Stenamma diecki

This widespread species lives in a wide variety of habitats and is extremely variable in form.

Identification
Snelling (1973) carefully explained how to identify these ants:

Over much of the western range it is sympatric with the equally tolerant and variable Stenamma occidentale.

Stenamma species can be difficult to tell apart. The following is modified from Snelling 1973. It discusses differences between Stenamma diecki and Stenamma occidentale and also highlights some of the problems in identifying Stenamma collections to species.

Stenamma diecki: the median lobe of the clypeus possesses a fine transverse carinula below the summit of the declivity between the two lateral carinulae; the median lobe, in profile has definite dorsal and anterior surfaces, the latter clearly "undercut" below; the sides of the pronotum are sharply longitudinally rugulose, with distinctly shiny interspaces; the mesopleura, although with numerous fine punctulae, is moderately shiny and with several distinct oblique rugulae; the upper half of the side of the propodeum bears several longitudinal rugulae; and the anterior face of the metanotal groove, in profile, is vertical or nearly so, the groove flat along the length.

Stenamma occidentale: the median clypeal lobe lacks a clearly defined transverse carinula and in profile the lobe is evenly, strongly convex, without defined dorsal and anterior surfaces, the latter not sharply "undercut"; the sides of the pronotum are densely punctate, longitudinal rugulae largely effaced; the mesopleura barely shiny, densely punctate and without evident rugulae; the upper half of the propodeal sides closely punctate, dull and without defined longitudinal rugulae; the metanotal groove, in profile, either broadly V-shaped, or if long, the anterior slope oblique rather than vertical.

These differences between the two types, while perfectly obvious in many specimens, are subject to much variation and many specimens are thus difficult to correctly place. The variation of greatest significance is perhaps that of thoracic punctation. The sides of the pronotum are longitudinally rugulose in both species, the rugulae very fine and irregular in Stenamma occidentale, coarse and sharp and widely spaced in Stenamma diecki. The interspaces are, in Stenamma diecki, usually concave, smooth, shiny and without evident punctulae. Specimens do show up, however, which do have punctulae in these interspaces; the punctulae are fine, often visible only in oblique lighting, and most often separated by more than a puncture diameter. Those specimens which do have punctulate propleurae have the punctulae confined to the lower half or less and the punctulae do not obscure the decidedly shiny overall aspect of the segment. In those insects which I interpret to be Stenamma occidentale the entire propleura is closely punctulate and the punctures evenly distributed and the overall appearance is of a dull, or barely shiny, segment.

The mesopleura of Stenamma diecki is variable, but is usually more or less closely punctulate, varying from moderately to strongly shiny, dependent upon the density and depth of the punctulae. Although occasionally reduced or absent, rugulae are usually present and distinct. These rugulae may be strictly longitudinal or may extend obliquely from the upper anterior angle toward the lower posterior corner. In Stenamma occidentale the mesopleura is barely shiny, closely and deeply punctulate. Usually absent, occasionally one or two fine, indefinite longitudinal rugulae may be present.

The propodeal sides of Stenamma diecki always bear coarse longitudinal rugulae over the entire surface; these are sometimes reduced over the posterior portion of the upper half. The interspaces are punctulate, the punctulae most often fine and sparse, the surface moderately shiny. In this species, too, the basal face of the propodeum is definitely shiny, with fine, obscure, sparse punctulae. Sometimes fine transverse rugulae may be present and in a few specimens the basal face is reticulate-rugulose. Dense punctation on both lateral and basal faces is characteristic of Stenamma occidentale, the lateral face rarely with a few fine, obscure longitudinal rugulae on the sides at or below the level of the spiracular opening.

The profile of the metanotal groove is variable in both species, but in Stenamma diecki it is usually long. The posterior slope of the mesonotum, which sets off the groove anteriorly is usually vertical, or nearly so, as is the anterior slope of the basal face of the propodeum. In this species, the "floor" of the metanotal groove is, in profile as long as the groove is deep or longer. In Stenamma occidentale the groove is most commonly broadly V-shaped in profile, without a definite "floor." Some specimens have been seen in which this is not true; in these, however, the posterior slope of the mesonotum is strongly oblique, rather than vertical, and the anterior slope of the basal face of the propodeum is similarly inclined. In a few specimens the basal face of the propodeum almost entirely lacks the anterior slope and the mesonotal groove is thus very poorly delimited and is quite shallow.

Other differences exist between the workers of these species, but are too variable to be used reliably. Eyes tend to be larger in Stenamma diecki, the OMD correspondingly less; cephalic rugulae are usually coarser in Stenamma diecki; the pronotal humeri are usually more pronounced in Stenamma diecki and in this species the nodes of the petiole and postpetiole are usually shiny and occasionally lightly rugulose. These conditions are usually contrasted in Stenamma occidentale which is also, on the average, a smaller species.

Range
CANADA, USA, MEXICO.

Biology
Smith (1957) noted the following about this species:

This widely distributed species is highly adaptable to various types of ecological habitats. It can be found at altitudes at least as high as 8,000 feet. Stenamma diecki appears to be especially common in southern Canada and the most northern tier of our States. In southeastern United States (Virginia, North Carolina, and Tennessee) Stenamma diecki seems to be confined to rather high altitudes. Colonies nest in the soil and in rotting wood. Nests in the soil are usually under cover of stones, logs, humus, moss, etc. Colonies may be found nesting in boggy or swampy areas to rather dry areas and from unshaded conditions through open woods to dense woods. There appears to be only a single queen per colony. The largest colony that has come to my attention contained, on July 31, 97 workers, 98 worker pupae, 1 dealate female, 56 alate females, 28 males and 1 male pupa; the adults alone totaling 182 individuals. Data based on a number of colonies indicate that males and virgin females are produced from midsummer to early fall. Mating flights may take place from spring to fall. In the older colonies it is common for the males and virgin females produced during the current year to remain overwinter in the parental nest and take their nuptial flights in the spring or later the following year. Nests in the soil are usually shallow, seldom being more than a foot or so from the surface of the soil. There are only a few chambers. The workers are commonly of a timid disposition and on occasion will feign death when disturbed.

Cole (Tenn. Acad. Sci. J. 25:297, 1950) under the name impressum, gives an interesting account of the habitats and biology of Stenamma diecki: He studied 18 colonies found in red spruce-balsam fir forest near Newfound Gap, elevation 5,250 feet, in the Great Smoky Mountains National Park, Tennessee. The stands were not dense but the trees close enough to provide continuous dense shade; high soil moisture and low soil and atmospheric temperatures. Underbrush was exceedingly sparse but the ground was covered with decaying logs and a heavy carpet of moss. Each nest consisted of a small superficial chamber in the surface soil just beneath a stone or piece of wood. He found that when disturbed although the workers were sluggish they moved their small batches of brood to cover. Only a single egg mass was noted per nest and this mass contained from 16 to 32 eggs. Eight complete colonies consisting of 12 to 29 workers, a queen and brood were taken to ,the laboratory for further study. Males emerged from some of the colonies from August 7 to 19, but no females developed in any of the nests. Living collembolans and thysanurans were placed in the nest for food and their internal contents were devoured by the workers.

W. L. Brown and E. O. Wilson, who collected a number of colonies of Stenamma diecki in the White Mountains National Forest of Maine, made the following remarks in a letter to me,

"the Stenamma diecki colonies were taken in rotten wood and under stones at the summit of Evans Notch (a pass through the mountains) and on the east side in both dark beech woods and woods predominantly spruce-fir at somewhat lower altitudes. All localities were shaded and moist; pupae of sexes were present in many nests at this date (mid-August)and eclosed with full pigmentation a few days later in the laboratory. Other White Mountains collections were taken in mixed spruce-fir-birch forest in widely separated localities on the lower slopes on both sides of the Presidential Range. The colonies are quite abundant in many places, though inconspicuous. Few other ants are found in such situations, the most common being Formica subnuda Emery, F. neorufibarbis Emery, Camponotus herculeanus (L.) one or two Myrmica spp., and one or two less abundant species.

The Stenamma feign death for a short time but are quick and persistent at removing the brood to safety. The nest population of Stenamma diecki are estimated to average about 40 to 60 adult workers each in the White Mountains area. We have not been able to find nests of these or other Stenamma species at all in eastern Massachusetts though one can easily find stray workers under stones and in leaf litter during the months at the beginning of the season. We cannot account for the very striking difference in collecting between localities near Boston and those in the White Mountains.

At Evans Notch Brown and Wilson found larvae of Stenamma attached and actually feeding on a dipterous larva which was determined by W. W. Wirth as possibly an empidid. Under the same conditions but in another colony they found what they thought was a Tomocerus springtail."

Taxonomy
Stenamma westwoodi subsp. diecki Emery, 1895c: 300 (w.q.) CANADA. Smith, M.R. 1957b: 161 (m.); Wheeler, G.C. & Wheeler, J. 1953b: 50 (l.). Subspecies of brevicorne: Forel, 1901e: 347. Raised to species: Creighton, 1950a: 136. Senior synonym of impressum: Smith, M.R. 1957b: 158. See also: Snelling, 1973c: 18.

Etymology
Patrynom. The specimens Emery (1895) used to describe the species were collected by Dr. George Dieck.