Morphological terminology follows Bolton (1994), on which much of this key is based. The term mesosoma is used in preference to alitrunk, to refer to the body part formed from fusion of the thorax and the first abdominal segment (i.e., thorax + propodeum). The promesonotum is that part of the mesosoma composed of the pronotum and the mesonotum. Metasoma refers to the apparent abdomen, comprising the segments posterior to the propodeum, i.e., abdominal segment 2 and succeeding segments. In ants abdominal segment2 forms a node- or scale-like petiole, which is separated by a constriction from the rest of the metasoma. In some species abdominal segment 3 is also node-like and in this case it is said to form a postpetiole.
The key has been designed to apply only to those ant species occurring in California but it should also work for most of western North America, excluding Arizona and New Mexico where additional genera occur.
1 Postpetiole present: abdominal segment 3 separated from segment 4 by a strong constriction and distinctly smaller in size, so that anteriorly the metasoma has two nodelike segments, the petiole and postpetiole .................................................................... 2
- Postpetiole absent: abdominal segment 3 separated from segment 4 by a weak to moderate constriction or by none at all, and when viewed in profile not distinctly smaller in size (height) than segment 4; metasoma anteriorly with a single, isolated node-like or scale-like segment ....................................................................................................... 26
2 Eye reduced to a single ommatidium or absent; antennal insertions fully exposed in a full-face view of head ( Ecitoninae ) ........................................ Neivamyrmex Borgmeier
- Eye very rarely reduced, usually consisting of multiple ommatidia; antennal insertions not fully exposed, covered partially by frontal lobes or medial extensions of the antennal sclerites, when the head is observed in full-face view ........................................... 3
3 Eye very large, eye length about one-half of head length (excluding mandibles); pronotum freely articulating with mesonotum ( Pseudomyrmecinae ) ............................. ....................................................................................................... Pseudomyrmex Lund
- Eye less than one-half head length; pronotum fused with mesonotum ( Myrmicinae ) ... ....................................................................................................................................... 4
4 Antenna with 6 segments, including a 2-segmented club ............................................. 5
- Antenna with 10 segments, including a 2-segmented club ......... Solenopsis Westwood
- Antenna with 11 segments; club variable ..................................................................... 6
- Antenna with 12 segments; club variable.. ................................................................ 14
5 Mandible elongate and linear, with an apical fork of two spiniform teeth ...................... ...................................................................................................... Strumigenys F. Smith
- Mandible short and subtriangular, with a multi-denticulate masticatory margin ........... ............................................................................................................... Pyramica Roger
6 Postpetiole attached to the dorsal surface of the following abdominal segment; petiole dorsoventrally flattened, not node-like in profile ........................... Crematogaster Mayr
- Postpetiole attached to the anterior face of the following segment; petiole node-like in profile, not dorsoventrally flattened ............................................................................. 7
7 Head in lateral view with a diagonal carina running from above the eye down toward the mandibular insertion; promesonotum with conspicuous tubercles or spines ......... 8
- Head in lateral view lacking such a diagonal carina; promesonotum without conspicuous tubercles or spines .................................................................................................9
8 Frontal lobes expanded laterally and covering the sides of the head below the eyes, in full-face view; body lacking erect pilosity.................................... Cyphomyrmex Mayr
- Frontal lobes not expanded laterally to cover the sides of the head; body with erect pilosity.............................................................................................. Acromyrmex Mayr
9 Antenna with a distinct 2-segmented apical club............................... Wasmannia Forel
- Antenna lacking a distinct 2-segmented apical club, either 3-segmented or indistinct.. .....................................................................................................................................10
10 Eye absent or rudimentary; propodeum unarmed, basal face rounding into declivitous face ................................................................................................ Solenopsidininew genus
- Eye well developed, with multiple ommatidia; propodeum angulate or spinose.......11
11 Lateral portions of clypeus, in front of the antennal insertions, developed in the form of a raised ridge or shield-wall; frontal carinae extending almost to the posterior margin of the head......................................................................... Tetramorium Mayr (part)
- Lateral portions of clypeus not developed as a raised ridge or shield-wall; frontal carinae very short or absent...............................................................................................12
12 Eye with short erect setae projecting between the ommatidia........ Formicoxenus Mayr
- Eye lacking erect setae................................................................................................13
13 Median portion of clypeus with a smooth, longitudinally excavate surface, and lacking carinae .............................................................................................. Leptothorax Mayr
- Median portion of clypeus with several longitudinal carinae......................................... ................................................................................................. Temnothorax Mayr (part)
14 Hind tibial spur finely pectinate (as seen at 50-100x magnification).........................15
- Hind tibial spur simple or absent.................................................................................17
15 Metanotal groove absent or very weakly impressed, not breaking the dorsal profile of the mesosoma; psammophore usually present............................ Pogonomyrmex Mayr
- Metanotal groove present and interrupting the dorsal profile of the mesosoma; psammophore absent...........................................................................................................16
16 Propodeum unarmed; mandible with more than 12 teeth ....................... Manica Jurine
- Propodeum armed with a pair of spines; mandible with 6-10 teeth .. Myrmica Latreille
17 Lateral portions of clypeus, in front of the antennal insertions, developed in the form of a raised ridge or shield-wall; apex of sting with triangular lamellate appendage...... ................................................................................................. Tetramorium Mayr (part)
- Lateral portions of clypeus not developed as a raised ridge or shield-wall; apex of sting without triangular lamellate appendage .....................................................................18
18 Petiole short and sessile, lacking well differentiated anterior peduncle and dorsal node; ventrolateral margin of head with sharp, longitudinal carina extending from mandibular base to posterolateral corner of head........................................... Myrmecina Curtis
- Petiole with anterior peduncle and dorsal node; ventrolateral margin of head without sharp, longitudinal carina ...........................................................................................19
19 Dorsum of head and mesosoma without standing pilosity .......... Cardiocondyla Emery
- Dorsum of head and mesosoma with standing pilosity ............................................... 20
20 Anteromedian portion of clypeus notably elevated and bounded by a pair of carinae that diverge anteriorly ................................................................................................ 21
- Anteromedian portion of clypeus not abruptly elevated and lacking a pair of anteriorly diverging carinae ........................................................................................................ 23
21 Propodeum unarmed ....................................................................... Monomorium Mayr
- Propodeum armed with a pair of teeth or spines ........................................................ 22
22 Antennal club 3-segmented; propodeal spiracle large and located close to the declivitous face of the propodeum, separated from latter by no more than the diameter of the spiracle .................................................................................................. Rogeria Emery
- Antennal club 4-segmented; propodeal spiracle relatively small and separated from the declivitous face of the propodeum by more than the spiracle diameter .......................... ....................................................................................................... Stenamma Westwood
23 Antennal club 3- (rarely 4-) segmented ....................................................................... 24
- Antenna lacking a distinct club ................................................................................... 25
24 In profile promesonotum domed and distinctly elevated above the propodeal dorsum; workers dimorphic ......................................................................... Pheidole Westwood
- In profile entire mesosoma dorsum flat to weakly convex, promesonotum not domed or markedly elevated above the level of the propodeum; workers monomorphic .......... ................................................................................................. Temnothorax Mayr (part)
25 Head narrow, longer than broad; mandible slender and triangular, outer margin not strongly curving towards the midline; psammophore absent ...... Aphaenogaster Mayr
- Head broad, subquadrate; mandible short and thick, outer margin strongly curving towards the midline; psammophore usually present ................................. Messor Forel
26 Pygidium (last visible abdominal tergite) flattened and bordered laterally with a row of peg-like teeth or spines that converge distally ( Cerapachyinae ).... Cerapachys F. Smith
- Pygidium (last visible abdominal tergite) convex and rounded, lacking a row of teeth or spines ..................................................................................................................... 27
27 Distinct constriction between abdominal segments 3 and 4; terga and sterna of abdominal segments 3 and 4 laterally fused ......................................................................... 28
- No constriction between abdominal segments 3 and 4; terga of abdominal segments 3 and 4 overlapping the corresponding sterna, not laterally fused with them ............... 30
28 Articulation of petiole (second abdominal segment) to third abdominal segment very broad; petiole without a distinct posterior face ( Amblyoponinae ) .................................. ..................................................................................................... Amblyopone Erichson
- Articulation of petiole (second abdominal segment) to third abdominal segment narrow; petiole with a distinct posterior face .................................................................. 29
29 Pronotum freely articulating with the mesonotum; abdominal tergite 4 not strongly enlarged and not curved ventrally; apex of metasoma directed posteriorly ( Ponerinae ) ....................................................................................................... Hypoponera Santschi
- Pronotum fused immovably to the mesonotum; abdominal tergite 4 strongly enlarged and curved ventrally; apex of metasoma directed anteriorly ( Proceratiinae ).................. .......................................................................................................... Proceratium Roger
30 Apex of metasoma with a circular orifice, often fringed with short setae (the acidopore) ( Formicinae )...................................................................................................31
- Apex of metasoma with a slit-shaped orifice ( Dolichoderinae )..................................38
31 Antenna with 9 segments ............................................................. Brachymyrmex Mayr
- Antenna with 11 segments ................................................................. Plagiolepis Mayr
- Antenna with 12 segments .........................................................................................32
32 Metapleural gland absent; antennal insertions well separated from the posterior clypeal margin; in profile mesosoma dorsum usually evenly convex........................ Camponotus Mayr
- Metapleural gland present; antennal insertions adjacent to the posterior clypeal margin; in profile promesonotum separated from the dorsal face of the propodeum by a distinct impression .....................................................................................................33
33 Maxillary palp segments 3 and 4 greatly elongated, segment 3 (counting from base) half the head length or more; psammophore present.............. Myrmecocystus Wesmael
- Maxillary palp segments 3 and 4 not greatly elongated, segment 3 much less than half the head length; psammophore absent .......................................................................34
34 Ocelli present; propodeal spiracle elliptical to oval....................................................35
- Ocelli absent or indistinct; propodeal spiracle circular to subcircular.......................36
35 Mandible triangular, with seven or more distinct teeth on the masticatory margin........ ............................................................................................................ Formica Linnaeus
- Mandible falcate (sickle-shaped) and lacking distinct teeth............ Polyergus Latreille
36 Dorsum of head and mesosoma with coarse setae, arranged in distinct pairs; eye situated in relatively anterior position, at or in front of midlength of side of head ............. ............................................................................................ Paratrechina Motschoulsky
- Pilosity on dorsum of head and mesosoma variable, but not arranged as coarse setae in pairs; eye situated in relatively posterior position, behind midlength of side of head ... .....................................................................................................................................37
37 Mandible with six teeth; antennal scape long, surpassing posterior margin of head by more than half its length; mesonotum in dorsal view strongly constricted behind pronotum .............................................................................................................. Prenolepis Mayr
- Mandible with seven or more teeth; antennal scape shorter, surpassing posterior margin of head by less than a third its length; mesonotum in dorsal view not strongly constricted behind pronotum...................................................................... Lasius Fabricius
38 Propodeum with a prominent conical tooth at the junction of the dorsal and declivitous faces; maxillary palp segment 3 elongate, subequal in length to segments 4-6; apical mandibular tooth much enlarged...................................................... Dorymyrmex Mayr
- Propodeum rounded or subangulate at the junction of the dorsal and declivitous faces, but without a conical tooth; maxillary palp segment 3 short, subequal in length to segments4; apical mandibular tooth not notably enlarged .............................................. 39
39 Mesosoma dorsum lacking standing pilosity .............................................................. 40
- Mesosoma dorsum with standing pilosity .................................................................. 41
40 Petiole flattened, plate-like, and without a conspicuous, dorsally protruding scale (petiole often overhung by the succeeding abdominal segment); dorsal face of propodeum much shorter than the declivitous (posterior) face ............................ Tapinoma Foerster
- Petiole with a well developed, dorsally protruding scale; dorsal face of propodeum subequal in length to declivitous face .............................................. Linepithema Mayr
41 In profile mesosoma dorsum without an impressed metanotal groove, the promesonotum and propodeum forming a continuous surface; workers variable in size within a colony ........................................................................................................... Liometopum Mayr
- In profile mesosoma dorsum interrupted by a well marked metanotal groove; workers showing little intra-colony size variation .................................................................... 42
42 Petiole lacking an erect scale; side of mesosoma with conspicuous microreticulate sculpture; dark brown-black, with contrastingly paler tarsi .......... Technomyrmex Mayr
- Petiole with well developed erect scale; side of mesosoma without conspicuous microreticulate sculpture; varying in color from yellowish-orange to dark brown, but without contrastingly paler tarsi ............................................................ Forelius Emery
A few genera are divided into parts, reflecting distinct characters sets outlined in Table 1.
1. Two distinct, long, narrow spines or lobes present on apical portion of abdominal sternum IX (Fig. 3a) or, if absent, then mandibles extremely elongated, distinctly longer than head, and volsella massive, claw-shaped, directed dorsally. Pygostyles absent........................................................... Cerapachyinae
- Spines or lobes absent on apical portion of abdominal sternum IX or the apical portion bilobed, with each lobe very wide (Fig. 3b). Mandibles not elongated, distinctly shorter than head. Volsella moderate, not claw-shaped, not directed dorsally. Pygostyles present or absent......................................................2
2. Abdominal segment III much smaller than segment IV in lateral view (Fig. 3c)....................................3
- Abdominal segment III nearly as large as segment IV in lateral view (Fig. 3d)......................................4
3. Hind tibia with two spurs (Fig. 3e)......................................................................... Pseudomyrmecinae
- Hind tibia with one spur or without spurs (Fig. 3f) ............................................................. Myrmicinae
4. Anal region of hind wing vestigial. Oblique mesopleural furrow reaching pronotum close to its posteroventral corner (Fig. 4a)...................................................................................................... Proceratiinae
- Anal region of hind wing well developed. Oblique mesopleural furrow not reaching pronotum, its anterior termination well separated from the pronotum (Fig. 1a)..................................................................5
5. Petiole (abdominal segment II) broadly and dorsally attached to abdominal segment III; dorsal constriction between petiole and abdominal segment III very shallow or indistinct in lateral view (Fig. 4b......... ........................................................................................................................................ Amblyoponinae
- Petiole (abdominal segment II) narrowly and ventrally attached to abdominal segment III; dorsal constriction between petiole and abdominal segment III deep in lateral view (Fig. 6a)...6
6. Scuto-scutellar suture usually longitudinally sculptured. Forewing clearly with cross vein 2rs-m (Fig. 1b); if vein weak then at least with vestigial branches on Radial sector and Media. Scape short, not reaching posterior margin of head in full-face view (Fig. 5c). Constriction between abdominal segments III and IV present in some cases.............................................................................................. Ponerinae
- Scuto-scutellar suture not longitudinally sculptured. Forewing usually without any trace of cross vein 2rs-m (Fig. 4c). Scape short (Fig. 4d) or long (Fig. 2a). Constriction between abdominal segments III and IV absent...........................................................................................................................................7
7. Many minute, serrate teeth present on masticatory margin of mandible (Fig. 4d), or, if teeth absent, then scape not reaching posterior margin of head in full-face view........................................ Dolichoderinae
- Several larger teeth present on masticatory margin of mandible (Fig. 2a). Scape long, distinctly exceeding posterior margin of head in full-face view (Fig. 2a)........................................................ Formicinae
