Probolomyrmex procne Brown, 1975

Varghese, Thresiamma, 2024, of a rare ant species Probolomyrmex procne (Formicidae Proceratiinae) from the Indian Institute of Science campus, with description dealate queen, Halteres (Oxford, England) 14 (1), pp. 1-16 : 5-10

Probolomyrmex procne Brown, 1975 ( Figs. 1–2).

Probolomyrmex procne Brown, 1975: 56 . Holotype worker. Type-locality: Palni Hills, 39 km. E Kodaikanal, Tamil

Nadu [Madras] India [MHNG].

Worker diagnosis. P. procne clearly fits in the longinodus group by the following characters: petiole clearly longer than high ( Fig. 1D) and subpetiolar process is low with poorly developed postero-ventral portion ( Eguchi et al. 2006). Since the worker caste was thoroughly described by Brown (1975), only the most important characters are included here. Head narrow, sides convex, no eyes; mesosoma very feebly but evenly convex from front to rear in side-view outline, propodeal teeth or angles almost rectangular. Lateral petiolar index about 136; dorsal surface convex behind as well as in front; seen from above with almost perfectly straight sides diverging caudad and ending in a pair of acute angles, with the posterior border broadly and rather deeply concave. Erect pilosity restricted to a few hairs on the mandibles; pubescence of the very fine, opaque “pruinose” kind, overlaid by larger punctures of foveolate that are particularly distinct on the 2 main gastric segments, where the integument is slightly more shining; larger punctures are a little larger and more distinct on node. Colour medium ferruginous with more yellowish antennae and legs ( Brown 1975). The specimen in this study agrees with the above description as well. It agrees well with the other diagnostic features as mentioned earlier: frontal lobes fused to each other and forming a vertical plate; antennal socket located in shelf-like fronto-clypeal region that overhangs mandibles; mandible small, elongate-triangular, with acute apical tooth followed by a series of small denticles; outer surface of mandible with several thick and short setae as seen above. Morphometric data provided by Brown (1975) in parenthesis () mostly agrees with the fresh data: HL 0.58 (0.58), HW 0.36 (0.375), SL 0.37 (0.40), WL 0.66 (0.75), PRNW 0.27 (0.29), PTL 0.28 (0.32), PTW 0.18 (0.21), PTH 0.22, TL 2.36 (2.40), CI 63 (65), SI 103, LPtI 133 (136).

Antennal sensilla. The nature and arrangement of sensory hairs throughout the body are distinctive and those on antennae are remarkably diverse. The antennal scape, pedicel, and especially the antennal funiculi are with abundant pubescence, sensory hairs of varying shapes and sizes in addition to varying number of pits ( Fig. 3A–B). The last flagellar segment shows the most diversity and the following composition of antennal sensilla ( Fig. 3C). A unique shaped, very long appressed hairs, which are exceptionally longer and thicker than all other hairs, set in deep longitudinal grooves, are present in quite abundant numbers on flagellum of P. procne as seen in Fig. 3C, 1 (inside markings). Those in longitudinal grooves are in two different shapes and sizes, the second type is distinctly long, thin and ridged and is similar to sensilla chaetica ( Fig. 3C, 2) and this type of sensilla are most commonly seen in almost all species of ants’ flagellar segments.

The long and thin appressed hairs are present from first funicular segment onwards, whereas the very long and exceptionally thick appressed hairs are present from the second funicular segment onwards only. The long appressed hairs resemble the “LAH” reported in many species of Probolomyrmex in the study by Oliveira & Feitosa (2019), though they are not classified into any of the known sensillum types.

The long, dagger like basiconica seems to be very diverse as shown in the image ( Fig. 3C, 3a, 3b). The most prominent, long, and thick basiconica, which are broader at base and uniformly narrowing towards the tip are abundant and uniformly distributed on the ventral surface of the last flagellar segment, whereas they are absent on dorsal surface. The second type of basiconica, which are narrow at the base and tip, with a broader middle portion like a spearhead are at least in three subtypes, one long, thick and seen inclined at an angle to the antennal surface, whereas the second type is very short, but are of the same shape, seen at similar angles. The third possible subtype, which are scarce are placed vertically on the surface as seen in Fig. 3C.

There are varying number of antennal pits on the surface of different flagellar segments as seen in Fig. 4. Additionally, there observed a few remarkably thicker and larger sensilla towards the border of the last flagellar segment, both inside and outside surface, with a very broad base and narrowed rounded tip, which projects beyond all other sensilla ( Fig. 3C, 5) and might belong to the basiconica type of sensilla. The sensilla of this shape were not reported in any other ant species.

The most abundant and diverse type of sensilla noticed on the last flagellum are trichodea ( Fig. 3C, 6, 6a, 6b), both on the dorsal and ventral surface. The first type is long, thin and straight, whereas the second type is curved, and this type seems to be of two types, one thin, curved and quite abundant. These type of sensilla are present throughout the surface and form a pattern surrounding the LAP hair, as if it protects the LAP sensilla. The second curved type, trichodea curvata is flattened, broad from base and very curved and fewer in number compared to other two types of trichodea curvata sensilla. Another not so abundant type of sensilla, similar to companiformia in other species of ants are observed at the joints ( Fig. 4A, 7).

In addition, there are some unique types of sensilla at the basal segments of the antennae in P. procne (these are present only on the basal segments, lesser or absent on 12th flagellar segment), which are broad with transverse ridges, but the tips are narrowed and rounded mostly. These types may be classified into at least six types ( Fig. 4A, 8, 8a, 8b, 8c, 8d, 8e). Some of these are narrow at base, medially broad, uniformly narrowing towards the tip, more or less lanceolate in shape and are upright ( Fig. 4A, 8), whereas a few other sensilla are of the same shape and size but are slanted ( Fig. 4A, 8a). A further little narrower form than this slanted subtype is also seen abundantly ( Fig. 4A, 8b). Another subtype has a similar narrow base, but medially very broad and attenuated distally with an oblique tip ( Fig. 4, 8c). While a few are similarly shaped, attenuated, but much shorter and narrower ( Fig. 4, 8d). However, some others are of the same shape, but larger than the above mentioned two types ( Fig. 4A, 8e). These type of sensilla are very abundant at the basal segments ( Fig. 4B–C), but the number decreases towards the tip ( Fig. 4D–E, 5A) and visibly, they are overlaid by the generous array of trichodea sensilla.

The sensilla coeloconica are deep, wider and conspicuous with distinct peg in the middle as seen in earlier Fig. 4A, 9. Another not so abundant types of sensilla are possibly sensilla ampullacea as showed in earlier Fig. 4A, 9a. Among the abundant, diverse sensilla on surface of basal antennal segments, some are similar to previously described types ( Fig. 4A, 8–8e) but longer and narrower than most of them are also there as seen in Figure 4A, 10–10a). Furthermore, there are a few clusters of B ӧhm bristles at the basal segment, which are very thin and short ( Fig. 5B, 11). The most common and abundant types of sensilla are those types described above and many of them are known in other species of ants and other insects as well.

Distribution. Karnataka (Mysore) and Tamil Nadu (Coimbatore, Palni hills) ( Brown 1975; AntWeb 2024).

Differential diagnosis. As per Eguchi et al. (2006), P. procne is similar to P. dammermani Wheeler and P. longinodus Terayama & Ogata but differs from them by the nature of the petiolar node and by the differences in morphometry. SEM images of P. procne , further signify body sculpturing, hairs, spines, spurs, teeth, and claws of the worker and dealate queen. The whole body is furnished with abundant hairs, including the metapleural gland and legs, most of which are in foveae and micropits ( Fig. 5C–E). Tip of the gaster possess a small number of teeth as in many species of ponerinae and cerapachyinae ( Fig. 5F). Also, tibial spurs are pectinate, whereas hairs on legs are abundant, long, thin, flattened and greatly pointed like a knife, towards the tip ( Fig. 5G–H).

and metapleural gland; E. close up view of propodeal spiracle; F. gaster tip showing teeth; G. pectinate tibial spur; H. distribution and abundance of hairs and setae on leg.

Queen diagnosis. The queen is similar in colour and morphology to the worker caste, but the queen differs majorly from workers in having well developed eyes, ocelli, and wings (Fig. 6A).

Description. Dealate measurements: HL 0.58, HW 0.41, SL 0.43, WL 0.78, PRNW 0.31, PTL 0.28, PTW 0.23, PTH 0.22, GL 1.03, TL 2.68, CI 71, SI 104, LPtI 127.

Body ferruginous. The body sculpturing, structure of head, appendages, and metasoma are as in workers. Additionally, dealate queen possess well-developed eyes, three ocelli of similar size. Head 1.41 × longer than wide, sides broadly convex, sclerites of frons and clypeus are fused and form shelf-like projection (Fig. 6B–D), which overhangs mandibles as in worker castes. Antennae 12-segmented; inserted on a shelf-like projection; antennal insertions are fully open as in workers; their bases are separated by narrow, vertical carinae. Antennal scape short, extending a little beyond the level of posterior margin of median ocellus. Mandibles short and triangular, with acute apical teeth followed by 6–7 undefined teeth. Pronotum subtriangular in lateral view. Metanotum convex (Fig. 6E). Petiole little longer than its height with well-developed subpetiolar process ( Fig. 7A).

Antennal sensilla. The diversity and abundance of sensilla showed variations across scape, pedicel and flagellar segments as observed in worker antennae ( Figs. 7–8). Evidently, the density and diversity of sensilla across the whole antennae were significantly reduced compared to those in worker antennae. Those very thick LAP sensilla were observed from 3rd funicular segment onwards, whereas the thin, LAP sensilla were observed from first funicular segment onwards as in workers, though in very few numbers on all segments. Additionally, three variants of possibly, trichodea and basiconica, were also observed in fewer numbers and there are a few probably queen specific sensilla ( Figs. 8H, 9A–C), which are present from 7th segment onwards. As described above, the shape, size, abundance, and distribution of these sensilla differs between the antennae of dealate queen and worker as well. As observed in the case of sensilla on antennal flagellar segments, sensory hairs are lesser on the cuticular surface of dealate queen as well.