Stenamma brevicorne

From Smith (1957): Stenamma brevicorne is not only one of the most widely distributed but the oldest and perhaps best known of our North American species of Stenamma. Although apparently preferring to nest in wooded areas, the species is also found in meadows and other places. It usually nests in moderately dense to dense woods of such composition as maple, mixed oaks, oak-maple, red and white oaks, and beech-maple. Here the ants more commonly nest in the soil under stones, logs, humus, moss or other debris, as well as in the rotting wood of logs and stumps. Colonies are small, consisting of only a few dozen to a hundred or so adult individuals. The largest that has come to my attention was found at Tiffin, Ohio, on August 21, 1948, by Talbot and Headley and was composed of 105 workers, 8 alate females, and 12 males. Although no one has made a detailed study of Stenamma brevicorne, random observations indicate that this species is timid and sluggish, subterranean or hypogaeic. The workers are undoubtedly carnivorous, but may also be predaceous. Wheeler, so far as I am aware, was the first individual to advance the theory that, in some colonies at least, winged females and males overwinter in the parental nest, these sexual castes having been produced from late summer to early fall. None of the evidence I have seen disproves this, but no doubt the age of the colony has a great deal to do with whether or not sexual castes can be produced. As the colonies are not large, it is likely that there is only one mother queen per colony. This also seems to be the case in many, if not most of our own species. The overwintering sexual castes apparently start emerging from the parental colony in spring or early summer of the following year. It is believed that it is then that mating and the forming of new colonies are started. H. V. Weems, Jr., captured a copulating male and female on May 14, 1950. Since alate females have been captured at light traps and males taken while flying at dusk, it appears that both castes fly freely at night, like many other ants. Although altitudunal records are lacking for Stenamma brevicorne, it appears that this form lives in low lands or areas of only moderately high elevation.

Identification
The worker of Stenamma brevicorne may be distinguished by its moderately large eye, which usually bears 8-10 ommatidia in its greatest diameter; the strong mesoepinoraI impression which is frequently twice as long as deep; base of epinotum usually with a transverse welt following the mesoepinorai impression; the distinct spines which are approximately one-fourth to one-third the length of the base of the epinotum; petiolar node subconical, when viewed from behind; postpetiolar node, from above, stout, as broad as, or broader than long; head, thorax, petiole and postpetiole subopaque; promesonotum largely coarsely longitudinaIly rugulose; body usually brown with the gaster light at the base and apex and darker medianly.

Smith 1957

Distribution
This taxon was described from the United States.

Range
CANADA, USA. From Nova Scotia, Quebec, and Ontario south to Tennessee and west to Nebraska and Minnesota.

Missouri
Talbot (1965) offers the following observations about Stenamma brevicorne from Missouri:

Stenamma brevicorne, together with Lasius flavus, seemed the most typical ant of the low field habitat. It was common and widespread here (23 colonies on 18 plots), but had been collected only once before on the Reserve (at swamp edge). Three other species of Stenamma (S. impar, S. diecki, S. schmitti) present on the Reserve did not live in the field.

Stenamma brevicorne workers foraged over the ground in cool damp weather, and the finding of workers just below the surface indicates that they also foraged below ground in the crumbly topsoil. Nest entrances were simply small openings hidden among leaves and grasses, and were not found unless workers were seen entering them. An attempt was made to collect whole colonies; therefore nests were dug completely when possible. This was not always a successful process, but the general pattern of nest structure seemed clear. Usually there was a chamber within 2 inches of the surface, which might be empty during dry weather. Some nests appeared to have no chambers in the crumbly surface soil but started with a chamber 4 inches down in more solid clay. No colony had more than 4 chambers, and most had only 2. Lowest chambers ranged in depth from 3 to 13 inches, with most at 6 inches. Large colonies dug down to soil moisture, while small colonies did not. Chambers were surprisingly large and were often only partly filled with brood. Those measured ranged from 1/2 X 1/2X 1/4 in. to 1 1/2 X 1 1/2 X 1 in. with most being larger than an inch in diameter and a half inch high. The range of size of the 10 most complete population counts is recorded in Table II. Eggs, larvae and worker pupae were present from before June 23, when the first colonies were dug, through the rest of the summer, although the September 10 colony had only one worker pupa. Twelve colonies which had winged adults or winged pupae averaged 20.6 alates per colony, of which 11.2 were females and 9.4 were males. Winged pupae were found in colonies from July 9 to August 11, and adults from August 10 to September 10 when observations ceased. It is known that S. brevicorne has flights in the spring (Smith, 1957; Kannowski, 1958). It may also fly in the late fall when conditions are right.

This species generally inhabits open areas, with either a soil nest that opens underneath an object resting on the ground or within the litter horizon in debri or other objects where they can aggregate within chamber like structures.

Reproduction
Kannowski (1958) offers these detailed observations on a mating flight:

On the afternoon of May 2, 1955, I observed swarming of Stenamma brevicorne (Mayr) alates near "Blanchard's Pond" on the outskirts of Ann Arbor, Michigan. The pond, about an acre in size, occupied a shallow depression surrounded by a field which had not been cultivated in the last five years. Around the periphery of the pond were isolated clumps of shrubs and small trees, of which willows (Salix spp.) and hawthorns (Crataegus spp.) were the most numerous. About 150 yards to the northeast of the pond was a large oak-hickory woodland ("Eber White Woods").

I discovered the swarms about 2:45 p.m. It was a cloudy and moderately warm day (about 70 F.) with very little wind. The swarms had formed at the tops of 8 to 12 foot high hawthorns and willows at the edge of the pond. The hawthorns were in bloom. When I first sighted the swarms from a distance, I assumed that they were composed of bees attracted to the pollen of the flowers. Closer examination revealed that they were alate ants. The alates were not in contact with the trees, but always remained slightly above the tree tops. About a dozen such swarms were found at the tops of these trees. Usually, there was only one swarm per tree top, but several large trees had two swarms each.

Each swarm was composed of 15 to 50 males. These swarms were observed for an hour and a half, but no females were seen. At 4:15 p.m. it was necessary for me to leave the area, and the observations were terminated. No new swarms were seen to form during the time of observation. Each swarm hovered in approximately the same position during this time. The size varied from swarm to swarm, but each occupied an area that was roughly cuboidal in shape and measured from one to two feet on a side. Movement of an entire swarm seemed restricted to a distance of about one foot in any direction. Within each swarm the males were in continual motion without moving in any set direction.

Stenamma brevicorne colonies seldom comprise more than 200-300 individuals. Probably not more than a few dozen alates are produced each season in a single colony. Thus, the large number of males in these swarms a total of at least 250 indicates that they came from a number of colonies. I attempted to find the sources of these alates without success. I had previously found this species nesting only in wooded areas in southeastern Michigan. However, parts of the field near the pond were cursorily examined for Stenamma nests, but none was found. I next examined a small area in Eber White Woods where I had previously collected S. brevicorne. Again I found no evidence of the species. Nevertheless, I believe that these alates most likely came from colonies in Eber White Woods and/or in other nearby woods.

The occurrence of exclusively male swarms is significant. Perhaps they developed because only males took part in flights on this day. The first flights of the season of some species of ants are mostly, if not exclusively, male ones. Possibly this is also true of S. brevicorne. On the other hand, both sexes may have taken part in the flights with the males forming the swarms first and the females being attracted later in the day (The only female of S. brevicorne that I have taken in flight was collected over a lawn at the Edwin S. George Reserve, Livingston County, Michigan, about 6:15 p.m. on May 30, 1956). If the latter hypothesis has any validity, it is necessary to account for the activities of the females from the time of flight until the time they join the swarms.

The swarms were observed in an environment quite distinct from that in which nests of this species were located in this region. Evidently the primary attraction for these alates was the top of a tree, not the major environment. Perhaps swarms form at the tops of trees in any local environment. They would, however, be most noticeable where the trees were small and scattered. Additional swarms may form over the trees in the woodlands, but, because of the height of the trees and the dense foliage, they would be difficult to observe. If the alates were attracted to the tree-tops as prominences, the fact that the trees surrounding the pond were isolated may be important. In a more or less open environment any tree-top would likely be a conspicuous promontory. In a woods probably only those individual trees which towered above the general canopy level would attract the alates.

Type Material
Described by Mayr from workers and alate females collected by Theodore Pergande from beneath a stone in a locality presumably near Washington, D.C.

The Gustav Mayr collection in the Naturhistorisches Museum in Vienna Austria. A cotype worker and female are in the U. S. National Museum as well as two workers and four females apparently belonging to the original series.

Smith 1957

Type Locality Information
Virginia. USA.

Etymology
Morphological. Presumably brevi L., short and corn L., horn, are a reference to the spines on the propodeum.

Additional References

 * Kannowski, P. B. 1958. Swarming of the ant Stenamma brevicornis (Mayr). Entomological News. 69:231-233.
 * Mayr, G. 1886. Die Formiciden der Vereinigten Staaten von Nordamerika. Verhandlungen der Kaiserlich-Königlichen Zoologisch-Botanischen Gesellschaft in Wien. 36:419-464.
 * Smith, M. R. 1957. Revision of the genus Stenamma Westwood in America north of Mexico (Hymenoptera, Formicidae). American Midland Naturalist. 57:133-174.
 * Talbot, M. 1965. Populations of ants in a low field. Insectes Sociaux. 12:19-47.
 * Wheeler, G. C. and J. Wheeler. 1986. The ants of Nevada. Natural History Museum of Los Angeles County, Los Angeles.