E. A. Andrews.
Sequential Distribution of Formica exsectoides Forel.
Psyche 33:127-150, 1926.

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PSYCHE
VOL. XXXIII. DECEMBER No. (i
SEQUENTIAL DISTRIBUTION OF FORMICA EXSEC- TOIDES FOREL.
Johns Hopkins University
Observations here recorded, indicate that the mounds made by this ant arise and pass away in rhythms harmonious with phases of forestation.
The continued life of this species may indeed be dependent upon the migrations to new growths of trees. Trees that when young furnish food for the ant, when mature may cut off sun- light needed to make the mound a successful incubator for the young.
Thus in fifteen years there has been found a migration of mounds comparable to the moving on of some primitive peoples dependent upon newly cleared forests.
According to the book upon ants, and to other writings of that foremost student of these animals, Professor William Morton Wheeler; the most common mound building ant of North Amer- ica belongs in the species of Formica exsectoides Forel, and its mounds have been observed in Nova Scotia, Ontario, Maine, New Hampshire, Massachusetts, Connecticut, New York, New Jersey, Pennsylvania, Maryland, North Carolina, Georgia, Wis- consin, Illinois and Colorado, though there may be doubt as to all the extreme western forms being of the same species as the others.
In general these ants are spread along the Appalachian region and are to be looked for in hilly or mountainous regions where the land is wooded more or less. It is thus exceptional to find these mounds near sea level as Wheeler observed them in Staten Island and as I have seen a few in Massachusetts ten feet



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above low tide and but few hundred feet from open salt water and again near Baltimore in the region of Cowenton on the neck between Bird River and the Gunpowder at an elevation of much less than one hundred feet.
One important factor in restricting these mounds to hilly country may well be not so much the elevation as the probable association of good cultivation with level and lower lying land while in the hills there are greater opportunities for partly wooded regions, especially abandoned fields and clearings growing up with new woods and left comparatively free from live stock and human interferences, so that the colonies of ants may find both food and freedom from disturbance for long periods of years. The classical account of the mound building ants of America is that of the enthusiastic student of ants, the late Rev. Henry C. McCook, who camped out for a week in August 1876 amongst the ant communities one mile north-east of Hollidaysburg near Altoona, Pennsylvania, where there were some fifty acres of ant mounds on the southwest base of Bush mountain belonging to the Cambria Coal and Iron Co. This region of sandy, stoney soil grown up with open woods of oak and few pines was known to the people of the region as the "Ant City": and at the present date the trolley station there is labelled "Ant Hills." In this com- munity or city were no less than 1700 dwellings; 25 to 33 to the acre. In other neighboring regions: Warriors Mark and Pine Hill: there were in the former 30 to the acre (but some were abandoned and moss grown) and in the latter 1800 dwellings at, the rate of 30 to 59 per acre.
Not only were the mounds so numerous but some were of great size, 10-12 feet around the base and 2%-3 feet high. A photograph published by McCook shows a mound that was 25 feet around, 6 ft. 9 in. up the West front, 3 ft. 6 in. up the East, 4 ft. 4 in.
on the South and 4 ft. 3 in. down the North face. Another photograph represents a mound 24 inches in vertical height and a third one 32 in height.
At Warriors Mark and Pine Hill he records even larger mounds. A fine cone was 12 feet across over the top and 30 feet around the base. While the largest of all three thousand or more seen was 42 inches in vertical height and 58 feet around the base



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19261 Sequential Distribution of Formica exsectoides For el 1 29 while a line across its top measured 24 feet. This was one of a pair and the other measured 15 feet across the top and 47 feet about the base. Both these great structures were built up upon an old level charcoal hearth; and hence their age was limited. No such large accumulation of these vast works of mound- building ants seems to have been recorded elsewhere; but Wheeler figures a mound in New Jersey 1 m. x 3.25 meters and a mound of another species in Belgium as 2.15 x 9.8 meters. In Maryland the mounds made by this ant are not un- common in the wooded mountains of cent,ral and western coun- tries where the summer visitor often evicts them with little regard for any claims that might be set up by these red and black original inhabitants so that in time not only the demands of agriculture but the thoughtless aspects of enjoyment of nature may combine to exterminate the present mound builders. In eastern Maryland one may find the mounds here and there in Balt'imore County, near Baltimore, as along the hills West of the limestone valley in which lies Cockyesville, up the Beaver Dam Run, and north of Green Spring valley where a fine grass-grown mound was measured and photographed in 1906 by Professor Philip H. Friese, along the North Run, a mile north of Stevenson. As described in a letter of that date this mound was about thirty inches in height and a perfect cone except for rounded top and evidently owed its steepness to being covered with grasses, some of which were not represented among the grasses of that neighborhood. And as above noted there are quite a number of small medium mounds near the shore below the Piedmont Plateau, in the region near Cowenton. While most of these mounds stand alone or a few in a group, an unusually populous settlement of ant's mounds was found near Lutherville and Timonium, some nine miles north of Baltimore, by the late Professor Basil Sellers who called attention of the Baltimore Naturalists Field Club to this favorable place for study of these mound builders.
However these ants were earlier known to the late Professor Philip Uhler, sometime Associate in Natural History in the Johns Hopkins University and Provost of the Peabody Institute, who told me that in his boyhood, when he lived at LutherviUe,



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130 Psyche [~ecember
these ant mounds were less numerous but individually larger. That period was probably about 1850. This is in harmony with the existence in 1905 of a few very large circular regions in- dicating old mounds long since washed away. The region in which these ant mounds of Timonium occur is a large tract of deserted land of some 600 acres, roughly a mile on each side, bounded on the West by the York turnpike, on the East by a road leading out of the old Dulaney Valley and Sweet Air Turnpike, on the North by the "cinder" road and on the South by the open farming country of Long Quarter. This region is largely given up to young woods and used as wood lots, not pastured nor fenced for the most part, having been formerly used as source of bog-iron ore for the Ashland Iron Ore Co. of Ashland, Md. which left various large and small excavations abandoned at different dates down to 1888 when the last work was done. It is represented by diagram one, which shows the positions of the ant mounds, but with the size exaggerated. The land slopes gently from elevation of 400 feet in the northwest to elevations of 300 toward the southeast and two springs give rise to a little run which flows toward the Gunpowder. The soil is poor with gravel and at the south a small outcrop of crystalline limestone with two large excavations. The geo- logical formation is said to be Potomac or lower Cretaceous. A reconnaissance of this area made in December 1905 showed that the ant mounds were located in two regions, a larger "town" near the York road [above in the diagram] and a smaller "village" to the south [to the right and below in the diagram] separated from the larger settlement by a third of a mile of woods in which however a few faint indications of the former existence of large mounds suggested that at one time the two settlements might have been connected. Ants carried in the following summer from the mounds of the larger to the mounds of the smaller set- tlement did not seem to excite hostile responses but were im- mediately allowed to run into the mounds without being fought by the inhabitants. Ant,s taken April 14, 1906 from the "village" and put in a mound in "town" did not start up a fight, but the queens were seized and dragged along into holes in the strange mound. This may be taken as some indication that the ants in



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1926J Sequential Distribution of Formica exsectoides Forel 131 the two settlements were not of remotely different origin, but probably all of one general society.
In the larger town 157 mounds were enumerated of which 95 were occupied and 62 deserted while in the village only 27 mounds were found; of these 22 were occupied, and 5 deserted. The larger mounds were in the larger settlement, while the village had the appearance of being a more recent growth from immigration.
In size the mounds varied from 1 to 8 feet in diameter and 4 to 24 inches in vertical height.
Three general types of architectural effect were noted. Some mounds were free from vegetation and showed fine conical forms the resultant of the untiring efforts of the ants to carry up onto the mound mouthfulls of earth and bits of stick and dead leaf as well as large bits of stone and other objects to be found on the surface of the ground near the mound, counteracted more or less by the down rolling and washing of the materials in the usual process of denudation of hills of all sizes. When the sub- soil is red the mounds are red, when white, white, and again the collections of small sticks may give grey effects. A second type of mound common in grassy glades presents more abrupt sides and artificial, tower-like contours from the combination of the above factors complicated by the upgrowth of certain grasses and other plants, as peppergrass, which tend to holding the down-rolling materials in steep slopes. A third and rare aspect is that of the mound partly coated over with moss which makes the natural surface more resistant to denu- dation and tends to emphasize differences in slope between the faces of the mounds that do and do not support moss. While the typical mound is nearly circular many are much elongated as if made up of the fusion of two formerly separate cones: while others elongate down steep slopes of the ground. The census of the mounds of the entire region made in 1905 showed 184 mounds of which 117 were occupied and 67 deserted. A second census made by several students in 1920 recorded



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132 Psyche [ ~ecember
193, of which 182 were occupied and 10 or more desert'ed. The
impression, however, was that many mounds were smaller, that there were not as many great mounds as fifteen years before; and this is in harmony with t,he opinion of Professor Uhler that the mounds of 1850 or so had been larger and fewer than fifty years after that.
Diagram 1




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19261 Sequential Distribution of Formica exseetoides Forel 133 Actual survey showed that the many mounds were dis- tributed differently from the mounds of 1905. The following
table shows the changes in distribution: North Middle
South
Active Deserted
Active Deserted Active Deserted
1905 40 24
55 38 22 5
1920
33 9 65 1
84 1
Thus the deserted mounds of the north section had largely disappeared, and so had those of the middle region and of the southern region; however, the census of deserted mounds was not as accurate in 1920 as in 1905.
The increase in active mounds was most pronounced in the southern settlement or village where the number jumped up from 22 to 84, nearly a 400%) increase.
There was also an increase from 55 to 65 in the middle region and a diminution in the north from 40 to 33. The increase was greatest in the south, great in themiddle, and negative in the north.
The shift of population from the north and the occupation of new mounds in the middle and south is shown at a glance in the first diagram, which shows by dots the mounds that were active in 1905 and by x marks the mounds deserted in 1905, and by triangles the active mounds of 1920 and by squares the in- active mounds of 1920.
Many of the active and very many of the deserted mounds of 1905 were not extant at all in 1920. We see here that the more easterly mounds of the comn~unity had been abandoned, all the mounds of 1920 being to the west or to the south, with few ex- ceptions. None of the deserted mounds became reinhabited as far as ascertained. After 15 years but two of the deserted mounds were still discernible as traces as indicated by a square enclosing an x. In 1920 three of the active mounds of 1905 had been de- serted as indicated by squares enclosing dots. After 15 years, some 25 mounds still remained active as indicated by triangles around dots. But very many of the active mounds, shown by the plain triangles, were new developments not directly con- nected with the mounds of 1905.
The nearly pure constellations of these new mounds are,



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134 Psyche [~ecem ber
near the York Road in the middle region, called the flats, and the larger one in the south, village, region. The area embraced by the population in 1920 is no more extensive than in 1905 and the number of active mounds about the same, but there was a shift of relative location of many domiciles: the north losing, the middle and especially the south village gaining many new mounds.
There had been increase in density of housing: in the north and middle concentration had resulted from occupancy of about one fourth the former area.
Concentration is accompanied by
abandonment of some areas and migration into others but the
entire area is not abandoned nor fundamentally altered in its interior, but rather one edge fades away as the opposite advances. After fifteen years the northwest part is reorganized; there is recession all along the east and great protrusion in the middle west portion; while the south village expanded in all directions. In the town the centre of population moved to the west; destruction
of some suburbs being compensated for by new growths elsewhere.
Yet the entire occupied area has shrunken while holding about the same number of domiciles.
Before considering possible causes of these shifts in popula- tion, some details of change in the northwest region may be considered. Of 61 nests mapped in 1905 only 18 were found, of these 8 were still active and 10 dead; but three of these had been dead in 1905, thus of the 18 found, 8 had continued 15 years and 7 had failed in that time. Of the 61, 21 were apparently deserted in 1905.
Thus, of 40 active nests, fifteen years left 7 remaining as vestiges and 8 as still active. Also there were still
remaining vestiges of three that were dead in 1905. Table of amount of growth in 6 of the 8 that survived 15 years.
Mound No. Gain in Height In Diameter.
3 2 in. 34 in.
54 2 5
54 twin 8 18
5 8 12 50
59 3 12-36 N. S.
60 9 18-20 N. S.
-
Average 6 Average 23-27




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19261 Sequential Distribution of Formica exsectoides Forel 135 , The mounds that continued active through 15 years had grown variably, from 2 to 12 inches in height and from 5 to 50 inches in width. Each of the above eight mounds was a foot or more in height in 1905 and doubtless of some years' standing. Special attention should be called to the pair of mounds No. 54 on the edge of a gravel pit which were always conspicuous for dark color and coarse sandy surface due to the special en- vironment; there being hereabouts little but subsoil and a growth of false indigo which in the fall yielded dark blackish leaves that were collected on the nests to such an extent as to make these appear very dark. The hills in 1905 were small but not ap- parently young; one 9 by 24, the other 4 by 18 inches. In the whole 15 years the former grew to be 11 by 29 and the latter 12 by 36. They had grown almost to touch at the base and one had advanced a little over the edge of the cliff. From that period up to the present, these two mounds remain but little changed, being of very slow growth, apparently resulting from poor con- ditions of soil and of vegetation. '
In contrast, the mound No. 59 which was very large in 1905, being 2 feet by 5 feet, had in fifteen years grown to 32 inches by 96 inches, measuring 115 inches over its surface and with a circumference of 22 feet. This mound in the midst of Japanese honeysuckle has always been nearly clear of all but a little grass and the vine has stopped rather abruptly at the moat or clear area about the base, being restricted in growth by the ants of this very successful nest. This mound still continues, and though in the past few years it showed signs of weakness in lack of growth and poor upkeep it is greatly recovered in 1926. Many other cases were recorded showing marked indivi- duality in the mounds, not only in architecture and location but in longevity, rate and character of growth and decay. The individuality of each mound is a result of interaction of the environment and the special internal states of each com- munity, as failure or success depends upon both external and internal factors.
The mound is not only the abode of the adults but pre- eminently the incubator for the young and thus the means of securing the perpetuation of the race. The mound is not a tem-



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136 Psyche [~ecem ber
porary seasonal affair like the nest of a wasp but may be of very considerable permanence, one of the most enduring architectural results of insect communism. In considering possible reasons for differential successand failure as between mounds near to- gether, it must be born in mind that these ants are long-lived amongst insects. Ants were kept in captivity by Lubbock for five or six years as workers and up to fifteen years of age in the case of a female ant.
In nat'ure ants of this species lie dormant about four months of the year and it may be that thus they live longer than in cap- tivity; nevertheless it is probable that the mound may outlast the lives of the original builders and be possessed throughout the generations.
Some of the mounds mapped in 1905 were already large and remained active in 1920 and even in 1926. To attain that large size probably requires several or many years judging from the measurements elsewhere recorded (Andrews, Growth of Ant Mounds, Psyche, 32, 1925); so that a mound already large in 1905 may at this present writing be over thirty years old which is in harmony with the estimates of McCook as to the time that one mound may endure and in agreement with the fact that Fore1 had a prosperous mound of a related European ant under obser- vation for forty years. The final end of the existence of a mound may be like that of a human city, variable, complex and to be known only by detailed history-which has not as yet been written up to the last day of any ant mound. That the mound may persist longer than the original found- ers of the mound is probable also from a described habit of this ant to seize upon young female ants after swarming and to get them into the old mound in some cases so that many mothers of different ages are actually found in a well advanced mound. Hence the deficit of population from old age may be compensated for and the tribe or family be able to live on in the same old mound, if all goes well.
As to empty mounds: beside desertion by migration any one mound may lose its inhabitants either from internal or ex- ternal causes, (sufficient disease or epidemics are not yet known) but old age of the inhabitants would lead to an empty mound if



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19.261 Sequential Distribution of Formica exsectoides Forel 137 the above process of substitution of younger for older did not succeed in any one mound. Of external inimical factors there are many, such as man's culture of the field, his domesticated animals tramping on ants and the mounds, his direct hostile acts and the attacks from animals that feed on ants, as the skunk and the woodpecker, also adverse influences of mosses and other vegetation. In this Timonium region, the direct causes of the extinction of life in mounds are not at all known. On the other hand the differential dying off of many mounds in one part of the area and the coincident appearance of new mounds in another section of the area is a phenomenon that may be correlated with environmental factors. As far as known important enemies of the ants are largely absent from the whole area and as impotent in one as in another section of the region and the success or the failure of groups of mounds would seem more likely due to some factors that have changed slowly through the many years.
As in general Formica exsectoides is found where there are trees but not dense old forests, it may be regarded as a dependent upon certain stages of forestation.
In the north region of the diagram during the slow dis- appearance of so many mounds there was greater growth of the trees and increase in their age. In the middle region the inrush of new mounds in fifteen years has been accompanied by theup- springing of a new growth of young trees. This correlation of many new mounds with new trees and many empty mounds with old trees may well be significant. It is supported by such facts as: the vestiges of old mounds in the region between the ant "town and village" where the woods are dense and old; the failure of a mound transplanted to the large woods of "Homewood" and its better success when the ants migrated spontaneously to the open edge of the wood in Wyman Park; the great success of colonies in the Holidaysburg region where mining operations kept the forest cut down in part; the present flourishing of a large colony in the cut-over forests of the neighboring Warriors Mark. (Andrews, Ent. News, 1925)) and the observation that this ant in early spring is more active in regions of sunny exposure than in older woods.



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138 Psyche [~ecernber
If the trees and ants are interconnected possibly one or the other of the two basal factors of animal success may be involved; the gaining of food for energy of the individual ant or else the proper conditions for reproduction and the continuance of the species. One mound may not obtain food enough to keep up the depletion in population, while another mound may obtain a sur- plus of food and be able not only to maintain itself but actively to colonize the surrounding neighborhood with new and rapidly growing mounds.
Unfortunately little is known about the food of this par- ticular mound building ant. In this region it is observed that the ants climb the trees near their mounds and go out on the branches and'leaves; it is observed that they get honey dew from some kinds of aphids or plant lice and from the black leaf hopper Vanduzea arcuata Say. They are seen to drag various dead insects into their mounds.
It may be assumed that these ant.s depend greatly upon trees for their food supply which is partly at least carbohydrate in nature. In artificial formicaries many kinds of ants can be kept very long periods when fed chiefly sugar and water or honey. If it be granted that ants derive their energies from food supplied by trees, there may be an inverse ratio between the