|Myrmica sordidula, now Crematogaster sordidula
A subgenus of Crematogaster. Blaimer (2012b) revised the Malagasy species in this subgenus, initially dividing its fauna into three species groups. Additional species groups were subsequently added by Hosoishi and Ogata (2016) for some species occurring beyond the Malagasy region.
Blaimer (2012) - The two-subgenera system as proposed follows a deep molecular divergence event between Orthocrema and Crematogaster sensu stricto, tracing back into the Mid-Eocene (ca.40–45 Mya) when these two clades last shared a common ancestor (Blaimer, in press). Recognizing these two groups from each other on a morphological basis is fairly easily achieved, although a diagnosis of the Crematogaster sensu stricto is more problematic given the high morphological variability. An obvious further improvement to the system presented here would have been the recognition of the global Crematogaster clade and the Australo-Asian Crematogaster clade as separate subgeneric entities, as they certainly show enough molecular divergence to justify a three-subgenera-approach. I refrained from such a formal distinction since it was not possible to assign species to these groups on a morphological basis. I primarily aimed to develop a practical, ‘user-friendly’ classification. Petiole and postpetiolar characters that are useful for the distinction of Crematogaster sensu stricto and Orthocrema become hypervariable within the two sensu stricto clades and are not informative at deeper phylogenetic levels. Similar character patterns, for example the presence/absence of a median longitudinal impression on the postpetiole appear to have evolved multiple times within the genus, but the selective pressures acting on morphological evolution in Crematogaster have yet to be revealed. More biological data will need to be collected to investigate these questions.
Biology, Morphology, and Classification
Hosoishi and Ogata (2016) - Orthocrema ants have generally been considered to be ground dwellers, and they have frequently been collected by general sampling methods in faunal surveys (e.g., Malsch, Rosciszewski & Maschwitz, 2003; Eguchi et al., 2005; Pfeiffer et al., 2011).
Workers of the subgenus are roughly distinguished even in the field, by having a yellow-colored body and acrobatic style. The subgenus has been characterized based on the following features: two-segmented antennal club; petiole with subparallel sides; postpetiole without median sulcus (Santschi, 1918; Emery, 1922; Wheeler, 1922).
In his review of Asian Crematogaster ants, Menozzi (1935) also introduced a chaetotaxy system for the dorsal mesosomal surface among members of the subgenus Orthocrema. Our collections suggest that workers of Asian Orthocrema species generally possess a pattern of rows of long erect setae on the body segments (head, mesosoma, petiole, postpetiole and fourth abdominal tergite), and that chaetotaxy in Asian Orthocrema species is informative for both species-level identification and phylogenetic interpretation. We generally follow Menozzi (1935) and redefine each seta on the mesosoma surface. The chaetotaxy system of Asian Orthocrema species proposed in the present study is summarized in Figure 7 (Menozzi, 1935: fig. 1) and the nomenclature is given in Table 1.
As suggested by Sorger & Zettel (2009), although chaetotaxy is important for species identification, counting setae is often difficult in old or damaged specimens. We therefore did not rely on only the number of setae to distinguish between species.
As in the studies on Tetraponera by Ward (2001), the species richness of Asian Orthocrema is highest in Borneo (10 spp.), followed by Sumatra (7 spp.) and Peninsular Malaysia (6 spp.); indeed, it appears that the Sundaic region is the center of diversity for Asian Orthocrema species.
The subgenus Orthocrema is generally considered to contain ground dwellers that nest in soil, but some species are arboreal and nest in dead twigs on trees (Blaimer, 2012b). Although information on nesting biology of Asian Orthocrema species is limited, our collections show that a variety of nesting sites is used. Crematogaster baduvi and Crematogaster storki were collected by using fogging machine, and one nest of C. baduvi was found in decayed wood (Yamane, 2013); these species are considered to nest in dead twigs or in leaf litter mats on trees. Conversely, Crematogaster longipilosa, Crematogaster bandarensis, Crematogaster biroi and Crematogaster osakensis are all typical ground dwellers that nest in soil on the ground or in leaf litter. Crematogaster quadriruga, Crematogaster suehiro, Crematogaster fritzi, Crematogaster reticulata all nest in dead twigs or in dead leaves on trees. Cladistic analysis also showed that nest site selection is not fixed in each species group, and that variations occur within species groups.
Hosoishi et al. (2010) revised the subterranean species from the Asian Orthocrema fauna. Crematogaster masukoi and Crematogaster myops have reduced compound eyes (approximately 6–10 ommatidia) and depigmented yellowish bodies. The two species are similar in that they have reduced eyes, but they are quite different from each other in other morphological aspects (Hosoishi et al., 2010). Our cladistic analysis showed that C. masukoi belongs to the C. biroi group, whereas C. myops belongs to the C. quadriruga group, suggesting that their subterranean mode of life evolved independently and is an example of convergence.
Most Orthocrema species are supposed to be general predators. Interestingly, in the subterranean species (C. masukoi and C. myops), their mandibles show different character states from other species. The basal tooth of the mandibles is arranged away from the third apical one in those species, although their teeth on masticatory margin are arranged at an equal distance in most species. Little information is known about the biology of those species, but the unique arrangement of the mandibles may be related to their predatory behaviours (Bolton, 1988).
Workers in the subgenus Orthocrema are generally monomorphic in size, but intermediate workers have been found in some species in North America (Heinze et al., 1999), Madagascar (Blaimer, 2012b), Brazil (Longino, 2003; Quinet et al., 2009) and Taiwan (Peeters et al., 2013). These workers are intermediate in size between workers and queens. It is reported that the intermediate workers laid unfertilized trophic eggs for larvae (Heinze et al., 1999; Peeters et al., 2013). Histological studies reveal that the intermediate workers have developed ovaries but lack a spermatheca, it is suggesting that they are a soldier caste with a specialized trophic function (Peeters et al., 2013). In the Neotropical species, Crematogaster missouriensis (as Crematogaster minutissima smithi), many of the unfertilized eggs are eaten by the queen and larvae, but some eggs can develop into males (Heinze et al., 2000; Oettler, Dijkstra & Heinze, 2013). In this revision of the Asian taxa, we found intermediate workers in five species, C. biroi, C. reticulata, Crematogaster schimmeri (See Peeters et al., 2013), Crematogaster vieti and C. quadriruga. The intermediate workers are clearly separated from ordinary workers by the following features: larger body size, developed ocelli (sometimes vestigial), highly convex mesonotum. The former four belong to the C. biroi group, while the last belongs to the C. quadriruga group, suggesting that intermediate workers are widely scattered across Asian Orthocrema species.
The subgenus Orthocrema is abundant among Asian ground ant fauna in the temperate to tropical regions. Crematogaster osakensis was one of the most abundant species collected by using Winkler extractions in grasslands of temperate regions, Japan (Hosoishi et al., 2015). Several colonies of C. vieti and C. bandarensis were frequently collected as nest series in northern Vietnam (Eguchi et al., 2005) and Bogor, Indonesia (Ito et al., 2001), respectively.
Crematogaster baduvi group
Hosoishi and Ogata (2016)
Relatively long scape (SI 98–118). Basal flagellar segment (antennal segment III) longer than broad. Propodeal spines long and directed laterally. Postpetiolar dorsum highly convex in lateral view; postpetiole distinctly higher than petiole in lateral view.
This species group is easily distinguished by the propodeal spines directed laterally and highly convex postpetiolar dorsum from other Asian Orthocrema species.
Crematogaster binghamii group
Hosoishi and Ogata (2016)
Basal flagellar segment (antennal segment III) longer than broad. Posterior margins of mesonotum forming short triangle-shaped process in lateral view. Propodeal spines undeveloped, or developed and directed posteriorly. Standing pilosity abundant on body surface.
This species group is easily distinguished by the short triangle-shaped process on posterior margins of mesonotum and long and abundant standing pilosity from other Asian Orthocrema species.
Crematogaster biroi group
Hosoishi and Ogata (2016)
- Crematogaster biroi
- Crematogaster fritzi
- Crematogaster luzonensis
- Crematogaster masukoi
- Crematogaster ocellata
- Crematogaster osakensis
- Crematogaster reticulata
- Crematogaster schimmeri
- Crematogaster udo
- Crematogaster vieti
Relatively short scape (SI 70–89). Basal flagellar segment (antennal segment III) broader than long. Anterior margin of pronotal collar almost straight in dorsal view. Dorsal surface of propodeum striated with rugulae. Mesosoma sculptured. Subpetiolar process developed acutely. Subpostpetiolar process developed acutely.
This species group is easily distinguished by the subpostpetiolar process developed acutely and sculptured surface of mesosoma from other Asian Orthocrema species.
Worker diagnosis of the Crematogaster madecassa-group: Very small species (HW 0.48–0.60, WL 0.44–0.69). Masticatory margin of mandibles with 4 teeth; posterior margin of head in full face view usually laterally rounded, sometimes medially slightly depressed; occipital carinae well pronounced; antennal scape length variable; midline of eyes situated well above midline of head in full face view; eyes large (OI 0.22–0.28) and distinctly protruding.
Pronotum laterally subangular; mesonotum laterally with distinct, raised carinae that are confluent with lateral carinae bordering metanotal groove and propodeum; in lateral view outline of promesonotum moderately convex; mesonotum transversely concave, without a distinct posterior face and gradually sloping into metanotal groove; metanotal groove in dorsal view constricted by bordering lateral carinae, propodeal spines short to medium-sized (SPI 0.10–0.26), form variable; dorsal face of propodeum very short; petiole in dorsal view ovo-rectangular, with dorsolateral margins increasingly carinate posteriorly, ending in small posterolateral denticles; subpetiolar process variable: from small, but distinct and acute tooth to reduced angular dent; postpetiole globular, faintly impressed posteriorly, no trace of median impression; subpostpetiolar process present or absent.
Sculpture overall reduced; head shiny; mesosoma dorsally mostly shiny, carinulate laterally; meso- and metapleuron mostly shiny, with some reticulations; dorsal face of propodeum carinulate, posterior face shiny; dorsal face of petiole shiny; helcium dorsally carinulate; postpetiole dorsally feebly reticulate; lateral and ventral face of petiole and postpetiole reticulate; face with 4–8 erect, long flexuous setae, and abundant shorter, subdecumbent pubescence; promesonotum usually with 4–6 erect, long flexuous setae: 2 humeral setae, and 2 setae at anterior and usually also 2 setae at posterior end of mesonotal carinae; additional long erect setae, and scattered shorter erect setae may be present dorsally on promesonotum; petiole with a single stiff, erect seta on each posterolateral tubercle; postpetiole with a pair of erect dorsoposterior setae; abdominal tergites and sternites 4–7 with fairly abundant, erect long pilosity (> 20 setae) and sparse decumbent pubescence throughout. Color pale to medium yellow, or yellowish-brown.
Queen diagnosis of the Crematogaster madecassa-group: (C. telolafy unknown) Very small (HW 0.80–1.10, WL 1.28–1.74). With worker characters, except as follows. Masticatory margin of mandibles with 5 teeth; antennal scapes not, or just reaching posterior margin of head; eyes large (OI 0.29–0.37) and protruding, situated slightly above midline of head in full face view; head wider than long (CI 1.11–1.21) and widest just posterior to eyes, posterior margin of head straight.
Mesosoma more compact (MSNI 1.55–1.82, WL 1.28–1.74); mesoscutum in dorsal view almost or as wide as long; dorsal face of propodeum absent, and posterior face very sharply and almost vertically sloping; propodeal spines present, much shorter than in workers (SPI 0.02–0.14), sometimes reduced to minute dents; petiole and postpetiole as in workers.
Sculpture smooth and shiny throughout, except metapleuron and anteriormost part of propodeum carinulate; erect pilosity very abundant on head, dorsal side of mesosoma and on metasoma, but finer and shorter than in workers; petiole with 1–3 pair(s) of long flexuous setae posterior to denticles; postpetiole with abundant erect pilosity. Color similar to respective workers, but often metasoma darker.
Crematogaster moatensis group
Two apical flagellar segments not differentiated in coloration. Pronotum distinctly higher than pronotal collar. Metanotal groove not covered by lamellate ridges. Propodeal spiracles oval. Petiole long and slender.
This species group is unique in having the metanotal groove not covered by lamellate ridges, oval-shaped propodeal spiracles and long and slender petiole from other Asian Orthocrema fauna.
Crematogaster quadriruga group
Hosoishi and Ogata (2016)
- Crematogaster bandarensis
- Crematogaster celebensis
- Crematogaster gavapiga
- Crematogaster javanica
- Crematogaster myops
- Crematogaster philippinensis
- Crematogaster quadriruga
- Crematogaster suehiro
- Crematogaster sundalandensis
Relatively long scape (SI 81–100). Basal flagellar segment (antennal segment III) broader than long. Subpetiolar process developed. Subpostpetiolar portion wholly convex in lateral view. Mesosoma generally smooth and shining.
This species group is distinguished by the subpostpetiolar portion wholly convex and smooth and shining surface of mesosoma from other Asian Orthocrema species.
Worker diagnosis of the Crematogaster volamena-group: Very small to medium sized species (HW 0.51–0.98, WL 0.56–0.92). Masticatory margin of mandibles with 4 teeth; clypeus with or without several irregular vertical carinae; posterior margin of head in full face view laterally subangular, often medially slightly depressed; occipital carinae indistinct; antennal scapes just barely (small workers) or not reaching (larger workers) posterior margin of head; midline of eyes situated well above midline of head in full face view; eyes small (0.18–0.22) and fairly flush with head.
Pronotum laterally subangular; in lateral view, anterior part of mesonotum often angular or denticulate, posteriorly at least weakly carinate until meeting metanotal groove; in lateral view outline of promesonotum fairly flat; dorsal face of mesonotum flat, posterior face distinct or indistinct; metanotal groove very constricted by bordering lateral carinae, a third as wide as pronotal width; propodeal spines short (SPI 0.06–0.12), upwards directed sharp points; length of dorsal face of propodeum about a third of posterior face; petiole in dorsal view ovo-rectangular, dorsolateral margins angulate, ending in small posterolateral denticles; subpetiolar process variable, from well pronounced acute tooth to reduced angular dent; postpetiole short and broad, appearing bilobed, with diffuse, broad median impression; subpostpetiolar process absent.
Sculpture overall reduced; head shiny to aciculate; mesosoma dorsally mostly shiny, meso- and metapleuron mostly shiny, rugulose in some parts; dorsal and posterior face of propodeum shiny with some carinulae; dorsal face of petiole shiny to carinulate; helcium dorsally reticulate; postpetiole dorsally shiny; lateral and ventral face of petiole and postpetiole feebly reticulate; face with very abundant silken erect to suberect pilosity of variable length, usually hereof 6–12 longer setae; promesonotum with at least 6 erect, long flexuous setae: 2 humeral setae, 2 setae at anterior end of mesonotum and 2 setae on mesonotal tubercles or denticles; additional long erect setae, and scattered shorter erect setae may be present dorsally on promesonotum; longer, erect pilosity present or absent from propodeum; petiole with a single flexuous setae on each posterolateral tubercle; postpetiole with a pair of long flexuous dorsoposterior setae, and several shorter setae dorsally and laterally; abdominal tergites and sternites 4–7 with dense erect pilosity (> 50 setae) of medium length, interspersed with a subdecumbent shorter pubescence. Color, pale or golden yellow, or medium brown.
Queen diagnosis of the Crematogaster volamena-group: Large (HW 1.48–1.72, WL 2.61–2.70). With worker characters, except as follows. Masticatory margin of mandibles with 5 teeth. Antennal scapes not surpassing posterior margin of head, reaching to about level of median or lateral ocelli; occipital carinae well pronounced or indistinct; eyes medium-sized to large (OI 0.23–0.27), situated at midline of head in full face view; head wider than long or slightly longer than wide, posterior margin of head straight.
Mesosoma compact to slender (MSNI 1.64–1.77, WL 2.61–2.70); propodeal spines absent; petiole oval or subquadrate and lacking denticles; postpetiole broad, but lacking median impression; broad subpetiolar process present, but lacking distinct tooth.
Head sculpture aciculate or carinulate-aciculate; sculpture on mesosoma and metasoma aciculate, except dorsal face of propodeum transversely carinulate and metapleuron carinulate. Erect pilosity somewhat less abundant than in workers, but denser on mesoscutum and scutellum; petiolar and postpetiolar pilosity as in workers. Color brown with yellow markings on meso-, metasoma and legs, or reddish brown.
The following information is derived from Barry Bolton's Online Catalogue of the Ants of the World.
- ORTHOCREMA [subgenus of Crematogaster]
- Orthocrema Santschi, 1918d: 182 [as subgenus of Crematogaster]. Type-species: Myrmica sordidula, by original designation.
- Orthocrema raised to genus: Soulié, 1964: 398.
- Orthocrema junior synonym of Crematogaster: Hölldobler & Wilson, 1990: 13.
- Orthocrema subgenus of Crematogaster: Bolton, 1995b: 40.
- APTEROCREMA [junior synonym of Crematogaster (Orthocrema)]
- Apterocrema Wheeler, W.M. 1936c: 45 [as subgenus of Crematogaster]. Type-species: Crematogaster (Apterocrema) atitlanica, by monotypy.
- Apterocrema junior synonym of Orthocrema: Blaimer 2012: 52.
- EUCREMA [junior synonym of Crematogaster (Orthocrema)]
- Eucrema Santschi, 1918d: 182 [as subgenus of Crematogaster]. Type-species: Formica acuta, by original designation.
- Eucrema junior synonym of Orthocrema: Blaimer 2012: 52.
- [Emery, 1919b: 62, Emery, 1922e: 137, Soulié, 1964: 389 and Soulié, 1965: 78 incorrectly treat Eucrema as a junior synonym of subgenus Crematogaster sensu stricto. This stems from Emery's (1912d: 272) unjustified subsequent designation of Formica acuta as type-species of Crematogaster; see above under that genus.]
- MESOCREMA [junior synonym of Crematogaster (Orthocrema)]
- Mesocrema Santschi, 1928b: 33 [as subgenus of Crematogaster]. Type-species: Crematogaster rasoherinae, by subsequent designation of Donisthorpe, 1943f: 661.
- Mesocrema junior synonym of Orthocrema: Blaimer 2012: 52.
- NEOCREMA [junior synonym of Crematogaster (Orthocrema)]
- Neocrema Santschi, 1918d: 182 [as subgenus of Crematogaster]. Type-species: Crematogaster distans, by original designation.
- Neocrema junior synonym of Orthocrema: Emery, 1922e: 130.
- Neocrema raised to genus: Soulié, 1964: 398.
- Neocrema junior synonym of Crematogaster: Hölldobler & Wilson, 1990: 13.
- Neocrema subgenus of Crematogaster: Wheeler, W.M. 1922a: 662; Kempf, 1972a: 82; Bolton, 1995b: 39.
- Neocrema junior synonym of Orthocrema: Blaimer 2012: 52.
- RHACHIOCREMA [junior synonym of Crematogaster (Orthocrema)]
- Rhachiocrema Mann, 1919: 318 [as subgenus of Crematogaster]. Type-species: Crematogaster (Rhachiocrema) wheeleri, by original designation.
- Rhachiocrema raised to genus: Soulié, 1964: 398.
- Rhachiocrema junior synonym of Crematogaster: Hölldobler & Wilson, 1990: 13.
- Rhachiocrema subgenus of Crematogaster: Bolton, 1995b: 45.
- Rhachiocrema junior synonym of Orthocrema: Blaimer 2012: 52.
- TRANOPELTOIDES [junior synonym of Crematogaster (Orthocrema)]
- Tranopeltoides Wheeler, W.M. 1922e: 10. Type-species: Tranopelta huberi, by original designation.
- Tranopeltoides junior synonym of Crematogaster: Kempf, 1960c: 173.
- Tranopeltoides junior synonym of Orthocrema: Blaimer 2012: 52.
Orthocrema Diagnosis - Malagasy region
Blaimer (2012): Diagnosis of the subgenus Orthocrema in the Malagasy region
1. Very small to medium-sized (HW 0.43–0.98, WL 0.44–0.95).
2. Antennae 11-segmented, antennal club 2-segmented.
3. Promesonotal suture absent.
4. Lateral margins of promesonotum with at least 4 long, erect setae.
5. Propodeal spiracle circular or subcircular (Figure 2).
6. Petiole in dorsal view rectangular (Figure 3) or ovo-rectangular (Figure 4A).
7. Petiole with dorsoposterior lateral denticles or tubercules that each bear an erect seta (Figures 3 and 4A).
8. Postpetiole either more or less globular (Figure 4B), without median longitudinal impression, or weakly bilobed with a broad impression (Figure 5).
9. Postpetiole with at least one pair of long, dorsoposterior setae (Figure 4B).
10. Subpetiolar process present (form variable).
11. Sculpture overall reduced, mostly aciculate, small regions areolate or reticulate.
A combination of characters 6, 7, 8 and 9 will unequivocally separate workers of Orthocrema species from the remaining Crematogaster species in the Malagasy region.
1. Very small to large (HW 0.73–1.72, WL 0.83–2.70).
2. Antennae 11-segmented, antennal club weakly 2-segmented.
3. Occipital carinae mostly present (Figure 6).
4. Propodeal spiracle circular (Figure 7) or subcircular.
5. Petiole in dorsal view rectangular (Figure 8A), ovo-rectangular (Figure 9A), oval (Figure 10A) or subquadrate (Figure 11A).
6. Postpetiole more or less globular, without distinct median longitudinal impression (Figures 8–11B).
A combination of characters 3, 5 and 6 will unequivocally separate queens of Orthocrema species from the remaining Crematogaster species in the Malagasy region.
Orthocrema Diagnosis - Asian region
Hosoishi and Ogata (2016) - We present the following characteristics for Asian fauna:
1. Mandibles with four teeth on the masticatory margin; the teeth located at equal distance each other in most species, but basal tooth located apart from other teeth in subterranean species (Hosoishi et al., 2010).
2. Palp formula 5, 3 (Bolton, 2003).
3. Anterolateral margins of clypeus not protruded anteriorly (Hosoishi & Ogata, 2012).
4. Clypeus usually striated with longitudinal rugulae; the length of rugulae variable in different species.
5. Antenna 11-segmented.
6. Antennal club 2-segmented. Apical two flagellar segments clearly distinguished in coloration from other flagellar segments especially in the C. baduvi group.
7. Sensilla basiconica and sensilla trichodea curvata distributed in the apical two flagellar segments (Hosoishi & Ogata, 2009c).
8. Compound eye generally distinct and large, but strongly reduced in subterranean species (Hosoishi et al., 2010).
9. Ocelli rarely present in intermediate workers.
10. Occipital carina distinct.
11. Mesonotal ridges developed dorsolaterally, but weakly developed in C. masukoi and C. moatensis.
12. Ventrolateral katepisternal ridge distinct (Hosoishi, 2015).
13. Propodeal spines usually developed, but undeveloped or small tubercles in C. binghamii.
14. Petiole usually with subparallel sides.
15. Petiole usually with short process posteriorly.
16. Postpetiole not bilobed. If weakly bilobed, usually without distinct longitudinal median sulcus.
17. Fourth abdominal tergite with erect to suberect setae.
18. Body principally yellow, but sometimes partly brown to black in C. binghami group, C. biroi group, C. moatensis group and C. quadriruga group, but brown in C. baduvi group.
19. Monomorphic in size; intermediate workers occasionally present.
20. Nesting in soil; some species nesting in dead twigs on lower vegetation.
- Blaimer, B.B. 2012. Taxonomy and species-groups of the subgenus Crematogaster (Orthocrema) in the Malagasy region (Hymenoptera, Formicidae). ZooKeys, 199, 23-70. doi:10.3897/zookeys.199.2631
- Bolton, B. 2003. Synopsis and Classification of Formicidae. Mem. Am. Entomol. Inst. 71: 370pp (page 239, Orthocrema as subgenus of Crematogaster)
- Brown, W. L., Jr. 1973b. A comparison of the Hylean and Congo-West African rain forest ant faunas. Pp. 161-185 in: Meggers, B. J., Ayensu, E. S., Duckworth, W. D. (eds.) Tropical forest ecosystems in Africa and South America: a comparative review. Washington, D.C.: Smithsonian Institution Press, viii + 350 pp. (page 183, Orthocrema as junior synonym of Crematogaster [provisional])
- Emery, C. 1922c. Hymenoptera. Fam. Formicidae. Subfam. Myrmicinae. [part]. Genera Insectorum 174B: 95-206 (page 130, Orthocrema as subgenus of Crematogaster)
- Hölldobler, B.; Wilson, E. O. 1990. The ants. Cambridge, Mass.: Harvard University Press, xii + 732 pp. (page 13, Orthocrema as junior synonym of Crematogaster)
- Hosoishi, S. and K. Ogata. 2016. Systematics and biogeography of the ant genus Crematogaster Lund subgenus Orthocrema Santschi in Asia (Hymenoptera: Formicidae). Zoological Journal of the Linnean Society. 176:547–606.
- Peeters, C., LIN, C.-C., Quinet, Y., Martins Segundo, G. and Billen, J. 2013: Evolution of a soldier caste specialized to lay unfertilized eggs in Crematogaster ants (subgenus Orthocrema). Arthropod Structure & Development. 42:257-264.
- Santschi, F. 1918d. Sous-genres et synoymies [sic] de Cremastogaster (Hym. Formic.). Bull. Soc. Entomol. Fr. 1918: 182-185 (page 182, Orthocrema as subgenus of Crematogaster)
- Smith, D. R. 1979. Superfamily Formicoidea. Pp. 1323-1467 in: Krombein, K. V., Hurd, P. D., Smith, D. R., Burks, B. D. (eds.) Catalog of Hymenoptera in America north of Mexico. Volume 2. Apocrita (Aculeata). Washington, D.C.: Smithsonian Institution Press, pp. i-xvi, 1199-2209. (page 1376, Orthocrema as junior synonym of Crematogaster)
- Soulié, J. 1964. Sur la répartition géographique des genres de la tribu des Cremastogastrini. Bull. Soc. Hist. Nat. Toulouse 99: 397-409 (page 398, Orthocrema as genus)
- Wheeler, W. M. 1922i. Ants of the American Museum Congo expedition. A contribution to the myrmecology of Africa. VII. Keys to the genera and subgenera of ants. Bull. Am. Mus. Nat. Hist. 45: 631-710 (page 662, Orthocrema as subgenus of Crematogaster)