Wm. S. Creighton.
Additional Studies on Pseudomyrmex apache (Hymenoptera: Formicidae).
Psyche 61:9-15, 1954.

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ADDITIONAL STUDIES ON
PSEUDOMYRMEX APACHE
(HYMENOPTERA : FORMICIDAE)
BY WILLIAM S. CREIGHTON
Department of Biology, City College, New York When Pseudomyrmex apache was described in this Jour- nal in 1952 (I), the writer called attention to the fact that the incidence of this ant appeared to be greatest in the mountains of southeastern Arizona and that the incidence seemed to decrease sharply in areas south of that region. Additional field work in northern Mexico during the spring of 1953l has shown that this view is correct. Moreover, these studies indicate that the range of apache does not enter the tropics at all. This is a noteworthy distribution for a member of the genus Pseudomyrmex. The majority of the species in that genus are strictly confined to the Neo- tropical region and the few species which range into the southern United States are usually more abundant south of the Tropic of Cancer than north of it. The unique geo- graphical position of Ps. apache is, therefore, a matter of unusual interest.
The new records for apache are presented below, together with a map showing the known range of this species: TEXAS : Arsarca Canyon, Chinati Mts., (4800') one colony Quercus grisea.
NUEVO LEON : China (600') one colony in Prosopis jzdif lora. 'Field work done on a Guggenheim Fellowship. CHIHUAHUA: 3 miles south of Encinillas (4900') one col- ony in Q. emoryi; 16 miles west of General Trias (5800') two colonies, one in Q. emoryi, the other in a deciduous oak (sp?) ; 23 miles south of Parral (5500') one colony in Q. santaclarensis; 34 miles south of Par- ral (5800') one colony in Q. santaclarensis. DURANGO: Villa Ocampo (5700') one colony in Q. fusifor- mis or closely related species.
It should be noted that, except for the two colonies taken Psiffte 61:9- 15 11954). http //psychr mLclub.mg/fil/6l-WI him)



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west of General Trias, none of the stations in Chihuahua yielded more than a single colony. Many of the oak groves in Chihuahua, particularly those near the larger towns, have been denuded by wood-cutters. Despite this there are numerous areas where the groves are untouched. Those south of Parral are as extensive as any that the writer has encountered. Since apache will nest in at least three of the oaks present in this region, it follows that there are abund- ant nest sites available for it in central and southern Chi- huahua. Yet the incidence of apache in these groves is low. In many groves the writer failed to find any specimens of apache and the few colonies which were secured are the result of repeated visits to the station at which they were finally taken. It may be recalled that identical collecting procedure in the oak groves of southeastern Arizona often produced from three to ten colonies per station. The survey which gave the above records was carried south into Jalisco and Guanajuato and west through the Sierra Madre Occidental in Durango to the eastern border of Sinaloa. Except for the single record from Villa Ocampo, a small town five miles south of the Durango-Chihuahua border, no colonies of apache were taken south of the state of Chihuahua. The three southernmost records for apache, Villa Ocampo (Durango), China (Nueva Leon) and Monte Alto (Texas) are all near Latitude 26'. China lies about twenty-one miles to the south of the parallel, the other two stations lie a little to the
north of it. Hence, there is a
distance of at least one hundred and fifty miles between each of these stations and the Tropic of Cancer. The writer has repeatedly collected in the region between Latitude 26' and the Tropic of Cancer. The eighty-nine stations which have been visited extend from Tamaulipas through Nuevo Leon and southern Coahuila to the western border of Du- rango. Oaks were present at many of the stations and these oaks frequently contained arboreal ants. But the only record for apache coming from this region is the China record cited above. It is certain, therefore, that the incidence of apache south of Latitude 26' is even lower than it is in Chihuahua and it is probable that this ant is absent over much of the region between Latitude 26' and the tropics.




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19541 Creighton - Pseudomyrmex apache 11 There are now enough records to show that apache oc- curs in a comparatively narrow band of territory, about twelve hundred miles long, which extends northwestward from the mouth of the Rio Grande River to southern Cali- fornia. Because of the skew of this band to the northwest it is difficult to give satisfactory northern and southern limits for the range of apache. If only latitude is considered the range runs from Lat. 33' 25' to Lat. 25O 48', a north- south extent of approximately 512 miles. But this method of delimiting the range is confusing, for it leaves out of account the fact that at any point along the east-west axis the width of the range is much less than five hundred miles. Indeed, in most places the band seems to be no more than two hundred miles wide and its maximum width does not exceed 370 miles. The distribution of apache throughout this long, narrow band is not uniform. The figures below show the total number of stations and colonies in each of the states where apache occurs.
Stations Colonies
California 1 2
Arizona
11 39
Chihuahua
5 10
Texas
3 4
Nuevo Leon
1 1
Durango
1 1
The very marked abundance of apache in the region near the southeastern border of Arizona is even more striking when it is considered that half of the ten colonies secured in Chihuahua came from a station in the extreme north- western corner of the state (Nogales Ranch) which is only about thirty miles south of the U. S.-Mexico border. Since the most favorable part of the range of apache appears to be the region at the northern end of the Sierra Madre Occidental, it is instructive to consider the environmental conditions in this area.
The majority of the Arizona records for apache come from what Shreve (2) has called the "western xeric ever- green forest where oaks are dominant." This association is closely similar to LeSeur's (3) "santaclarensis consocia- tion" in Chihuahua. There seems to be no essential climatic difference in the two biomes, the principal distinction be-



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12 Psyche [March
ing the dominant oak involved. In Arizona this is Q. emoryi, in Chihuahua it is Q. santachrensis. Both these oak associations appear to reach their maximum develop- ment in areas where the average minimum annual temper- ature is not less than 15å¡F or more than 20å¡F In such
areas light winter snows are not uncommon and minimum January temperatures as low as - 6 O F . have been recorded at several weather stations (4). The average annual rain- fall in such areas is from 15 to 20 inches. Of this total more than half falls during the period from the first of July to the middle of September. Spring rains are excep- tionally light, seldom comprising more than 10% of the total annual rainfall. Over most of the year there is a difference of at least 40å¡F between the daily minimum and maximum temperatures. The humidity is low and the evaporation rate very high, since the area has an unusually large percentage of cloudless days.
Fig. 1. Map showing the known distribution of Pseudomyrmex apache in the southwestern United States and northern Mexico. It seems clear from the above data that apache can tol- erate lower temperatures than most of the other species of Pseudomyrmex which occur in the southern United States.



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19541 Creighton - Pseudomyrmex apache 13 For the range of Ps. elongata is confined to areas where the average annual minimum temperature is 30å¡F or more (southern Florida). That of Ps. gracilis mexicanum is limited to areas where the above temperature is 25OF. or more (southern Texas). Ps. brunnea is restricted to areas where the average annual minimum temperature does not go below 20å¡F. hence, while it occurs as far north as the Carolinas and thence south through the Gulf States into Mexico, it does not occur in southern Arizona. Only Ps. pallida, whose tolerance for low temperatures is equal to that of apache, does so, and pallida occurs through the Gulf States and north to the Carolinas.
The question immediately arises as to why, since apache can tolerate low temperatures as well as palla, does not apache occupy the same area as the latter species. The restricting factor in this case appears to be rainfall rather than temperature. Although apache can live in areas where the average annual rainfall is as low as ten inches, it has never been taken in an area where the annual average rain- fall is more than twenty-four inches. Beginning in west Texas (10 inches) and running east to Alabama (65 inches) there is a gradient of rainfall which increases to the east. It is interesting to note that the line which marks the area where the average annual rainfall passes 25 inches lies only a few miles north and east of the known eastern limit of the range of apache. It is further inter- esting to note that there is a comparable gradient of rain- fall which increases southward from the Rio Grande Val- ley down the coastal plain of Mexico. This gradient begins with 25 inches in the Brownsville area, rises to 45 inches at Tampico and reaches 64 inches at Vera Cruz. As has been noted elsewhere, apache appears to be absent in this area. This might be expected if the ant is unable to toler- ate an average annual rainfall in excess of 24 inches. If, as seems to be the case, it is rainfall rather than temperature which plays the major part in determining the range of apache, some interesting speculations can be advanced as to how apache reached its present geographical position. It may be taken as axiomatic that apache came to southern Arizona from tropical sources. Since three of the species of Pseudomyrmex which occur in the southern



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[March
United States also occur along the coastal plain of eastern Mexico, this region has served as a pathway for northern migration of some members of this genus. It is possible that apache might have come north by this route and reached southern Arizona by turning west up the Rio Grande Valley. But if this has been the case then the climatic conditions along the eastern coastal plain in Mexi- co must have been different from what they are now or apache must have acquired its low tolerance for annual rainfall after it turned west from the coastal plain. In either case it is difficult to see why apache should have stopped its migration along the coastal plain at the Rio Grande River. For, on either count, areas north of the Rio Grande along the Gulf Coast should have been available to it. It can be argued that subsequent climatic change elim- inated apache from the Gulf Coast except for the narrow strip of arid territory in the Rio Grande Valley area. The writer finds it difficult to believe that there would not be some traces of apache left in other parts of the Gulf Coast region under such circumstances. A much more acceptable explanation of the present range of apache can be made if it is assumed that the insect migrated north along the Mexican Plateau. The traces of this northern progress are present along the western edge 'of the Plateau, for the de- creasing incidence of apache south through Chihuahua may be regarded in this light. There is additional evidence from the responses of apache that it has had a long and extensive acquaintance with conditions on the Plateau. It has been shown elsewhere (1) that apache customarily nests in sizeable limbs or the trunk of the tree, and that it rarely, if ever, nests in hollow twigs as do many species of Pseudomyrmex. It has also been shown (5) that larger limbs, particularly those stubs which point upward, ac- cumulate much moisture after a rain fall. It seems clear that apache has lived under arid conditions long enough to have restricted its nesting habits to the parts of the tree which provide the maximum conservation of moisture. Coupled with this is the large tolerance of apache for vari- ous sorts of trees as nest sites. To date this ant has been taken from six species of evergreen oak, two species of deciduous oak and mesquite. It is interesting to note that



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19541 Creighton - Pseudomyrmex apache 15 there is a succession of different species of oaks from north to south along the Plateau and that these oaks by no means form a continuous belt throughout this region. There are many areas where the oaks are replaced by mesquite. It may be no more than a coincidence that apache accepts several species of 'oaks as well as mesquite as nest sites but, at least, this behavior is precisely what would be ex- pected if apache had migrated north along the Mexican Plateau.
LITERATURE CITED
Creighton, W. S., Psyche, Vol. 59, No. 4, p. 141, December, 1952. Shreve, F., in Kearney and Peebles, U.S.D.A. Misc. Pub. No. 423, May 1942.
LeSeur, H., Univ. Texas Publication No. 4521, June 1945. Smith, H. V., Univ. Ariz. Agric. Exp. Sta. Bull. 197, July 1945. Creighton. W. S.. Psyche, Val. 59, No. 4.. p. 161, December 1952.



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