Aenictus species groups

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The following species groups and their descriptions have been presented by Jaitrong and co-authors (see the references selection at the end of this page) in an ongoing series of papers that are methodically revising the entire genus.

These species groups are based on worker morphology.

Aenictus ceylonicus group

Key to southeastern Asian Aenictus ceylonicus group species

Diagnosis. Antenna 10-segmented; scape reaching or extending beyond half of head length, but not reaching the occipital corner of head in full-face view. Mandible linear; its basal and lateral margins almost parallel; masticatory margin with large apical tooth followed by medium-sized subapical tooth; between subapical tooth and basal tooth 0–6 small denticles present. With mandibles closed, a gap present between mandibles and anterior margin of clypeus. Anterior clypeal margin weakly concave or almost straight, lacking denticles. Frontal carina short and thin, reaching or slightly extending beyond the level of posterior margin of torulus; anterior curved extension of frontal carina reaching or extending beyond the level of anterior clypeal margin in full-face view; parafrontal ridge absent. Promesonotum usually convex dorsally and sloping gradually to propodeum. Subpetiolar process developed.

Head and first gastral tergite smooth and shiny. Body yellowish, reddish or dark brown; typhlatta spot absent.

Distribution. India, Sri Lanka, southernmost part of Japan (?), S. China, Taiwan, Vietnam, Thailand, Borneo (Sabah and Sarawak), Philippines, Aru Island (Indonesia), New Guinea (Papua), and Australia (Queensland).

The A. ceylonicus group occurs in the Oriental, Indo-Australian and Australasian regions (Wilson 1964; Shattuck 2008; Jaitrong and Yamane 2011, 2013). Twenty four worker-based species of the group are recognized from Southeast Asia. Among them 14 species are found in Indo-China (continental Southeast Asia), 3 in the Malay Peninsula, 2 in Sumatra, 3 in Borneo, 2 in Java, 1 in Bali, 1 in the Philippines and 3 in Wallacea. Examining the distribution of individual species of this group in Southeast Asia most species show more or less limited ranges, being found in small areas or in special habitats. This may be because all Aenictus queens remain flightless throughout their lives and found new colonies by splitting from a mature colony and traveling on the ground with a large retinue of workers to a nearby location. This mode of colony foundation makes long-distance dispersal across barriers like mountain ranges, dry zones and seas highly unlikely.

All the Southeast Asian species of the A. ceylonicus group are confined to primary forests (currently very often isolated) or areas close to them. This is confirmed by Matsumoto et al. (2009), who found that the encounter rates with six Bornean Aenictus species were highest in continuous and isolated primary forests compared with young secondary forests or young fallows. Thus, the great loss of the forests in Southeast Asia nowadays may have a serious negative effect on the survival of Aenictus species.

The A. ceylonicus group is a unique group easily separated from the other groups by the following characteristics: mandible linear; a gap present between mandibles and anterior margin of clypeus when mandibles are closed; anterior clypeal margin almost straight or feebly concave, lacking denticles. Our concept roughly agrees with Wilson’s (1964) definition of the “ceylonicus group”, but three species, A. biroi, A. javanus and A. piercei, should be removed from his list since they have triangular mandibles and different conditions of the anterior clypeal margin. All these species belong to three different species groups.

The species of the Aenictus ceylonicus group show continuous size variation or sometimes weak dimorphism among workers within a colony. The workers can be roughly sorted into two size classes, but they are not sharply differentiated. There is a general tendency for smaller specimens to have weaker punctation, more elongate head and shorter antennal scape than larger specimens. This size variation is the same as that observed in the other species groups comprising small species such as the Aenictus javanus and Aenictus minutulus groups (see Jaitrong and Yamane 2012, Jaitrong and Hashimoto 2012). In contrast, the single species of the A. inflatus group, Aenictus inflatus has discrete polymorphism in the worker caste, with major workers (with inflated propodeum and long antennal scape), minor workers (with normal propodeum and very short antennal scape) and intermediate workers (Yamane and Hashimoto 1999, Jaitrong and Yamane 2011). Thus, the identification of these species must be carefully done with colony series.

(this list includes the 2015 addition of Aenictus yangi and hoelldobleri)

Aenictus currax group

Key to Aenictus currax group species

Diagnosis. Head in full-face view with occipital corner convex and with a distinct protuberance, which gives the head a unique “horned” appearance; occipital margin forming a carina. Antenna 10-segmented. Anterior clypeal margin roundly convex, lacking denticles. Mandible subtriangular; its masticatory margin with a large apical tooth followed by a medium-sized subapical tooth, and 4–6 denticles. Frontal carina short, extending a little beyond posterior margin of torulus; parafrontal ridge feeble and incomplete or almost absent. With mesosoma in profile promesonotum convex dorsally and sloping gradually to metanotal groove. Legs slender. Subpetiolar process present; its anteroventral corner always angular, and directed forward and downward.

Head and first gastral segment entirely smooth and shiny. Body black, dark brown to reddish brown; typhlatta spot present, always located at the occipital corner of head.

Distribution. Vietnam, Laos, Myanmar, Thailand, Malay Peninsula (S. Thailand and W. Malaysia), Sumatra Indonesia, Borneo (Sabah, Sarawak, Brunei, and E. Kalimantan), Sulawesi, New Guinea, and Australia.

Remarks. Our concept agrees well with Wilson’s (1964) definition of the “currax group”. This species group is closely related to the A. leptotyphlatta group and A. laeviceps group, all bearing typhlatta spots on the worker head, and also sharing the black or dark brown to reddish brown body, and entirely smooth and shiny head (Jaitrong & Eguchi 2010). The A. currax group is distinguished from the latter two by the following characteristics:anterior clypeal margin roundly convex, lacking denticles; head in full-face view with occipital corner convex and with a distinct protuberance, which gives the head a unique “horned” appearance; in profile “typhlatta spot” always located at occipital corner; subpetiolar process present, triangular with the apex always directed forward and downward.

Aenictus hottai group

Diagnosis. Antenna long, consisting of 10 segments; scape long, reaching posterolateral corner of head. Anterior clypeal margin roundly convex, lacking denticles. Mandible subtriangular, with very dense punctures; its masticatory margin with a large and sharp apical tooth followed by a medium-sized subapical tooth and 18–20 small inconspicuous denticles. Frontal carina not reaching midlength of head, well developed anteriorly and poorly developed posteriorly; parafrontal ridge present not reaching midlength of head; seen in profile its anteriormost part well developed and raised as a subtriangular process, and poorly developed posteriorly. Occipital margin of head forming a collar or carina. Propodeal junction angular; declivity of propodeum concave, encircled with a rim. Subpetiolar process well developed, posteroventrally produced. Entire head, mesosoma, petiole and postpetiole very densely puncto-recticulate and opaque. Punctoreticulation on antennal scape, coxae, femora, tibiae, and basitarsi similarly dense but weaker. First gastral segment with dense but weak and superficial micropunctation, subopaque and slightly shiny. Body dark brown to reddish brown; typhlatta spot absent.

Distribution. Malay Peninsula (S. Thailand and W. Malaysia), W. Sumatra (Indonesia), and Borneo (Sarawak).

Remarks. This species group is closely related to the A. pachycerus group and A. philippinensis group in that all have a well-developed frontal carina and parafrontal ridge. The A. hottai group can be separated from other groups by the first gastral segment being densely micropunctate, subopaque and slightly shiny, and by the well developed subpetiolar process which is posteroventrally produced (in the other groups the first gastral segment is smooth and shiny).

Aenictus inflatus group

Aenictus inflatus

Diagnosis. 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. The single species of this group, Aenictus inflatus Yamane et Hashimoto, 1999, 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).

Distribution. Borneo (Sarawak).

Remarks. Yamane and Hashimoto (1999) erroneously mentioned that the antenna is 12-segmented when it was in fact 10-segmented. The smallest worker of this group is most similar to the worker of the A. 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.

Aenictus javanus group

Key to Aenictus javanus group species

Diagnosis. Head in full-face view with occipital corner convex; occipital margin lacking collar. Antenna 10-segmented; antennal scape short extending only half length of head. Anterior clypeal margin roundly convex bearing 6–10 denticles. Mandible subtriangular; masticatory margin with 3 teeth including the large apical tooth. Frontal carina short, not extending beyond the level of posterior margin of torulus. Parafrontal ridge absent. Mesosoma in profile with dorsal margin almost flat; dorsal face of mesosoma meeting with lateral face at a right angle; propodeal junction angular; propodeal declivity encircled with a thin rim. Subpetiolar process developed, triangular or subrectangular.

Head and first gastral segment entirely smooth and shiny except the base of gastral tergite I and sternite I that have dense small punctures. Body reddish brown to yellowish brown; typhlatta spot absent.

Distribution. Vietnam, Laos, Thailand, and W. Java (Indonesia).

Remarks. This is a group of relatively small ants measuring 1.38–3.40 mm in total length. It is similar to the A. piercei group in terms of body size and coloration, but in the former the anterior clypeal margin has several denticles, while it lacks denticles in the latter. Though Wilson (1964) treated Aenictus javanus as a member of the A. ceylonicus group, it has a quite different set of characteristics as mentioned above.

Aenictus laeviceps group

Key to Aenictus laeviceps group species

Diagnosis. Head in full-face view with occipital corner rounded; occipital margin forming a carina. Antenna 10-segmented; antennal scape relatively long, usually attaining posterior corner of head. Anterior clypeal margin roundly convex, bearing 5–12 denticles. Mandible subtriangular; its masticatory margin with a large apical tooth followed by a medium-sized subapical tooth and 5–8 denticles. Frontal carina short, extending slightly beyond posterior margin of torulus; parafrontal ridge feeble and incomplete or almost absent. With mesosoma in profile promesonotum convex dorsally and sloping gradually to metanotal groove. Legs slender; subpetiolar process well developed, triangular; its apex usually directed backward and downward. Head and first gastral segment entirely smooth and shiny. Body black, dark brown to reddish brown; typhlatta spot present, usually located anterior to occipital corner.

Distribution. India, Vietnam, Cambodia, Thailand, Malay Peninsula (S. Thailand and W. Malaysia), Sumatra (Indonesia), Borneo (Sabah, Sarawak, Brunei, and E. Kalimantan), Java (Indonesia), Philippines, and Sulawesi (Indonesia).

Remarks. Our concept agrees well with Wilson’s (1964) definition of the laeviceps group. This group is closely related to the A. currax group and A. leptotyphlatta group (see under A. currax group). The A. laeviceps group is distinguished from the latter two by the following characteristics: anterior clypeal margin roundly convex with several denticles; head in full-face view with occipital corner rounded; in profile typhlatta spot usually located anterior to occipital corner; subpetiolar process well developed, with the apex directed downward and backward.

Aenictus leptotyphlatta group

Aenictus leptotyphlatta

Diagnosis. Head in full-face view with occipital corner rounded; occipital margin lacking collar. Antenna 10-segmented; antennal scape short, reaching or extending slightly beyond midlength of head. Anterior margin of clypeus slightly convex medially, lacking denticles. Mandible subtriangular; its masticatory margin with a large apical tooth followed by 4–5 relatively large teeth. Frontal carina short and thin, extending posteriad, not beyond the level of posterior margin of torulus; anterior curved extension of frontal carina reaching or extending beyond the level of anterior clypeal margin. With mesosoma in profile promesonotum weakly convex dorsally and sloping gradually to propodeum. Legs slender. Subpetiolar process well developed, triangular, with its apex directed downward. Head and first gastral segment entirely smooth and shiny. Body black to dark brown; typhlatta spot present but not clear.

Distribution. Thailand.

Remarks. This group is closely related to the A. currax group and A. laeviceps group (see under A. curraxgroup). The A. leptotyphlatta group exhibits conditions intermediate between the A. currax group and A. laeviceps group. It has the anterior clypeal margin lacking denticles as in the A. currax group, but the occipital corner of the head is similar to that of the A. laeviceps group. Furthermore the typhlatta spot is less clearly demarcated from the background, and much paler in coloration than in the other typhlatta-bearing species groups.

Aenictus minutulus group

(Aenictus piercei group in Jaitrong et Yamane, 2011)

Key to Aenictus minutulus group species

Diagnosis. In an earlier paper (Jaitrong & Yamane 2011) this species group was defined as follows: head in fullface view with occipital corner convex, and posterior margin almost straight to shallowly and broadly concave; occipital margin lacking collar; antenna 10-segmented; antennal scape short reaching only midlength of head; anterior clypeal margin roundly convex, lacking denticles; mandible subtriangular; its masticatory margin with a large apical tooth, medium-sized subapical and basal teeth, and 2-6 denticles between them; basal margin of mandible with conspicuous denticles; frontal carina short; parafrontal ridge absent; with mesosoma in profile promesonotum convex dorsally and sloping gradually to propodeum; metapleural groove present or absent (mesonotum and propodeum fused); propodeal junction angulated; subpetiolar process well developed, triangular or subrectangular. Head and first gastral segment entirely smooth and shiny. Body yellowish brown to reddish brown; typhlatta spot absent.

Size variation occurs among individuals from single colonies.

Distribution. Southernmost part of Japan, Taiwan, Myanmar, Thailand, Singapore, Sumatra (Indonesia), and Philippines.

Remarks. This is a group of rather small species, measuring 1.80-3.20 mm in total body length. It is most similar to the Aenictus javanus group in terms of body size and coloration, but in the former the anterior clypeal margin lacks denticles, while it has several denticles in the latter.

The Aenictus piercei group was established by Jaitrong and Yamane (2011) to include five named species occurring in Southeast Asia: Aenictus changmaianus, Aenictus lifuiae, Aenictus minutulus, Aenictus peguensis and Aenictus piercei. However, after carefully examining the type material and an associated specimen of A. piercei we concluded that this species should be removed from this group and that it is a member of the A. javanus group (A. piercei has denticles on the anterior margin of clypeus). The A. piercei group is renamed here as the A. minutulus group.

After a careful examination of the smaller specimens of A. lifuiae collected from Iriomote-jima and Okinawajima, southern Japan, we found that the mandible is almost linear, the anterior clypeal margin is straight or weakly concave, and that a gap occurs between the mandibles and anterior clypeal margin. These characteristics are used to separate the A. ceylonicus group from the other groups of the genus Aenictus, and also these specimens possess other conditions shared with the A. ceylonicus group. We transfer A. lifuiae to the A. ceylonicus group.

Aenictus pachycerus group

Key to Aenictus pachyerus group species

Diagnosis. Antenna long, consisting of 10 segments; scape long, reaching or extending beyond posterolateral corner of head. Anterior clypeal margin roundly convex in the middle, lacking denticles. Mandible triangular, with very dense punctures; its masticatory margin with a large and sharp apical tooth followed by 4–12 small inconspicuous denticles, which gradually reduce in size toward basal angle of mandible. Frontal carinae fused at the level of antennal base to form a single carina, and extending less than half length of head, and well developed anteriorly and poorly developed posteriorly; parafrontal ridge present, reaching less than half length of head; seen in profile its anteriormost part well developed and raised as a subtriangular process. Occipital margin forming a collar or carina. Promesonotum distinctly convex or very weakly convex dorsally and sloping gradually to propodeum; propodeal junction angular; declivity of propodeum concave, encircled with a rim. Subpetiolar process weakly developed.

Head entirely sculptured or smooth and shiny. Petiole and postpetiole densely punctate in at least Southeast Asian species. First gastral segment entirely smooth and shiny, or rarely superficially shagreened, except the base of the tergite and sternite that has small, dense punctures. Body black, dark or reddish brown to light or yellowish brown; typhlatta spot absent.

Distribution. India, Sri Lanka, S. China, Vietnam, Thailand, Malay Peninsula (S. Thailand and W. Malaysia), Sumatra (Indonesia), Borneo (Sabah, Sarawak, Brunei, and E. Kalimantan), Philippines, New Guinea (Papua), and Australia (Queensland).

Remarks. The A. pachycerus group consists of relatively large species in terms of body size (TL 3.20–4.65 mm). Wilson (1964) pointed out that this group is closely related to the A. philippinensis group, but can be distinguished from the latter by the mesonotum not visibly demarcated from the mesopleuron, and the metanotal groove almost absent or indistinct (mesopleuron clearly demarcated from metapleuron by a deep groove and from promesonotum by a distinct carina and metanotal groove relatively deep and distinct in the latter). This species group is also related to the A. hottai group in having developed frontal carina and parafrontal ridge. See under the A. hottai group.

Aenictus philippinensis group

Key to Aenictus phillippinensis group species

Diagnosis. Antenna 10-segmented; scape not reaching the posterolateral corner of head. Anterior clypeal margin convex in the middle, lacking denticles. Mandible triangular, with very dense punctures; its masticatory margin with a large and sharp apical tooth followed by 6–8 small inconspicuous denticles. Frontal carinae fused at the level of antennal base to form a single carina, extending less than half length of head, and well developed anteriorly and poorly developed posteriorly; parafrontal ridge present, not reaching midlength of head. Occipital margin forming a collar or carina. Mesosoma in profile with promesonotum convex dorsally and sloping gradually to metanotal groove; mesopleuron clearly demarcated from metapleuron by a deep groove and from promesonotum by a distinct carina; metanotal groove relatively deep and distinct; propodeal junction angular; declivity of propodeum concave, encircled with a rim. Subpetiolar process weakly developed.

First gastral segment entirely smooth and shiny except the base of both tergite and sternite which has dense small punctures. Body reddish brown to dark brown; typhlatta spot absent.

Distribution. Philippines.

Remarks. This group consists of relatively large species measuring 4.05–4.60 mm in total body length, and is closely related to the A. pachycerus group and A. hottai group. However, the A. philippinensis group is separated from these by having the mesonotum demarcated from the mesopleuron by a conspicuous ridge and the metanotal groove being relatively deep and distinct. The sculpture of the head is variable, from entirely smooth to densely puncto-reticulate.

Aenictus silvestrii group

Key to Aenictus silvestrii group species

Diagnosis. Antenna thick, consisting of only 8 or 9 segments; scape somewhat flattened, broadened apically and strongly grooved below. Anterior clypeal margin roundly convex in the middle, without denticles. Mandible triangular, with very dense small punctures; its masticatory margin with inconspicuous denticles in addition to the sharp apical tooth. Frontal carinae fused at the level of antennal base to form a single carina; parafrontal ridge absent. Occipital margin forming a narrow collar. Declivity of propodeum concave, encircled with a rim; subpetiolar process weakly to well developed. Legs relatively short, with apical half of tibia weakly broadened and apical half of femur strongly broadened and somewhat flattened.

Head entirely sculptured but in one species smooth. Gastral segment I entirely smooth and shiny except the base of tergite I and sternite I which has dense small punctures; the punctured area usually dark colored. Head and mesosoma yellowish, reddish or dark brown; gaster paler, usually yellow; typhlatta spot absent.

Distribution. Thailand, Malay Peninsula (W. Malaysia), Sumatra (Indonesia), Borneo (Sabah, Sarawak, and Brunei), and W. Java (Indonesia).

Remarks. This is a unique group, which has the antenna with less than 10 segments. The groove on the ventral face of the scape, which is strongly flattened, and the enlarged femora of the legs are also useful for recognizing this group.

According to the literature (Wheeler 1929; Terayama & Yamane 1989; Shattuck 2008) and our own surveys, the species of this group seem to be confined to Southeast Asia. Little is known about their bionomics. Nothing is mentioned by Wheeler (1929) on the biology of A. silvestrii. Recent ecological studies on Southeast Asian Aenictus did not include any information on the A. silvestrii group (Rosciszewski & Maschwitz 1994; Hirosawa et al. 2000; Ito et al. 2001). However, all the series of A. latifemoratus were found in the daytime on the forest floor in relatively good rainforests. The single colony of A. jarujini was found under a stone in a disturbed area during the hottest and very dry season. No worker activ¬ity was seen around the stone and no immatures were found in the bivouac. This means that this group has two different types of species, one adapted to the rainforest and the other to sparse forest with a distinct dry season. Even with this scanty information, we tentatively conclude that this species group is essentially Southeast Asian, and differs in general biology from South Asian and African species, many of which are known to be hypogaeic (Schneirla & Reyes 1966; Gotwald 1976).

Aenictus wroughtonii group

Key to Aenictus wroughtonii group species

Diagnosis. Jaitrong & Ruangsittichai (2018) - (modified from Jaitrong and Yamane 2011). Head narrow, oval or ellipti¬cal; occipital margin lacking collar (distinct carina). Antenna short or long, comprising 10 segments; scape short, attaining mid-length of head or longer attaining or extend¬ing beyond the posterolateral corner of head. Anterior clypeal margin roundly convex with 5–10 denticles. Mandible triangular, with masticatory margin bearing 8–12 min¬ute inconspicuous denticles in addition to a large apical tooth with a sharp apex; basal margin of mandible lacking denticles. Frontal carina short; parafrontal ridge feeble and incomplete. Mesosoma narrow and elongate. Legs very slender. Propodeal junction in profile angulate or rounded. Subpetiolar process weakly developed or almost absent. Head and gaster entirely smooth and shiny. Body yellow, yellowish brown to dark brown; typhlatta spot absent.

Distribution. Greece, Iran, Israel, Turkey, Saudi Arabia, India, Sri Lanka, Southeast China, Taiwan, Vietnam, Thailand, Malay Peninsula (West Malaysia), Sumatra, Borneo (Sabah, Sarawak, and Brunei) and Philippines (Negros and Lu¬zon) (Aktaç et al. 2004, Jaitrong et al. 2010, Jaitrong and Yamane 2011, Sharaf et al. 2012, Staab 2014).

Remarks. This species-group is separated from the other groups by the following characteristics: yellowish and slender body; antennal scape long, usually attaining or extending beyond posterolateral corner of head; anterior clypeal margin roundly convex with several denticles. In general appearance, the species of the A. wroughtonii group are similar to the smallest worker of Aenictus inflatus. See under A. inflatus group.

Wilson (1964) treated Aenictus biroi as a member of the A. ceylonicus group, but Jaitrong et al. (2010) removed it from this group and transferred it to the A. wroughtonii group because of the presence of denticles on the anterior clypeal margin in the worker.

References