THE DISTRIBUTION OF THE THREE FORMS† OF THE MEAT ANT IRIDOMYRMEX PURPUREUS (HYMENOPTERA: FORMICIDAE) IN AUSTRALIA

Distributions of the three nominal forms of the meat ant Iridomyrmex purpureus (Smith), I. viridiaeneus Viehmeyer and J. sanguineus Forel have been studied. Each ranges widely, and seems characteristically to be developed in areas within prescribed limits of rainfall and vegetation. The most common form, purpureus, is found over one third of the Australian mainland; it is sometimes found in association with the other two forms and in places it appears to occupy a buffer zone between areas where viridiaeneus or sanguineus are common. Examples of apparent character displacement are given, with discussion of how the variation in nest structure could enable colonies to survive extreme conditions. Evidence suggesting effects of soil, altitude, rainfall and vegetation in limiting the distribution of each form is discussed.


INTRODUCTION
The meat ant Iridomyrmex purpureus (Smith) ( = I .detecrus (Smith)) and its nominal forms I. viridiaeneus Viehmeyer, and I .sanguineus Forel form a distinct group within the genus Iridomyrmex.They are very common ants of the Australian mainland and each form ranges very widely.
Information on the distribution of the three forms has been extracted from collections made during expeditions organized by the Division of Entomology, C.S.I.R.O.during the period 1935-1967.Many of these expeditions were in sparsely populated areas where collections were made at 10-20 mile intervals or at any point of major change in vegetation.
The material collected, together with specimens loaned by the principal museums and research institutions in Australia,was examined in 1967 by Dr. G. Ettershank.
Meat ant colonies may occupy a single unit nest.the ants of which are associated with one or more shrubs or trees; or a multiple nest system, comprising several nests with a number of food trees within the foraging area.Colonies of thepurpureus form often occupy a large central nest with several smaller subsidiary satellite nests (Greaves 1939(Greaves , unpublished data 1970)).
All three forms find their maximum abundance in open woodland country, where exudates from sap-sucking insects provide the bulk of their food.
Although similar in size and shape, the three forms differ in colour and nest structure: purpureus has a red head, thorax and petiolar node, over which there is a blue metallic sheen, the gaster is bluish-purple, sanguineus is yellowish with a bluish-purple gaster, while viridiaeneus is uniformly metallic bluish to purplish-black.Specimens of viridiaeneus from very low rainfall areas sometimes have a greenish metallic sheen on the head.There is some colour variation in each form.

DISTRIBUTION
The distribution of the three forms is shown in Figure I. Purpureus is the most wide-ranging form and is most common in savannah woodland within the 14 in.to 30 in.isohyets; it can persist in much drier areas where surface water and food trees are available, for example situations along river banks, around station properties and in irrigated areas.
Viridiaeneus is usually found within the 9 in.to 14 in.isohyets and is rarely found in very dry areas with a rainfall of less than 9 in.In these drier areas one finds the t The term form is here used to denote certain recognizable taxa whose status has not yet been  T. GREAVES purpureus form along river banks, etc.A surprising discovery by a collecting expedition in 1949 to the west and north of Tibooburra, N.S.W. was the occurrence of colonies of purpureus along Coopers Creek at Nappamerrie.The northern limit of the viridiaeneus form is lat.22''s-just north of Alice Springs, N.T.
The sanguineus form is most abundant in open woodland within the coastal littoral area of tropical Australia and it occurs away from the coast where moisture permits.In the drier and cooler parts of the tropics sanguineus is replaced by ants of purpureus-type.These forms may occur together in the Queensland tropics around Clermont and between Cloncurry-Camooweal, and south of Port Hedland in north western Australia (see Fig. 1).
Between the 10 in.and 14 in.isohyets the two forms purpureus and viridiuenrus are often found together.While purpureus is found associated with the two other forms, viridiaeneus has not been found with sanguineus.It will be seen in Figure 1 that between the northern boundary occupied by viridiaeneus and the southern boundary where sanguineus predominates there is a corridor across most of the continent in which the purpureus form alone is found.
CHARACTER DISPLACEMENT Throughout its range, purpureus forms large nests.Where purpureus and viridiaeneus are found together the nests are different, those of purpurrus being large and several feet in diameter and each with numerous entrance holes, while those occupied by viridiaeneus-type ants are small and flattened with between two and four entrance holes, often connected with other nests (Figs. 2 and 3).
In areas of less than 14 inches rainfall and where thepurpureus form is not present, the viridiaeneus form may occupy large nests with 20 or more entrance holes in man-made situations, such as along the banks of table drains or in formed roadworks.Similarly, where the purpureus and sanguineus forms are found together.their nests are quite different, sanguineus having a nesting habit similar to viricliueneus ; T. GREAVES however, in the regions bordering the Gulf of Carpentaria where the sanguineus form alone is present, it constructs large nests several feet across, with a series of turrets, each having one large hole near the apex, as shown in Figure 4.

FACTORS LIMITING DISTRIBUTION (i) Soil
None of the three forms occurs in large areas of Australia with quartz sand soils, although the climate would seem to be suitable for their occurrence.Possibly the soil type is a limiting distributional factor.
All three forms are common on lateritic ridges, granite outcrops and clay formations that provide well drained sunny situations.

(ii) Altitude and rainfall
In southern New South Wales where the purpureus form is found, the meat ant does not occur above 3,000 ft, and at this altitude colonies are usually associated with Eucalyptus rubida Deane and Maiden.In the New England area of northern New South Wales ants of this form are found at slightly higher altitudes.The other two forms are confined to areas of relatively low relief.
The sanguineus form is found in coastal areas of tropical Australia with purpureus in the cooler, drier or elevated areas, particularly on the banks of inland rivers.
Within the 10 in.isohyet the viridiaeneus form is characteristic except where purpureus-type populations occur along river banks, watercourses etc. where there are eucalypts.
(iii) Floods Some types of nests built by purpureus colonies can survive complete immersion in flood water for several days (Figs.2F and G).In the Darling and Murray River basins this form is usually common on the alluvial flats containing black box (E.largflorens F. Muell.) or river red gum (E.camaldulensis Dehn.).These flats may be flooded for long periods, resulting in the death of all meat ant colonies in the flooded areas.The repopulation of these areas from colonies at the boundary of the flooded areas, may take a number of years.
Professor W. L. Brown (personal communication) has observed that I. purpureus builds large mounds, shaped as in Figure 2 E, F and G, in the yate (E.cornutu Labill.)swamps on the sandplain around Esperance, Western Australia : periodic flooding is undoubtedly a factor in this situation.
(iv) Efect of vegetation Trees and shrubs in open situations are essential to all three forms, nests are rarely found on open treeless areas; and meat ants cannot tolerate the closedcanopy conditions of wet sclerophyll and rainforest.In eastern New South Wales, the most common trees in many areas occu ied by purpureus-type populations are Hemipterous insects such as scale insects, psyllids, leaf hoppers, etc. infesting host trees exude sweet secretions and are attended by meat ants and these exudations are apparently an essential part of the ants' food.
Large areas of coastal New South Wales and Victoria, which were originally covered with wet sclerophyll and rain forest apparently contained no meat ants.An area of over 1,000 square miles was evidently virtually free of the species before clearing.On the north coast of New South Wales the meat ant was not found between latitudes 29'50' and 32"O' S, a distance of bver 150 miles.With the establishment of railways and roads, creating open situations through forests, the purpureus form has spread northward from latitude 32"O' S to 31 '50' S and it appears to be only a matter of time before this ant will spread over most cleared areas within the region.
On the south coast of New South Wales, purpureus-type meat ants occur in dry heath land, but not on the heavily timbered slopes.Meat ants do not occur in wet sclerophyll forests in Western Australia.yellow box (E.melliodora A. Cunn.) and B P akely's red gum (E.blakelyi Maiden).
In north Queensland the sanguineus form is absent from the wet sclerophyll and rain forests in the higher rainfall zone between Townsville and Cape York, but is invariably present in open woodland of drier areas.
Meat ants are not found nesting on dense pasture.The Canberra suburb of Turner was built on a subterranean clover pasture where meat ants were absent.Within ten years, after shrubs and trees had been planted, meat ant colonies (purpureus form) were numerous around houses.

DISCUSSION
One or other of the three forms of the meat ant is present over most of the Australian mainland and some islands off South Australia.It has not been recorded from Tasmania.
Each form has a wide distribution within its own environmental limits.The typical purpureus form is by far the most widespread, occurring over one third of the continent.It replaces the viridiaeneus form where moisture is available in otherwise dry areas, and the sanguineus form in the cooler, drier areas of tropical Australia.Nel (1965) reported studies on the water loss by purpureus-type meat ants and showed that without a ready source of moisture the ants cannot survive hot dry conditions.This suggests a further reason for the field relationship between meat ants and trees and shrubs.
In a recent paper, Greenslade (1970) records data on the nest structure and foraging activities of viridiaeneus at Coober Pedy, South Australia.
FIG. 2.-Variation in nest structure of meat ant (Iridomyrmex purpureus form) colonies.(A) Fairly flat nest in open savannah.(D) Nests higher in open forest, near Canberra, A.C.T. (E) Nest in granite areas of New England; soil shallow, most of the galleries are above ground (F, G) On the bank of the Ward River, near Tallwood, Queensland; nests built up in areas liable to

FIG. 3 . 0 I
FIG.3.-General pattern of nests of meat ants, .sanguineu.r form and viridicimeu.\form, cornprlslng several small nests sites each with 2-4 entrance holes and connected by well defined trails.Note trails to trees.