Key to Monomorium rothsteini species complex

This worker key is based on: *Sparks, K.S., Andersen, A.N. and Austin, A.D. 2015. Systematics of the Monomorium rothsteini Forel species complex (Hymenoptera: Formicidae), a problematic ant group in Australia. Zootaxa. 3893:489–529. doi:10.11646/zootaxa.3893.4.2

Characters used in key and descriptions

In addition to traditional characters used in ant descriptions, the following characters require additional explanation and illustration.

Clypeal anterodorsal margin—in full face view the clypeus appears to have two frontal margins, one that is a continuation of the frontolateral margin of the head and one that sits ventrally. These are here referred to as the anterodorsal and anteroventral clypeal margins respectively. The presence of the anteroventral margin is most obvious in those species with a moderately to deeply concave anterodorsal margin.

Clypeal anteroventral margin median projection—in full face view the clypeal anteroventral margin can have a median projection giving the margin a broadly triangular shape.

Coarse lateral cephalic strigae—strigae that extend posteriorly from the mandibular articulation towards the eye. These strigae are generally coarse and well defined closer to the mandibular articulation and weaken towards the eye, but in some species remain coarse to the eye margin and beyond.

Propodeum dorsolateral angles—The shape of the propodeum varies greatly among species. In some the posterior face appears vertical giving a cuboid or rectangular impression of the overall shape of the propodeum. In these cases the dorsolateral angles approach a right angle. In a majority of species the posterior face is declivitous and the dorsolateral angles clearly form an obtuse angle.

Propodeum with a raised anterodorsal transverse carina—where the propodeum meets the metanotal groove there exists in many species a raised, sharply defined transverse carina.

This key is designed for the identification of the majority of the most commonly encountered species in the M. rothsteini complex. All of these species, with the exception of M. leda, were recognised as separately evolving lineages in the mitochondrial DNA analysis of Sparks et al. (2014). Additional morphotypes are known to exist within the complex, but there is insufficient evidence available at this time to formally describe them. For species identifications that are particularly important it is strongly recommended that the CO1 sequence data are obtained and compared with those deposited in Genbank. All species exhibit some level of intraspecific variation that can in some cases lead to incorrect identification when using this key. It is recommended that a range of specimens from a colony are examined to determine the character state that best represents the majority of specimens.

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 * Key to Australian Monomorium Species

1

 * Anterodorsal margin of the clypeus deeply concave, anterolateral carinae produced as acute angles, often with a wavy margin or minute preapical teeth medial to the acute angles . . . . . 2


 * Anterodorsal margin of the clypeus not as above, may be concave but without acute anterolateral angles or preapical teeth . . . . . 5

2
return to couplet #1
 * Petiole node, postpetiole and anterior half of T1 completely covered in a fine reticulate pattern; very large, robust species (HW and ML > 1mm for a majority of specimens) . . . . . Monomorium merepah


 * Petiole and postpetiole with a fine reticulate pattern that does not extend onto dorsal surface; smaller species (HW and ML < 1mm for a majority of specimens) . . . . . 3

3
return to couplet #2
 * First metasomal tergite (T1) with fine reticulate sculpture restricted to anterior third . . . . . Monomorium maryannae


 * T1 with fine reticulate sculpture covering at least anterior half . . . . . 4

4
return to couplet #3
 * A majority of specimens with mesonotum sculpture present over entire area posteriad of promesonotal suture, sculpture con-sisting of longitudinal rows of shallow alveolae, rarely with a small central area without sculpture; known only from the Kimberley region of Western Australia . . . . . Monomorium stagnum


 * Mesonotum sculpture restricted to promesonotal suture, posterior mesonotum and lateral curvature; central area always smooth; known only from the Cape York area of Queensland . . . . . Monomorium capeyork

5
return to couplet #1
 * Anterodorsal margin of the clypeus distinctly concave, with frontolateral carinae forming angular ridges that extend beyond anteroventral clypeal margin, forming obtuse angles . . . . . 6


 * Anterodorsal margin of the clypeus sinuous to almost straight, frontolateral carinae forming smooth ridges that do not extend beyond anteroventral clypeal margin . . . . . 15

6
return to couplet #5
 * Mesonotum sculptured over most of area posteriad of promesonotal suture, propodeum in lateral view with dorsolat-eral angles almost forming a right angle, such that propodeum appears cuboid . . . . . Monomorium mitchell


 * Mesonotum with sculpture restricted to at most posterior half of area posteriad of promesonotal suture; propodeum in lateral view with dorsolateral angles forming an obtuse angle or rounded . . . . . 7

7
return to couplet #6
 * T1 with fine reticulate sculpture over at least anterior quarter; coarse lateral cephalic strigae reaching anterior margin of eye . . . . . 8


 * T1 without sculpture, smooth and shining; coarse lateral cephalic strigae may or may not reach anterior margin of eye . . . . . 10

8
return to couplet #7
 * Anteroventral margin of the clypeus without an anterior median projection, petiole node broadly rounded in posterior view . . . . . Monomorium humilior (in part - also )


 * A majority of specimens with a median projection on the anteroventral margin of the clypeus; petiole node tapering to a rounded point in posterior view . . . . . 9

9
return to couplet #8
 * Mesonotum strigulate on lateral curvature, in metanotal groove and extending onto posterior mesonotum; petiole node narrow, width less than 2 x eye width when viewed from above; longitudinal strigae absent from dorsal surface of propodeum; known only from the Pilbara region of Western Australia . . . . . Monomorium pilbara


 * Mesonotum almost entirely smooth, a few strigulae on posterior and lateral portions of posterior mesonotum; petiole node of medium breadth, between 2 and 2.5 x eye width when viewed from above; propodeal longitudinal strigae very weakly present as broken lines on posterior half only; known from the Victoria River district of the Northern Territory . . . . . Monomorium hoffmanni

10
return to couplet #7
 * Coarse lateral cephalic strigae not reaching anterior eye margin and transverse strigae absent from dorsal surface of propodeum . . . . . 11


 * At least some coarse lateral cephalic strigae reaching anterior eye margin; propodeal transverse strigae present on dorsal sur-face, although may be very weak . . . . . 13

11
return to couplet #10
 * Medium to large species (ML > 1mm) with medium to small eyes (EW < 0.23 x LHW) . . . . . Monomorium leda


 * Relatively small species (ML < 1 mm) with large eyes (EW > 0.23 x LHW) . . . . . 12

12
return to couplet #11
 * Petiole node without sculpture and narrow ( < 2 x eye width when viewed from above); head and mesosoma light to dark brown with a reddish-orange tinge; anteroventral margin of the clypeus with a small median projection . . . . . Monomorium broschorum


 * Petiole node with fine reticulate sculpture laterally and a long posterobasal margin, of medium breadth (2–2.5 x eye width when viewed from above); head, mesosoma and legs amber orange to orange brown; anteroventral margin of the clypeus rarely with a small median projection . . . . . Monomorium kidman

13
return to couplet #10
 * Longitudinal strigae absent from dorsal surface of propodeum and longitudinal depression very shallow or absent; propodeum appears dorsally rounded, dorsolateral angles of the propodeum rounded; petiole node of medium breadth (2–2.5 x eye width when viewed from above) . . . . . Monomorium rothsteini


 * Longitudinal strigae present on dorsal surface of propodeum either as raised convergent ridges or irregular convergent lines; longitudinal depression present; petiole node narrow (< 2 x eye width when viewed from above) . . . . . 14

14
return to couplet #13
 * Longitudinal strigae on the dorsal surface of the propodeum converging anteriorly, forming a triangle with a carina that extends between the dorsolateral angles; head, mesosoma and legs dark chestnut brown, except trochanters light brown; known only from the Pilbara region of Western Australia . . . . . Monomorium tenebrosum


 * Longitudinal strigae on the dorsal surface of the propodeum not forming a triangle, carina absent between the dorsolateral angles; head and mesosoma dark brown with a reddish-orange tinge; known only from the stony plains region of northern South Australia . . . . . Monomorium oodnadatta

15
return to couplet #5
 * Dorsolateral angles of the propodeum almost forming a right angle, such that the propodeum appears cuboid; propodeum laterally alveolate with irregular strigae extending onto dorsal surface or on posterior half of lateral surface; mesopleuron with at least a few strigae extending anteriorly from metapleural groove . . . . . 16


 * Dorsolateral angles of the propodeum clearly forming an obtuse angle, laterally alveolate without irregular strigae extending onto dorsal surface or on posterior half of lateral surface; mesopleuron with or without strigae . . . . . 17

16
return to couplet #15
 * Mesonotum strigate-rugose over most of surface posteriad of promesonotal suture; sculpture on frons absent, a few strigae present only on antennal lobes; petiole node narrow, < 2 x eye width when viewed from above; colour of head and mesosoma predominantly dark chestnut brown . . . . . Monomorium eremum. . . . . Monomorium eremoides


 * Mesonotum rugulose-strigate on lateral curvature and in metanotal groove, smooth medially; frons with strigae that extend well above antennal lobes; petiole node broad, > 2.5 x eye width when viewed from above; colour of head and mesosoma predominantly dark amber orange . . . . . Monomorium geminum . . . . . Monomorium topend

17
return to couplet #15
 * T1 with a fine reticulate pattern over at least anterior third; colour of head and mesosoma predominantly dark amber orange . . . . . 18


 * T1 without sculpture, smooth and shining; colour of head and mesosoma predominantly light to dark brown, sometimes with a reddish tinge . . . . . 19

18
return to couplet #17
 * Petiole node of medium breadth, 2–2.5 x eye width when viewed from above; a majority of specimens with fine reticulate sculpture restricted to anterior half or less; known only from the central desert region of the Northern Territory . . . . . Monomorium humilior (in part - also )


 * Petiole node width broad, > 2.5 x eye width when viewed from above; all specimens with T1 completely covered in a fine reticulate pattern; known only from Cape York Peninsula . . . . . Monomorium hertogi

19
return to couplet #17
 * Anterior transverse carina prominent on propodeum . . . . . Monomorium bogischi


 * Anterior transverse carina on propodeum very weak or absent . . . . . 20

20
return to couplet #19
 * Dorsal surface of propodeum with a shallow longitudinal depression that runs most of its length; a majority of specimens with-out sculpture on the posterior mesonotum . . . . . Monomorium torrens


 * Dorsal surface of the propodeum appears flat or slightly rounded along most of its length, a depression present only between posterodorsal angles; a majority of specimens finely strigulate on posterior mesonotum . . . . . Monomorium speculum . . . . . Monomorium subapterum