate, the lobes separated by a broad V-shaped notch; also, lateral to each lobe is a small thumb-shaped lobe or knob extending dorsomesally (flexad) of the plane of the shield from its basal ridge. Maxillary palpi 4 - segmented, the basal segment short and cylindrical, the second segment Hat and attached to the basal segment at nearly a right angle by a peduncle that arises from the side of II; III and IV are short elliptical segments extending in line from II. This is the characteristic form of maxillary palpi in Proceratium . Labial palpi 3 - segmented.

Trunk convex from side to side, and from front to rear in side view, but the portion of the outline from mesonotum to propodeal declivity may be straight or may be interrupted by a feeble dip or saddle in the region of the obsolete metanotal groove. The propodeal angles may be smoothly rounded, as in Fig. 2, or may bear very low, obtuse corners that also render the sides of the declivity vaguely submarginate (30 March colony) as opposed to the rounded sides of the declivity in the 1 April series (Fig. 2), taken less than half a kilometer away. Seen from above, trunk broadly rounded in front. sutureless, tapering gently caudad, with a feeble suggestion of constriction behind mesonotum.

Petiole briefly pedunculate in front, the peduncle with strong anterolateral cornuae, the node varying from loaf-shaped with relatively steep, rounded anterior face (Fig. 2) and rounded sides to a lower version with more gradually sloping anterior face and sides less bulging. Postpetiolar (first gastric) segment varying in dorso-ventral depth, especially near the posterior quarter or third of its length, and the individuals with deeper postpetiole tend to have the tergum of this sgment more swollen (or humped as seen in side view) than in the paratype depicted in Fig. 2.

Integument overall smooth and decidedly shining, but head, trunk. node and postpetiolar (first gastric) segment thickly sown with circular, centrally-tuberculate and piligerous foveae or coarse punctures. These foveae tend to be larger and more densely arranged, even sub-contiguous in places (e. g., in the region behind the eyes), in the 30 March colony as compared with the series of 1 April, but the latter series vary greatly in this respect, even within samples from the same nest. The least strongly punctured individuals have the dorsa of trunk, node and postpetiole almost completely smooth and shining. the punctures here small and sparse. Antennae and legs much more finely and densely punctate, becoming more opaque apicad. Median clypeal lobe finely punctate-rugulose in a longitudinal direction; mandibles striate-punctate, but shining and coarsely punctate toward apicolateral margins. Second gastric (IV true abdominal) segment smooth, shining, with sparse piligerous punctulae. Gastric apex finely and densely punctulate, subopaque.

Pilosity abundant, moderate and uneven in length, of fine, sub-decumbent to suberect tapered hairs, a little longer but much less abundant than usual in Proceratium , grading into a much shorter, appressed to decumbent pubescence-like covering as one moves apicad on antennae, legs and toward gastric apex; also a fine, appressed. medially-directed pubescence in the space between eyes and frontal carinae.

Color (of fully mature individuals) rich bright ferruginous red, legs and scapes often lighter and more yellowish; region immediately around eyes often with darker pigment. Some specimens, presumably nearer to being callow, are lighter, more yellowish-ferruginous in general color.

Malpighian tubules numbered 6 each in 4 live workers dissected. and 4 in a fifth worker, the last probably representing a mutilated specimen (dissected under less than ideal conditions). Not crypto-nephric. The count of 6 agrees with a worker of P. goliath dissected in Costa Rica.

Queen, subergatoid (from 30 March colony): TL 5.1, HL 0.90, HW 0.92 (CI 102), ML 0.34 (mandibles slightly opened), scape L 0.83, greatest diameter of eye 0.14, WL 1.38, petiole L (from above) 0.57, petiole W. 0.58, L hind tibia 0.87, L hind metatarsus 0.71 mm. Head as in worker, of the shape with transverse, only feebly convex posterior border in full-face view; sides converging slightly from behind eyes toward mandibles, and bulging again slightly at lateral ends of clypeus. Mandibles with 5 moderately strong teeth and a small offset basal tooth. Ocelli small but distinct; compound eyes nearly circular in outline, moderately convex, with an estimated 70 distinct ommatidia, relatively smaller than usual in Proceratium queens. Scapes just barely overreaching posterior border of head when laid straight back.

Trunk somewhat reduced, but the usual sclerites of the ptero-thorax separated, except that the elements of the mesonotum are all indistinguishably fused into one gently convex continuous shield; metanotum distinct but narrow, forming the usual acute median process, but in short compressed form. Propodeum with slightly raised, but bluntly rounded angles and slightly concave, submargined declivity. Wing stumps present and blackened, but small.

Petiole shorter, broader and a little higher than in worker, the node rounded above and with a rather steep anterior face. Gaster much deeper and broader than in worker, extraordinarily voluminous, but undercurved as in worker. Sting present, stout, protrusible. Sculpture, color and pilosity much as in the worker over head. trunk and petiole, but the punctures here shallower, less distinct and sparser than in even the more weakly foveate workers; mesonotum and node with obsolescent foveae, almost completely smooth, shining. Gaster smooth and shining, without coarse punctures, but very finely and densely punctulate, with associated dense, short, appressed pubescence and a few short, delicate decumbent to erect hairs, mainly at the posterior borders of the first segment and on the undersurfaces. and becoming abundant near the gastric apex. Due to the great bulk of the gaster, the fusion of the mesonotal sclerites, and the small eyes, I doubt if this queen ever had flight-functional wings.

Male (from 30 March colony): TL 4.7, HL 0.76, HW (including eyes) 0.98, ML 0.29, scape L 0.64, greatest diameter of compound eye 0.40, WL 1.55, L petiole 0.55, W petiole 0.35, L hind tibia 0.93, L hind metatarsus 0.80, forewing 4.0 mm.

Head with high-domed vertex set with 3 large, clear ocelli; compound eyes large and strongly bulging, with convex inner margins as seen in full-face view of head. Median lobe of clypeus projecting, broadly truncate. Frontal carinae parallel, low but sharp, vertically raised. Mandibles triangular, each ending in a stout curved apical tooth, masticatory borders curved and cultrate. Scapes overreaching posterior border of vertex. Palpi segmented, as far as can be seen in the undissected specimen, as in worker.

Alitrunk with well-developed flight sclerites and wings; notauli lacking; scutellum semiglobose, protruding. Metanotum with the usual stout, acute median tooth. Propodeal dorsum short, convex, rounding obtusely into the declivity, which is indistinctly marginate on the sides.

Petiole long and very low, with the highest part of the gently convex node near midlength, but the greatest width at about the posterior third. Postpetiole anteriorly wider than petiole, and widening still more posteriad, but not quite as wide as the succeeding segment, which is vaulted ventrad, but not as strongly as in worker and queen.

Genitalia largely retracted (not dissected), but it can be seen that the parameres have thin, broadly rounded apices that are bent mesad toward each other.

Legs long and slender, with the usual single pectinate spur on each middle and hind tibial apex, and the tarsal claw slender and simple.

Wings of basic Proceratium pattern, with Rsf 2 - 3 completely lacking in the forewing; m-cu also gone; the enlarged " cubital " cell receives only one free vein distally (Rsf 5), because the apical free abscissa of M is gone. In the hind wing, the large cell receives a short remnant of Rs and the longer free apical abscissae of M and CuA; 1 A is missing beyond cu-a. Wings hyaline, with light brown veins. (In the newly-discovered males of the related P. stictum- - from Brookvale, Queensland, and P. goliath - - from Zent, Costa Rica, the venation pattern is similar to that of avium , but darker in goliath , still paler in stictum : m-cu is preserved in stictum , but in the hindwings of both, the free abscissa of Rs is reduced to obsolescence.)

Sculpture shining, with foveae much as in the worker, but also with rugulae and more opaque around the eyes, posterior scutellum, dorsum and sides of propodeum, metanotum and sides of petiole. Pilosity fine, rather short, moderately abundant, decumbent to sub-erect, generally distributed over body, scapes and legs. Color light yellowish-brown, gaster more brownish; vertex infuscated near ocelli and in back of compound eyes.

Holotype (from unnumbered colony, I April 1969) and paratypes (colonies ICA- 69, 30 March 1969; M- 252 and an unnumbered colony, plus strays, 1 April 1969) taken from Le Pouce (mountain), Mauritius, in native forest between 700 and 800 meters elevation on the plateau just below the peak (W. L. Brown, Jr.). Holotype and paratypes deposited in Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts, U. S. A. Paratypes in Cornell University Collection, British Museum (Natural History), Australian National Insect Collection, Canberra, Australia, and elsewhere.

P. avium belongs to the stictum group of Proceratium , containing two other species. P. stictum (Brown, 1958; 366) has been known only from a single worker from North Queensland. (Now I have seen males apparently belonging to stictum , taken at light by E. S. Ross and D. Q. Cavagnaro at Brookvale, Queensland; and R. W. Taylor showed me a series that I was only able to examine hurriedly, but which looked to me like stictum, from his collections made in North Borneo.) The second species of the stictum group is P. goliath (Kempf and Brown, 1968: 94 ff.) from Costa Rica. P. avium is distinct from both of these species in its much shinier and coarser sculpture and in details of the shape of the trunk, particularly its completely unarmed propodeum, in its longer antennae, and in its less strongly undercurved gaster. P. avium is also much larger than P. stictum , but smaller than P. goliath , and its eyes are relatively larger than in either of these species. In fact, P. avium might well go into a group of its own, but the structure of the clypeus, mandibles, and petiole are so much like those of P. stictum and P. goliath that the relationship of these three species is obvious.

The characters they share are also primitive for the genus Proceratium , and this with the widely discontinuous known distribution of the group suggests that the stictum group represents the relicts of an early dispersal wave of a primitive Proceratium stock that spread widely over the earth and was overtaken (except on Mauritius) by later waves of more advanced Proceratium groups. Possibly widespread extinction of stictum-group stocks ensued; although it is not easy to visualize significant competition among such rarely-collected species, I should point out that the adaptive zone (predation of spider eggs) is a limited one. Furthermore, advanced methods of collecting, at least in North America, have proven that some advanced Proceratium species are common in suitable microhabitats. These micro-habitats are, so far as we can tell, all cryptic ones - above all, in large masses of well-rotted wood. This may, of course, be an artifact of biased collecting methods, but I doubt it. On the northern fringes of the range of Proceratium - for example, in the environs of Boston - P. silaceum and P. pergandei are taken occasionally, but always under rocks in the soil. Farther south in the U. S., almost all collections are made deep in rotten wood or in humus and litter near rotten wood. The type series of P. goliath came from a rotten log in wet tropical forest.

The microhabitat of P. avium , to judge from the limited observations I have made, differs strikingly from that of other Proceratium . Certainly, the nest site is different. The first nest of P. avium taken on Mauritius came, it is true, from a hollow rotting stick lying on the forest floor. This may be a common kind of site for the species, yet very many such branchlets were examined on the day of collection without success, and it is possible that the branchlet had fallen recently from a tree above. In any case, I have never seen any of the eight other Proceratium species collected alive by myself in various countries nesting in such a large cavity in such an exposed site as was the colony in this stick. The other two P. avium nests seen were of course arboreal, with the foraging trails openly exposed for some meters over open soil and tree trunk. The ground and arboreal nesting sites for ants in wet tropical montane forest are rather academically distinguished in any case, but it seems to me that the exposure of the nests and foraging trails of P. avium is what is significant here. If other species of Proceratium forage so openly day or night, it has not been noted as far as I know, and indications are against it.

Of greatest interest is the contrast in habitus, especially that part owing to sculpture and pilosity, between P. avium and its congeners. In P. avium , the looser, coarser, more shining integument and its fairly long open pilosity may be compared with the finely rugulose-punctate or reticulopunctulate sculpture and very short, fine, more or less dense pilosity of the other Proceratium species. There is every reason to believe that these fine, crowded punctures and their associated hairs are a specialized evolutionary development (pushed further in Discothyrea ) stemming from a condition in which the integument was more coarsely sculptured, with larger foveae, each fovea bearing a hair on a central tubercle. The Old World (Indo-Melanesian) Gnamptogenys stocks include species (e. g., G. menadensis ) that meet these specifications and are known to be epigaeic, even arboreal, foragers (personal observations), but Gnamptogenys has reduced palpal segmentation and other characters that make it more likely a convergent than an ancestral stock to Proceratium .

I believe that Acanthoponera is nearer to the ancestral line of Proceratium because of the higher basic palpal segment number and the traces in some Proceratium species of a median carina on the frontal area and vertex of the head, characters of Acanthoponera (Brown, 1958: 188). But the main question we are led to consider is: Has P. avium preserved some version of the archetypal Proceratium sculpture-pilosity pattern, or is its present condition a secondary reversion from the fine-scale pattern characteristic of other Proceratium around the world?

Possibly we shall never have a clearly definitive answer to this question, but we do have one clue pointing toward the reversion hypothesis. This clue is the rather unusual eyes of P. avium - a rather large, glassy-looking orb on each side, backed by a perceptible amount of dark pigment, and relatively larger than the characteristically single-facetted " compound " eyes of other Proceratium species. I think we have to suppose that the ancestors of P. avium , and also of all other Proceratium , had already specialized for a cryptic existence to the point where minute single-facetted eyes, probably barely enough to sense the difference between light and dark, served adequately the lifeways of these animals. My guess is that such lifeways also had forced selection for the fine sculpture-pilosity pattern of Proceratium in the ancestral lineage of this genus. If my reasoning is correct, then P. avium is an interesting example of a " character-released " species on a remote oceanic island with a depauperate endemic ant fauna.

The known endemic ant fauna of Mauritius numbers only about 7 species, if we take Donisthorpe`s (1946, 1949) count as a base and deduct obviously introduced species, including synonyms, and add my collections. The endemics now appear to be restricted to the small areas of upland native forest; the cane fields and other culture areas are saturated with introduced ant species. In a sense, then, the mountain forests represent the " real " island (s) of Mauritius as far as the endemics are concerned.

Among the ants of the present endemic fauna, it is difficult to pick out any that might be serious competitors of Proceratium avium . Our assumption here, of course, is that P. avium subsists primarily on arthropod eggs, probably mainly the eggs of spiders. (But the assumption rests on only a few observations, which need augmentation.). Perhaps Solenopsis mameti , a much smaller ant that nests mainly in rotten wood in forest shade, would qualify as a competitor. This judgement is based on the generalized feeding habits of similar-sized Solenopsis elsewhere in the world, and we have absolutely no direct information on the food of S. mameti . At least, the species has not been seen foraging on open paths or tree trunks during the day.

The bright red color and open-trail foraging of P. avium suggests reduced predation pressure in the Mauritian native forest habitat, but the possible mimicry with Pristomyrmex bispinosus could on the other hand indicate that predator pressure is appreciable, and in some way answered by protective properties.

In summary, a reasonable hyothesis to explain the atypical " epi-gaeic characters " of P. avium assumes that the ancestral stock reached Mauritius a long time ago from Africa or Asia in a floating log or rotting branch, and established itself in an ant-poor environment that was perhaps also weak in the kinds of predators that attack open-foraging ants. Evolution in such an environment, it is argued, led to the reacquisition of characters that had been lost by the parent Proceratium stock during continental specialization to cryptic environments in which arthropod (spider?) eggs had become its main food.

Another hypothesis is that the Mauritian Proceratium retains a sculptural-pilosity pattern primitive for the genus, and that its arthropod egg-feeding proclivities were acquired in an open-foraging situation that elsewhere has since been modified under continental pressures of competition and predation.

In order to throw light on the question, it would be interesting to know exactly what animals the pre) ' eggs on Mauritius belong to. and where and how they are taken by the ants. It may be that we shall never find out, for the mountain forests of Mauritius appear to be teetering on the brink of extinction.