Steven O. Shattuck.
An analysis of geographic variation in the Pogonomyrmex occidentalis complex. (Hymenoptera: Formicidae).
Psyche 94:159-180, 1987.

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AN ANALYSIS OF GEOGRAPHIC VARIATION IN THE POGONOMYRMEX OCCIDENTALS COMPLEX
(HYMENOPTERA: FORMICIDAE)'
BY STEVEN 0. SHATTUCK
Department of Entomology,
University of Kansas,
Lawrence, Kansas 66045
While examining samples of Pogonomyrmex from southeastern Oregon, I encountered difficulty in segragating the two species reported as occurring in the area, P. owyheei Cole and P. salinus Olsen (Cole, 1968). Analysis of material from Oregon, Nevada and Utah suggested that the taxonomy of these two forms needed reeval- uation. Smith (1953) considered them a single species, but this view was subsequently rejected by Cole (1968). Cole (1963, 1968) described the occidentalis complex as com- posed of P. owyheei and salinus, together with P. anzensis Cole, brevispinosus Cole, occidentalis (Cresson), subdentatus Mayr and subnitidus Emery. MacKay (1980) described P. montanus from southern California, bringing the complex to a total of 8 species. Within the occidentalis complex, the species P. owyheei, salinus, occidentalis, subdentatus and montanus form a distinct, plausibly monophyletic group, united by the cephalic sculpturing, configura- tion of the antenna1 scape base, and thoracic sculpturing. This group is referred to here as the occidentalis subcomplex. Because they do not possess this combination of characters, and because monophyly of the occidentalis complex as a whole is uncertain, P. anzensis, brevispinosus and subnitidus were excluded from this group and the present study.
'Extracted from a thesis submitted to the University of Kansas in partial fulfillment of the degree of Master of Arts. Present address: Department of Entomology, Uni- versity of California, Davis, CA 95616. Contribution number 1953 from the Depart- ment of Entomology, University of Kansas. *Manuscript received by the editor February 8, 1987.



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160 Psyche [vo~. 94
Lending museums and individuals are referred to by the following abbreviations:
AMNH American Museum of Natural History, New York, New York.
LACM Los Angeles County Museum of Natural History, Los Angeles, California.
MDPC Mark B. DuBois, Washington, Illinois. ORST Oregon State University, Corvallis, Oregon. UKS University of Kansas, Lawrence, Kansas. SSPC Steven 0. Shattuck, University of California, Davis, California.
UALB
University of Alberta, Edmonton, Alberta, Canada. UCB University of California, Berkeley, California. UCD University of California, Davis, California. UCOL
University of Colorado Museum, Boulder, Colorado. UIDM
University of Idaho, Moscow, Idaho.
UNSM University of Nebraska State Museum, Lincoln, Nebraska.
USNM United States National Museum, Washington, D.C. UTST Utah State University, Logan, Utah. UWYL University of Wyoming, Laramie, Wyoming. WAST Washington St ate University, Pullman, Washington. Every attempt was made to record character data on worker spec- imens from the entire range of each species and to represent all areas with roughly equal numbers of individuals. In some cases this meant ignoring specimens from densely collected areas, when other areas lacked sufficient samples for analysis. If a single nest was repre- sented by many individuals, three individuals were selected at ran- dom and their characteristics recorded. Queens and males were not analyzed because they are poorly represented in collections. Speci- mens were examined with a dissecting microscope at 75X magnifica- tion. Measurements were made with an ocular micrometer and recorded to the nearest 0.01 mm. The collection sites for specimens studied are listed in Shatt~ck (1985).




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19871 Shattuck-Pogonornyrrnex occidentalis complex 16 1 THE P. OCCIDENTALS SUBCOMPLEX
This group is characterized by the presence of punctures in the interrugal spaces of the head; spines or denticles on the propodeum; postpetiole with a rounded venter; occipital corners with regular, uniform rugae; and frontal lobes only slightly expanded laterally, with a thickened border. The 5 nominal species were examined to find characters that varied geographically within and/or between forms. While many such characters where found, only six varied consistently enough to be of value. Each character was divided into character states (except Scape Index, which was quantified). Since these characters were continuous, division into states was somewhat arbitrary. However, the large variation of each character and the limited number of states recognized in each allowed ready place- ment of most specimens. Before states were assigned, specimens were compared with "standard" specimens established for each character state. Specimens were initially identified using the con- cepts of Cole (1968). Many specimens had been previously deter- mined by Cole (primarily from Cole's personal collection, now at LACM), and data were recorded using his identifications. Speci- mens not readily determined were recorded, but not utilized in the initial analysis. The characters are listed below by their abbrevia- tions, followed by an explanation of the character and a list of the recognized states.
Gaster: Base of the first gastric (fourth abdominal) tergum, near the insertion of the postpetiole, viewed dorsally. 1) Very shiny, with no sculpturing.
2) Very shiny, with faint reticulate sculpturing. 3) Shiny, with reticulate sculpturing.
4) Dull, with moderate reticulate sculpturing. 5) Dull, with heavy reticulate sculpturing. PetDor: Dorsum of petiolar node viewed dorsally. 1) Entire surface covered with fine punctations, transverse rugae absent.
2) Anterior surface with fine punctations; 1 to 2 weak, transverse rugae superimposed over punctation on posterior quarter.



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162 Psyche [vo~. 94
3) Anterior surface with fine punctations; 2 to 3 weak to moder- ate, transverse rugae superimposed over punctation on poste- rior quarter.
4) Anterior surface with fine punctations; 4 to 5 moderate, trans- verse rugae superimposed over punctation on posterior half. 5) Rugae (usually 6 to 7) superimposed over punctation on entire surface.
PpDor: Dorsum of postpetiolar node viewed dorsally. 1) Entire surface covered with fine punctations, transverse rugae absent.
2) Anterior surface with fine punctations; 1 to 2 light, transverse rugae superimposed over punctation on posterior quarter. 3) Anterior surface with fine punctations; 3 to 4 light, transverse rugae superimposed over punctation on posterior third. 4) Anterior surface with fine punctations; 5 to 6 moderate, trans- verse rugae superimposed over punctation on posterior half. 5) Rugae (usually 7 to 9) superimposed over punctation on more than half of surface.
PetVent: Lateral profile of anterior, ventral surface of petiolar peduncle with the petiole, postpetiole and gaster raised dorsally. 1) Tooth absent, entire profile smooth (Figs. 1 and 2). 2) Tooth absent, profile uneven (Figs. 3 and 4). 3) Tooth present but small or indistinct (Figs. 5 and 6). 4) Tooth distinct and moderately developed (Figs. 7 and 8). 5) Tooth well developed and broad (Figs. 9 and 10). 6) Tooth well developed, elongate and narrow (Figs. 11 and 12).
Scape: Base of the antenna1 scape in lateral view with the anten- nae parallel to and resting against the dorsal surface of the head. 1) Rounded; flange absent (Figs. 13 and 14). 2) Rounded; flange absent dorsally, but indicated laterally (Figs. 15 and 16).
3) Rounded; flange present, distinctly broader dorsally than ven- trally (Figs. 17 and 18).
4) Angular; flange present, slightly broader dorsally than ven- trally (Figs. 19 and 20).
5) Angular; flange present, uniform in thickness (Figs. 21 and 22).




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19871 Shattuck-Pogonomyrmex occidentalis complex 163 SI: Scape Index (SL X 100) / (HW) where: SL is the scape length (maximum length of the scape exclusive of the radicle).
HW is the head width (maximum width of the head in full face (dorsal) view, exclusive of the eyes).
Analysis of Character States
In this section the species concepts of Cole (1968) are maintained. Each character is considered separately by species, or pair of species where applicable. In the next section each species is considered individually and specific taxonomic changes proposed. Data are pooled and analyzed by county within each species or group of species. Figures 23 through 36 represent the character distribution data graphically. Each bar graph represents pooled data for one county, with the vertical axis representing frequency and the hori- zontal axis representing the character states (state 1 on the left, state 5 or 6 on the right). The number of specimens represented is indi- cated and each graph is located at the approximate center of the area pooled.
Pogonomyrmex occidentalis possesses Gaster states 1, 2 and 3 (Table 1) distributed randomly, with all states occurring in all areas of the range in roughly equal proportions (Fig. 23). Cole (1968) stated that in P. salinus the "base of dorsum of first gastric segment [is] frequently densely and coarsely punctate and opaque, sometimes only very finely punctate or densely shagreened and subopaque or shining," while for P. owyheei it is "as in occiden- talis," which has "light to moderate shagreening which does not dull the shining surface". These characterizations were confirmed in the present study. The gaster of P. salinus may possess any of the states, with state 1 the least common and state 5 the most common (Table 1). In P. owyheei, most specimens possess states 1,2 or 3, with states 4 and 5 represented in lower frequencies (Table 1). Thus while this character is of some value in distinguishing these two forms, there is frequent overlap with 80% of the P. owyheei and 44% of the P. salinus specimens showing Gaster states 2 or 3.



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164 Psyche [vo~. 94
Table 1.
Table of Character State Frequencies*
Character
/State Taxa
Gaster P. mon P. occ P. owy P. sal P. sub 1
2
3
4
5
PetDor
1
2
3
4
5
Petvent
1
2
3
4
5
6
PpDor
1
2
3
4
5
Scape
1
2
3
4
5
Sample Size:
*A11 values are percentages. Taxa abbreviations are: P. mon = P. montanus, P. occ = P. occidentalis, P. owy = P. owyheei, P. sal = P. salinus, P. sub = P. subdentatus. The geographic distribution of this character supports Cole's (1968) northern limit for P. salinus as southern Oregon (Lake, Har- ney and Malheur counties), but indicates P. owyheei as occurring throughout Nevada and Utah (Fig. 24) rather than northern Nevada and Utah northward.




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19871 Shattuck-Pogonomyrmex occidentalis complex 165 P. montanus has Gaster states 2 and 3 in equal proportions (Table l), but shows no geographic pattern. P. subdentatus possesses states 1, 2 and 3, with state 3 the most common (Table 1); the states are distributed randomly throughout the range of the species. All character states of PetDor are represented in P. occidentalis (Table 1). The character PpDor is represented by states 1,2,3 and 4 (Table 1). Both of these characters show random geographic distri- bution; all states occur in all areas of the range of P. occidentalis (Figs. 25 and 26).
Cole (1968) described the dorsum of the petiolar and postpetiolar nodes of P. salinus as "generally covered with numerous, strong, wavy, closely spaced, subparallel, usually transverse rugae" (Pet Dor states 4 and 5; PpDor states 4 and 5). In P. owyheei he described these structures as "not covered with numerous, strong, wavy, closely spaced, subparallel, transverse rugae, the nodes with or without irregular rugae, rugulae or striae" (PetDor states 1,2 and 3; PpDor states 1 and 2). In this study all states of PetDor and PpDor were found to occur in both P. salinus and P. owyheei (Table 1) in frequencies which agree loosely with Cole's (1968) descriptions of these species. P. salinus possesses primarily PetDor states 4 and 5 (85%), but substantial numbers have states 1 and 2 (10%). Similarly, PpDor states 4 and 5 occur in 39% of the specimens, but states 1 and 2 occur in 24%. In P. owyheei, PetDor states 1,2 and 3 account for 77% of the specimens, but states 4 and 5 occur in 23%. PpDor states 1 and 2 are represented in 80% of the specimens, while states 4 and 5 are only found in 7%. The broad overlap in the states of these characters makes them of limited value in discriminating these forms.
The geographic distribution of the states of these characters shows a pattern similar to that of the Scape states: decreasing fre- quencies of states 1 and 2 and increasing frequencies of states 4 and 5 from north to south, with a broad area of overlap from central Oregon into northern Nevada (Figs. 27 and 28). PpDor state 1 is found as far south as Washington County, Utah (loo%), while state 5 occurs only as far north as Harney and Lake counties, Oregon



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Figs. 1-12, states for ventral surface of petiolar peduncle (Petvent). Fig. 1, state
1 (P. salinus, 17 mi S. Eureka, Eureka Co., Nevada). Fig. 2, state 1 (P. salinus,
Lovelock, Pershing Co., Nevada). Fig. 3, state 2 (P. salinus, Aurora, Mineral Co., Nevada). Fig. 4, state 2 (P. salinus, Reno, Nevada). Fig. 5, state 3 (P. salinus,
6 mi E Wells, Elko Co., Nevada).
Fig. 6, state 3 (P. subdentatus, Gonzales, Monterey Co., California). Fig. 7, state 4 (P. salinus, Wabuska, Lyon Co., Nevada). Fig. 8, state 4 (P. subdentatus, Russell Reservation, Contra Costa Co., California). Fig. 9, state 5 (P. subdentatus, 18 mi S. Ravendale, Lassen Co., California).
Fig. 10, state 5 (P. salinus, Lovelock, Pershing Co., Nevada). Fig. 11,
state 6 (P. subdentatus, Santa Cruz Island, Santa Barbara Co., California). Fig. 12,
state 6 (P. subdentatus, Gonzales, Monterey Co., California). (16% and 14% respectively). PetDor state 1, however, occurs only as far south as Lassen County, California (33%) and Elko County, Nevada (9%), while state 5 extends northward into Lake (33%), Harney (28%) and Malheur (20%) counties, Oregon, and Elmore



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19871 Shattuck-Pogonomyrmex occidentalis complex 167 County (33%), Idaho. Thus neither of these characters fully sup- ports the distributional boundaries of P. salinus and P. owyheei cited by Cole (1968): PetDor indicates the northern extent of P. salinus as central Oregon rather than southern Oregon, while the southern boundary of P. owyheei (northern California and Nevada) is supported; PpDor supports the northern extent of P. salinus as southern Oregon but indicates P. owyheei as occurring throughout Nevada rather than limited to the northern portions. All specimens of P. montanus have PetDor state 5 and PpDor state 5 (Table l), thus showing no geographic variation. P. subden- tatus does show variation in these characters, with PetDor states 1, 3, 4 and 5 represented as well as all PpDor states (Table 1). How- ever, this variation is distributed randomly throughout the range of the species (Figs. 29 and 30).
In P. occidentalis, all states of PetVent are represented (Table 1). This agrees with the findings of Cole (1968), except for the presence of a prominent process (states 5 and 6) in 24% of the specimens from Dawson, Roosevelt and Rosebud counties, Montana. Pogonomyrmex owyheei possesses the first five states of PetVent in decreasing frequency and lacks the sixth state (Table 1). P. salinus shows a similar trend, but possesses all states (Table 1) and a higher proportion of individuals with higher states. P. subdentatus has all states represented but in increasing frequency from state 1 to state 6 (Table 1). The presence of a large process on the venter of the petiolar peduncle is often used as a diagnostic character for P. sub- dentatus (Creighton, 1950; Cole, 1968) and the presence of states 4, 5 and 6 in populations of P. salinus in western Nevada may indicate gene flow between these two species. States 4, 5 and 6 occur in P. salinus only in or near areas containing populations of P. subdenta- tus (south central Oregon, western Nevada), while in all other areas P. salinus exhibits states 1, 2 or 3 (Fig. 3 1). Other characteristics of P. subdentatus (cephalic sculpturing, propodeal spine develop- ment, thoracic profile) do not appear in western Nevada popula- tions of P. salinus, indicating only limited or no gene flow between P. salinus and P. subdentatus. There is a weak trend for state 1 to increase and states 2 and 3 to decrease as one moves outward from northwestern Nevada.




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Figs. 13-22, states for base of antenna1 scape (Scape). Fig. 13, state 1 (P. salinus,
2 mi N Montello, Elko Co., Nevada).
Fig. 14, state 1 (P. salinus, Sulphur, Hum- boldt Co., Nevada). Fig. 15, state 2 (P. salinus, T14N R6E, White Pine Co., Neva- da). Fig. 16, state 2 (P. salinus, T18N R64E, White Pine Co., Nevada). Fig. 17, state 3 (P. salinus, Raft River Narrows, Cassia Co., Idaho). Fig. 18, state 3 (P. salinus, Montgomery Pass, Mineral Co., Nevada). Fig. 19, state 4 (P. sub-
dentatus, Davis, Yolo Co., California). Fig. 20, state 4 (P. salinus, Kelton Junction, Box Elder Co., Utah). Fig. 21, state 5 (P. subdentatus, 18 mi S Ravendale, Lassen Co., California).
Fig. 22, state 5 (P. occidentalis, Ogden, Weber Co., Utah).



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19871 Shattuck- Pogonomyrmex occidentalis complex 169 Pogonomyrmex montanus was found to possess states 2 and 3 in equal proportions, a finding which agrees with the original description (MacKay, 1980).
The scape base of P. occidentalis varies from state 3 to state 5 (Table 1) but shows no geographic pattern, with the states occurring in all parts of the range in roughly equal proportions (Fig. 32). This agrees well with the observations of Cole (1968), although he did not recognize the extent of the variation.
All Scape states occur in P. subdentatus (Table 1). States 1 and 2, and 90% of the state 3 records occur in western Nevada; state 4 occurs equally in California and Nevada, and state 5 occurs prima- rily in California and Oregon, with a few (14%) records from Nevada (Fig. 33). Cole (1968) asserted that the scape base of P. subdentatus is equivalent to state 5, and distinct from that of P. salinus. This observation was apparently in error because some specimens found to have states 1 and 2 in this study were determined by Cole as P. subdentatus (specimens now in LACM).
Cole (1968) describes the scape base of P. salinus as "evenly and broadly rounded", while that of P. owyheei is "not evenly and broadly rounded." Cole's figures show the scape base of P. salinus as evenly rounded dorsally, without any trace of a basal flange, while that of P. owyheei has a distinct basal flange dorsally. Two aspects of this character are inconsistent with these statements. First, the geographic distributions of these character states are not concordant with the geographic distribution of the species supposed to possess them; and second, material determined by Cole (from LACM) does not always possess the appropriate states. The geographic distribution of these character states does not agree with the distribution of P. owyheei and P. salinus as stated by Cole (1968). Scape state 1 (indicative of P. salinus) occurs primarily in Nevada, but has also been found at Twin Falls, Montana; Red- mond, Cline Falls State Park, Summer Lake, and Hermiston, Oregon; and Burlington and Greybull, Wyoming (Fig. 34). Scape state 5 (indicative of P. owyheei) occurs in all areas expected except the northwestern portions of its range (Washington, British Colum- bia) where only Scape state 3 occurs. Scape state 5 occurs outside



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170 Psyche [vo~. 94
the putative range of P. owyheei at a site 3 miles southeast of Row- land, Nevada; at Bridgeport, California; and at several localities in Washington County, Utah. This would indicate that P. salinus extends north into central Oregon (Lake and Deschutes counties) with small populations farther north and east, while P. owyheei has populations in eastern California (Lassen and Mono counties) and southwestern Utah (Washington County).
The second inconsistency involves specimens determined by Cole. Numerous specimens with Scape state 1 were determined as P. owy- heei by Cole. These were collected in Nevada (Elko and Washoe counties), Oregon (Harney and Umatilla counties), and Wyoming (Big Horn County). Scape state 5 was found in two nest series determined as P. salinus by Cole from northern California (1 1.5 miles south of Termo and 18 miles south of Ravendale, Lassen County). These determinations show that Cole either allowed con- siderable variation in this character, or utilized other characters (alone or in combination) to identify these forms. The P. montanus collections show no geographic variation in this character.
SCAPE INDEX (SI)
The SI of P. occidentalis varies from 65 to 78, with a mean of 66.9. It shows a weak cline, with low values in the eastern part of the range (Colorado, Kansas) and high values in the western portions (Utah, Nevada) (Fig. 35).
Cole (1968) found the SI of P. owyheei to vary from 83.0 to 100.0, while that of P. salinus varied between 83.5 and 84.0. A random sample of material determined by Cole (from LACM) was mea- sured, and the SI varied from 69.0 to 79.0 (mean = 73.8, sd. = 2.20, n = 32) in P. owyheei and 70.0 to 84.0 (mean = 76.1, sd. = 3.18, n = 33) in P. salinus. The discrepancy between these measurements and those of Cole (1968) may be caused by differences in measure- ment techniques. Cole's range of SI values was greater for P. owyheei than for P. salinus, while material now available shows the opposite. This may be due to a small sample of P. salinus in Cole's study. Although the broad overlap in the ranges of SI values makes this character of little value in discriminating the taxa, the means are significantly different (t = 3.35, p = 0.0014).



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8 Shattuck-Pogonomyrrnex occidentalis complex 17 1 Figs. 23-26, distribution of character states. Fig. 23, Gaster states for P. occiden- talis. Fig. 24, Gaster states for P. salinus and P. owyheei. Fig. 25, PetDor states for P. occidentalis.
Fig. 26, PpDor states for P. occidentalis.