Aenictus inflatus

Remarkable for the genus in having a polymorphic worker caste, and an inflated propodeum (found filled with red liquid in the type colony) in larger workers. The smaller workers possess narrower heads with the sides almost parallel, very short antennae and legs, and normal propodea. Heterick & Kitching (2022) collected this species in the canopy of a lowland dipterocarp forest in Brunei.

Identification
Jaitrong and Yamane (2011) - The only member of the inflatus species group. Largest worker. Head in full-face view with occipital corner rounded; occipital margin lacking collar. Antenna long, consisting of 10 segments; antennal scape widened in apical half, reaching posterolateral corner of head in full-face view. Anterior clypeal margin roundly convex, lacking denticles. Mandible triangular; its masticatory margin with a large apical tooth, medium-sized subapical and basal teeth, with 4–5 denticles between subapical and basal teeth. Frontal carina very short, not extending beyond posterior margin of torulus; parafrontal ridge absent. With mesosoma in profile promesonotum convex dorsally and sloping gradually to metanotal groove; propodeum broader than pronotum, distinctly inflated. Legs slender. Subpetiolar process weakly developed or almost absent.

Head and first gastral segment entirely smooth and shiny. Body yellow to yellowish brown; head darker than other parts; typhlatta spot present, located at occipital corner. Variation. Aenictus inflatus is clearly polymorphic in the worker caste. Several very small workers were found among the type series and workers of other colonies. They are characterized by a relatively long head, short antennal scape reaching only midlength of head, and normal propodeum. The typhlatta spot is less pronounced in these specimens. Between the largest and smallest workers we have found a series of specimens that are intermediate in the development of propodeum and length of antenna and legs (see also Yamane & Hashimoto 1999).

The smallest workers are most similar to the worker of the Aenictus_wroughtonii_group in having a yellowish and slender body, long legs and weakly developed subpetiolar process. But in the former, the anterior clypeal margin is convex, lacking denticles and the antennal scape reaches only the midlength of the head, while in the latter, the anterior clypeal margin is roundly convex with 5–10 denticles and the antennal scape attains or extends beyond posterolateral corner of the head. The inflated propodeum of Aenictus inflatus contains a red liquid in living specimens; this liquid dissolves in alcohol

Key to Aenictus species groups

Distribution based on Regional Taxon Lists
Indo-Australian Region: Borneo, Brunei Darussalam, Indonesia, Malaysia.

Biology
Yamane and Hashimoto reported the following about the colony from which the types were collected: In March 1997 during a night walk in the jungle we found a raiding column of a strange yellowish army ant in the Lambir Hills National Park, Sarawak, Borneo. Surprisingly the workers had distinctly inflated propodea that were filled up with a red liquid. When we collected workers from the column running down from the canopy very small workers, though few in number, were witnessed. A close examination of these small individuals under a binocular microscope revealed that they are clearly distinguished from the majority of workers by the lack of propodeal inflation and possession of very short antennae and legs. Since intermediate specimens were also found, this species may not be typically dimorphic. But it must be the first polymorphic Aenictus species with the peculiar propodeal inflation in larger specimens.

Allometric growth pattern in some body parts (see description) shows that this species is polymorphic (and almost dimorphic) in its worker caste. The small workers are more similar to usual workers of Aenictus species than are larger ones. They are easily distinguished from small species such as Aenictus piercei, Aenictus minutulus etc. by the long head with parallel sides and weak body sculpture (only mesothorax distinctly sculptured). Their function is not known. However, since they are few in number (ca. 3.3% of all the workers captured), there is a possibility that they are a mere expression of ancestral form of this species owing to the short supply of food during their larval stage. This situation is quite different from that described by Topoff (1971) for Aenictus laeviceps in which the smaller workers predominate in number.

Castes
Known only from the worker caste. Aenictus inflatus is unusual within the genus as it has a distinctly polymorphic worker caste. The majority of Aenictus  species are monomorphic, with a few exhibiting some weak polymorphism.

Nomenclature

 * . Aenictus inflatus Yamane & Hashimoto, 1999: 428, fig. 1 (w.) BORNEO (East Malaysia: Sarawak).
 * Type-material: holotype worker, 100 paratype workers.
 * Type-locality: holotype Malaysia: Borneo, Sarawak, Lambir Hills Nat. Park, Miri, 2.iii.1977, at night (Sk. Yamane & Y. Hashimoto); paratypes with same data.
 * Type-depositories: FRCK (holotype); BMNH, MBSM, MNHA, MZBJ, USNM (paratypes).
 * Status as species: Pfeiffer, et al. 2011: 32.
 * Distribution: Malaysia (Sarawak).

Description
Larger worker (holotype). Body 2.2 mm long. Head 0.50 mm wide, 0.60 mm long excluding mandibles, and 0.35 mm high. Alitrunk 0.70 mm long; pronotum 0.30 mm wide; propodeum 0.34 mm wide. Petiole + postpetiole 0.35 mm long. Gaster 0.95 mm long and 0.51 mm wide in the widest part. Antennal scape 0.53 mm long. Fore femur 0.64 mm long; hind femur 0.85 mm long.

Head almost unsculptured, shining, distinctly longer than wide, with roundly convex sides, widest at the midlength, with almost straight posterior margin in full face view, and without a collar. Clypeus flat, anteriorly entire, roundly weakly produced, and unarmed. Right and left torular sclerites merged medially to form a single sclerite. Mandible triangular, approaching 'typical' sensu Wilson (1964); its masticatory margin with an acute, relatively long apical tooth followed by a much smaller preapical tooth; basal tooth slightly smaller than preapical one; space between them with 3 minute denticles; upper margin of mandible with at least 2 visible teeth that are slightly smaller than basal tooth on masticatory margin. Antenna 10-segmented. Scape relatively long, longer than head width, widened in the apical half; all the flagellar segments each longer than wide; apical segment slightly longer than the preceding two combined; flagellar segments, especially apical one, micro sculptured.

Pronotum distinctly narrower than head, unsculptured and shining. Mesonotum still narrower than pronotum; mesonotum and mesopleuron finely and very densely punctate and opaque; dorsal face sloping. Metanotal depression almost absent. Propodeum slightly wider than pronotum, distinctly inflated, almost unsculptured and shining; propodeal junction round.

Petiole and postpetiole nearly of the same size, each longer than wide, almost unsculptured except for ventral face; in profile posterior slope slightly steeper than anterior one in both the petiole and postpetiole; subpetiolar process almost absent. Gaster almost unsculptured; tergite 1 distinctly longer than wide; lateral margins convex.

Legs slender; basal portion of femora and tibiae distinctly narrowed; hind basitarsus slightly shorter than the rest of tarsus.

Body hairs relatively short, and recumbent to subrecumbent on head, alitrunk, petiole, postpetiole and legs; those on gastral sternites slightly longer and suberect; antennal scape, coxae and ventral face of cranium with a few rather long hairs in addition to normal ones.

Body yellow to yellowish brown. Head brownish; upper gena paler, but typhlatta spot absent. Propodeal cuticle transparent. Gaster yellowish; tergite 1 apically and the subsequent tergites darker.

Smallest worker (paratype). Body 1.75 mm long. Head 0.38 mm wide, 0.53 mm long excluding mandibles, 0.28 mm high. Alitrunk 0.75 mm long; pronotum 0.26 mm wide; propodeum 0.20 mm wide. Petiole + postpetiole 0.38 mm long. Gaster 0.40 mm wide in the widest part, and 0.68 mm long. Scape 0.23 mm long. Fore femur 0.33 mm long; hind femur 0.40 mm long.

In body color, the shape of petiole, postpetiole and gaster, and pilosity similar to the larger workers. But it strikingly differs from the latter in the relative lengths of antennae and legs, and the shape of head and propodeum. The following differences are noted: head relatively much narrower, more rectangular in full face view with almost parallel sides. Scape very short, scarcely reaching the midlength of head, much shorter than head width, and more strongly curved outwardly than in the larger workers; all the flagellar segments proportionally short. Alitrunk in profile with almost straight dorsal outline; propodeum scarcely inflated, distinctly narrower than pronotum; punctation on mesonotum and mesopleuron much finer and more superficial. Legs much shorter; all the femora strongly widened near the midlength; all the tibiae rather strongly widened in apical one-third.

Variation When the smallest and largest specimens are compared, they look like completely different species. However, in addition to the similarity between them mentioned above, we have found some intermediate specimens in the development of propodeum and lengths of antennae and legs. There is a general tendency that larger specimens have more inflated propodea, more rounded heads, longer antennae and legs. But the relative length of antennal scape abruptly and disproportionately decreases at a head width of about 0.4 mm and at a head length of 0.55-0.56 mm, producing two curves with different slopes. On the other hand, the curve for scape length and fore femur length is monophasic. Color pattern also varies; smaller specimens tend to be paler. In some larger specimens frons and vertex of head are rather dark brown in contrast with yellow upper gena. In this case the upper gena apparently represents a typhlatta spot.

Type Material
Holotype: large worker, 2 iii 1997, Lambir Hills National Park, Miri, Sarawak, Borneo from a raiding column in the night, Sk. Yamane and Y. Hashimoto leg. Will be deposited in the. Paratypes: 100 workers (both large and small) from the same column. Will be deposited in the, , , ,.

References based on Global Ant Biodiversity Informatics

 * Borowiec M. L. 2016. Generic revision of the ant subfamily Dorylinae (Hymenoptera, Formicidae). ZooKeys 608: 1–280.
 * Matsumoto T., T. Itioka, S. Yamane, and K. Momose. 2009. Traditional land use associated with swidden agriculture chnages encounter rates of the top predator, the army ant, in Southern Asian tropical rain forests. Biodivers. Conserv. 18: 3139-3151.
 * Pfeiffer M.; Mezger, D.; Hosoishi, S.; Bakhtiar, E. Y.; Kohout, R. J. 2011. The Formicidae of Borneo (Insecta: Hymenoptera): a preliminary species list. Asian Myrmecology 4:9-58
 * Yamane S.; and Y. Hashimoto. 1999. A remarkable new species of the army ant genus Aenictus (Hymenoptera, Formicidae) with a polymorphic worker caste. Tropics 8: 427-432