Key to western Nearctic Strumigenys species

This key to western nearctic Strumigenys species (workers and queens) is based on Booher (2021). Illustrations were prepared by C. Dandridge.

Diagnostic characters used to identify species and important for species description are illustrated in Figure 1. Most of the characters are typical of previous taxonomic work. New to this work is the division of the clypeal margin into anterior and lateral portions, delimited by the outer mandibular insertions, and the referencing of seta positions with respect to these portions.

1

 * Mandibles with at least some teeth interlocking or overlapping in the first half of mandible when fully closed (Fig. 2A); MI < 22
 * Mandibles with interlocking teeth limited to the apical half of mandible (Fig. 2AA); MI ≥ 25 (see also third choice below)
 * Mandibles with an elongate flat-topped basal lamella of the mandible followed by a second flat topped elongate lamella that extends to the middle of masticatory margin (Fig. 2AAA); MI ≥ 27–33 Strumigenys alberti (a single specimen collected, NV)

2

 * Four to six erect setae radiate from the medial anterior dorsum of clypeus and spread out like ribs of a fan, most easily observed when viewed from apex of clypeus (Fig. 3A)
 * Erect setae are not present on anterior dorsum of clypeus, or if present, these setae are not organized like ribs of a fan (Fig. 3AA)

3

 * Fan shaped clypeal setae expanded and bulbous at tips (Fig. 4A), curving posteriorly in lateral view Strumigenys ornata (OK, TX, and many eastern states)
 * Fan shaped clypeal setae are apically acute to slightly expanded (Fig. 4AA) and do not curve posteriorly in lateral view Strumigenys dietrichi (KS, OK, TX, and many eastern states)

4

 * Fringe of setae on margins of clypeus either absent or minute and appressed but never projecting outward away from margin (Fig. 5A)
 * Fringe of setae on margins of clypeus present and project outward from margin, these setae may be of various shapes from long fine and apically acute to spoon shaped (Fig. 5AA, AAA)

5

 * Mandible with transverse carinae on dorsal surface of mandible just anterior of clypeal margin (Fig. 6A) (Cosmopolitan tramp species) Strumigenys membranifera (AZ, CA, TX, and several southeastern states)
 * Mandible without transverse carinae on dorsal surface of mandible just anterior of clypeal margin (Fig. 6AA) Strumigenys superstes (AZ)

6

 * Setae on margins and dorsum of clypeus consist of only apically acute, elongate, simple fine setae that may be coarse, straight, curved, or irregularly flexuous but never expanded (Fig. 7A)
 * Setae on either clypeal margins or dorsum are expanded or flattened (Fig. 7AA)

7

 * In dorsal view, spongiform appendages absent on petiolar and postpetiolar nodes, and are replaced by smooth cuticular lamellae on lateral and posterior borders of nodes (Fig. 8A) Strumigenys subtilis (CA)
 * In dorsal view, spongiform appendages large and apparent as white processes on lateral and posterior borders of petiolar node and encircling exposed disc of postpetiole (Fig. 8AA)

8

 * Clypeus in lateral view with a broad conspicuous peripheral groove along free margins (Fig. 9A)
 * Clypeus in lateral view without a peripheral groove (Fig. 9AA)

9

 * Basalmost tooth not following a large diastemmic gap, overlapping when mandible closed and less than half the length of the following tooth (Fig. 10A). Setae on margins and dorsum of clypeus are coarse, straight or evenly and shallowly curved along their entire length, extending well past margin (Fig. 10B)
 * Basalmost tooth following a large diastemmic gap, not overlapping when mandible closed and less than half the length of the following tooth (Fig. 10AA). Setae on margins and dorsum of clypeus are fine, often curving strongly along apical portion and barely extending past margin (Fig. 10BB) Strumigenys collinsae (CO)

10
11A) Strumigenys laevinasis (KS, OK, TX, and many eastern states)
 * Setae on dorsum of clypeus inclined anterior at base and evenly curve so that their apices point upwards or posteriorly (Fig.
 * Setae on dorsum of clypeus inclined anterior at base and evenly curve so that their apices point upwards or posteriorly (Fig. 11B) Strumigenys brevisetosa (KS, OK, TX, and many eastern states)

11

 * With head in full-face view setae that project from the lateral clypeal margin fine, conspicuously J-shaped and curve posteriorly (Fig. 12A) Strumigenys pilinasis (KS, OK, TX, and many eastern states)
 * With head in full-face view setae that project from the lateral clypeal margin variable in form but not J-shaped and curve or are inclined anteriorly (Fig. 12AA) Strumigenys reliquia (CA)

12

 * The pair of setae positioned nearest the outer mandible insertions (Fig. 13A; sometimes on the anterior margin of clypeus) and often those occurring on the lateral margin of clypeus (Figs. 14A, 13B) curve posteriorly, with apices directed towards the back of the head and away from mandibles
 * The pair of setae positioned nearest the outer mandible insertions (Fig. 13AA) and those on the lateral margin of clypeus (Fig. 13BB) all curve ventrally or towards mandibles, without apices directed posteriorly

13

 * Two or more pairs of setae positioned nearest the outer mandible insertions Fig. 14A) or on the lateral margin of clypeus curve away from mandibles Strumigenys reflexa (KS, TX, and many other eastern states)
 * One pair of setae positioned nearest the outer mandible insertions (Fig. 14B) curve away from mandibles. Those on the lateral margin curve towards the mandibles Strumigenys missouriensis (possible, many eastern states; may be a complex of species; should also compare with eastern species)

14

 * Ventral surface of petiolar peduncle with no trace of spongiform tissue (Fig. 15A) Strumigenys margaritae (TX and many other southeastern states)
 * Ventral surface of petiolar peduncle with obvious lamella of spongiform tissue (Fig. 15AA)

15

 * Setae on the posterior half of clypeal dorsum (Fig. 16A) with several to many setae that curve and are directed posteriorly or posterolaterally
 * Setae on posterior half of clypeal dorsum (Fig. 16AA) consisting of anteriorly or anterolaterally directed setae; without posteriorly directed setae

16

 * Setae on the anterior margin of clypeus (Fig. 17A) curve away from midline Strumigenys lucky (IA, IL, MN, WI)
 * Setae on the anterior margin of clypeus (Fig. 17AA) curve ventrally or towards midline

17

 * Setae covering dorsum of clypeus short, simple to narrowly expanded, and all curving posteriorly (Fig. 18A) Strumigenys macgowni (TX)
 * Setae on clypeal dorsum expanded, spoon-shaped to scale-like and translucent; those on the anterior portion of clypeus curve anteriorly and posteriormost setae curve posteriorly (Fig. 18AA) Strumigenys chiricahua (AZ)

18

 * Clypeus with a wide peripheral groove along entire free margin (Fig. 19A); maximum width of groove more than 1/3 the maximum length of eye (usually 0.5 times or wider and most easily seen when clypeus is viewed in profile or from anterior apex of clypeus when mandibles are open)
 * Clypeus without a peripheral groove (Fig. 19AA) along margin or, if present, the maximum width of groove is less than 1/3 the maximum length of the eye (usually 0.25 times or less)

19

 * Setae on lateral margins of clypeus simple-linear to slightly and evenly expanded along apical portion (Fig. 20A), directed anteriorly, and not strongly curving towards the midline; setae on dorsum of clypeus (Fig. 20B) similarly shaped but shorter and inclined anteriorly Strumigenys brevisetosa (KS, OK, TX, and many eastern states)
 * Setae on lateral margins of clypeus spatulate (Fig. 20AA), directed antero-medially and strongly curving towards the midline; setae on dorsum of clypeus (Fig. 20BB) consisting of shorter spatulate setae Strumigenys clypeata (KS, OK, TX, and many eastern states)

20

 * Diastemmic gap absent (Fig. 21A) or if present it is shorter than the length of the first principle tooth (measured between anterior edge of clypeus to first tooth if apex of basal lamella is hidden by clypeus in full frontal view)
 * Diastemmic gap present (Fig. 21AA) and is equal to or longer than the length of the first principle tooth (measured between anterior edge of clypeus to first tooth if apex of basal lamella is hidden by clypeus in full frontal view)

21

 * Differentiated pronotal humeral setae absent (Fig. 22A), diastema apparent and nearly equal to length of first tooth (Fig. 22B) Strumigenys mendezi (AZ)
 * Pronotal humeral setae present (Fig. 22AA) although may be short, extremely fine and difficult to see; diastema obviously shorter than length of first tooth (Fig. 22BB)

22

 * Pronotal humeral (Fig. 23A) setae erect, simple, straight, and stiff (always apparent) Strumigenys rostrata (TX, and many eastern states)
 * Pronotal humeral setae flagellate (Fig. 23AA) and apparent or extremely fine and in some specimens difficult to see

23

 * With head in profile clypeus abruptly raised, interrupting the outline of head, with the frontal area impressed and concave (Fig. 24A) Strumigenys californica (CA)
 * With head in profile clypeus not abruptly raised and not interrupting the outline of head, the frontal area continuous with clypeus, straight and not concave (Fig. 24AA) Strumigenys arizonica (USA - TX, AZ, NM; MEX - Sonora, Chihuahua)

24

 * Differentiated flagellate setae absent on vertex margin of head or at apicoscrobal position of head (Fig. 25A)
 * Standing flagellate setae present at apicoscrobal position and usually another pair just posterior to the apicoscrobal position along the vertex margin, straddling the midline of head (Fig. 25AA)

25

 * First (basal) tooth about half the length of second tooth and not overlapping opposing tooth in closed position (Fig. 26A) Strumigenys moreauviae (NM)
 * First (basal) two teeth approximately equal in length and overlapping opposed teeth in closed position (Fig. 26AA)

26

 * At least one pair of setae on the anterior clypeal margin curve strongly away from the midline (sometimes two pairs) (Fig. 27A); posterior half of clypeal dorsum with few or no setae Strumigenys missouriensis (possible, many eastern states; may be a complex of species; should also compare with eastern species)
 * All setae on the anterior margin of clypeus curve towards the midline (Fig. 27AA); posterior half of clypeal dorsum with many setae Strumigenys talpa (part, specimens from OK)

27

 * First five teeth alternate in size between similarly large and small teeth, in some individuals basalmost tooth is a small adventitious tooth and in others the first tooth is a large well-developed tooth (Fig. 28A)
 * First and second tooth are nearly equal in size and much larger than third tooth, never alternating in size among the first five basal teeth (Fig. 28AA)

28

 * Setae on anterior margin of clypeus (Fig. 29A) curve away from midline; setae on anterior dorsum of clypeus curve laterally or posterolaterally (Fig 29B); setae are club shaped and opaque Strumigenys lucky (IA, IL, MN, WI)
 * Setae on anterior margin of clypeus (Fig. 29AA) curve ventrally or towards midline; setae on anterior dorsum of clypeus curve anteriorly or anterolaterally (Fig 29BB); setae spoon-shaped and translucent Strumigenys ananeotes (UT)

29

 * One to two pairs of setae on anterior clypeal margin curve away from midline of mandibles (Fig. 30A); fewer than five setae are present on lateral margin of clypeus (Fig. 30B) Strumigenys pulchella (KS, TX, and many eastern states)
 * All setae on anterior clypeal margin curve towards the midline (Fig. 30AA); more than five setae present on lateral margin of clypeus (Fig. 30BB) Strumigenys talpa (OK, TX, and many eastern states)

30

 * Mandibles short, MI 25–33, outer margins of mandibles convexly bowed (Fig. 31A)
 * Mandibles long, MI 50–70, outer margins of mandibles linear (Fig. 31AA)

31

 * Along apical half of mandible basal three teeth triangular (Fig. 32A); anterior and lateral margins of the clypeus angulate (Fig. 32B); scapes shorter, SI 65–69 (Fig. 32C) Strumigenys angulata (OK and many eastern states)
 * Along apical half of mandible second tooth following basal tooth bluntly rounded (Fig. 32AA); anterior and lateral margins of the clypeus rounded (Fig. 32BB); scapes longer, SI 65–69 (Fig. 32CC) Strumigenys pergandei (KS and many eastern states)

NOTE – species keying out in the rest of the key should also be checked with Bolton (2000) key to Neotropical Strumigenys.

32

 * Mandible without enlarged preapical tooth just basal of apicodorsal tooth (Fig. 33A) (however a minute denticle is present at midlength of mandible); all setae on anterior margin of scape directed towards apex (Fig. 33B); pronotal humeral seta short, stiff and stout (Fig. 33C) Strumigenys boneti (TX)
 * Mandible with enlarged preapical tooth just basal of apicodorsal tooth (Fig. 33AA) (with or without a minute denticle at midlength of mandible); one or more pairs of setae on anterior margin of scape directed towards base (Fig. 33BB); pronotal humeral seta flagellate (Fig. 33CC)

33

 * Smaller species (HW 0.33–0.37) with a minute preapical denticle located at about midlength of mandible (Fig. 34A) (global tramp species) Strumigenys silvestrii (CA, TX, and many southeastern states)
 * Larger species (HW 0.38–0.54) without a minute preapical denticle (Fig. 34AA), or if preapical denticle present it is located near the apical portion of the second third of mandible

34

 * Mandible with preapical denticle located near the apical portion of the second third of mandible (Fig. 35A) Strumigenys mixta (AZ)
 * Mandible without preapical denticle (Fig. 35AA) Strumigenys louisianae (OK, TX, and many eastern states)