The Ants Chapter 3

CHAPTER 3. THE COLONY LIFE CYCLE

The ant colony is an almost exclusively female society with the males remaining in the nest only until the time of their first, and invariably fatal nuptial flight. Also, the entire activity of the colony can be said to be pivoted on the welfare of the queen. It is, to paraphrase Samuel Butler's remark about the chicken and egg, the procedure by which a queen is used to make more queens. Seen in yet another way, the colony life cycle can be fruitfully analyzed as an orchestration of energy investments, in which workers are multiplied until such time as it is profitable to convert part of the net yield into new queens and males. In some extreme species this maturation point comes with the accumulation of only a few tens of workers, which are organized by means of the simplest caste and communication systems. For example, the average size of a colony of the fungus-grower Apterostigma dentigerum producing queens and males is 35 (Forsythe, 1981). The rare Central American myrmicine Basiceros manni reaches maturity at 50 workers (Wilson and Hölldobler, 1986). In others it is not attained until the worker population reaches tens of thousands and develops complex caste and communication systems. The extreme examples are the army ants, whose colonies do not divide until the worker populations exceed hundreds of thousands or even a million (Raignier and Van Boven, 1955; Rettenmeyer, 1963a).

The life cycle of a particular species can be viewed as the story of how the maturation point is attained with maximum combined speed and freedom from risk. Only by studying it as one strategy out of a great many possible can we expect to understand more deeply the way that a given species has adapted by social means to the particular environments in which it lives.

Stages of colony growth
Like the life cycle of the individual ant itself, the life cycle of an ant colony can be conveniently divided into three parts (Oster and Wilson, 1978). The founding stage begins with the nuptial flight. The virgin queen departs from the nest in which she was reared, leaving behind her mother, who is the queen of the colony, and her sisters, who are either sterile workers or virgin reproductives like herself. She meets one or more males and is inseminated. The males soon die without returning home, while the queen finds a suitable nest site in the soil or plant material and constructs a first nest cell. Here she rears the first brood of workers, drawing on her own tissue reserves to produce eggs and feed the growing larvae. Soon after reaching the adult stage, the workers take over the task of foraging, nest enlargement, and brood care, so that the queen is now permitted to confine herself to egg-laying. Over the coming weeks and months the population of workers grows, the average size of the workers increases, and new physical castes are sometimes added. The colony is now in the ergonomic stage: its activities are exclusively "ergonomic" in the sense that they are concerned with work devoted to colony growth, rather than with colony-level reproduction or dispersal (Figure Fig 3-1). After a period that ranges according to species from a single warm season to five or more years, the colony begins to produce new queens and males (reproductive stage). The sexual forms go forth to start new colonies, and the new colony life cycle has begun. As depicted in Figure 3-2, colonies of all known ant species are perennial. Like flowering plants, they issue a crop of seeds, then return to an interval of purely vegetative (i.e., worker) growth.

Substantial variation has been elaborated out of this elementary theme, especially with reference to details in the mode of colony founding and the number of egg-laying queens that coexist during the several stages of the life cycle. Figure 3-3 presents a classification of the variations and the relevant terminology. Monogyny refers simply to the possession by a colony of a single queen, as opposed to polygyny, which is the possession of multiple queens. The founding of a colony by a single queen is referred to as haplometrosis; when multiple queens start a colony the condition is called pleometrosis. The term metrosis refers generally to this biological variable. Monogyny can be primary, meaning that the single queen is also the foundress; or it can be secondary, meaning that multiple queens start a colony pleometrotically but only one survives. In a symmetric fashion, polygyny can be primary, in which multiple queens persist from a pleometrotic association, or secondary, in which the colony is started by a single queen and supernumerary queens are added later by adoption or fusion with other colonies. (The patterns of queen numbers will be discussed in greater detail in Chapter 6.)

Next, the mode of colony founding is subject to complicated variation among species. It can be accomplished by swarming, a process also called budding, hesmosis, or sociotomy, in which two or more forces of workers separate in the company of queens. We prefer to divide swarming into two types: the more common budding, in which a group of workers departs from the main nest with one or more queens and starts a new nesting unit; and fission of the kind used by army ants, in which queenright portions of the colony separate from each other and go their own way. Colony founding in ants is frequently claustral, meaning that the queen seals herself off in a chamber and rears the first brood in isolation. This is in fact the prevailing mode of independent colony formation in ants. However, the queens of such very primitive forms as Amblyopone and Myrmecia, as well as some more advanced ponerine genera, still forage outside their cells for food, the condition known as partially claustral colony founding (Wheeler, 1933b; Haskins and Haskins, 1950a,b, 1951). The same behavior has been observed in the myrmicines Acromyrmex, Manica, and some species of Pogonomyrmex, and the formicine Cataglyphis (Cordero, 1963; Le Masne and Bonavita, 1967; Fridman and Avital, 1983; Hölldobler, unpublished observations).

Nuptial flights and mating
The vast majority of virgin queens dies within hours after leaving the mother nest. Most are destroyed by predators (Figure 3-4) and hostile workers of alien nests, with the others being variously drowned, overheated, and desiccated. In species with large nest populations, such as the leafcutter ants (Atta) and fire ants (Solenopsis), it is not uncommon for one colony to release hundreds or thousands of the young winged queens in less than an hour. If the surrounding area is dominated by stable, mature colonies, only one or two of the queens might become the progenetrices of new colonies. Most of the rest will die before they can construct a first shelter--or even before they can find a mate. In an unusual study of its kind, Whitcomb et al. (1973) have produced a catalog of the many kinds of predators that decimate young queens of the red imported fire ant Solenopsis invicta. The few individuals that navigate all the dangers must also avoid breeding with males of other species, thus producing inviable or sterile offspring.

It follows that the brief interval between leaving the home nest and settling into a new, incipient nest is a period of intense natural selection among queens, a dangerous odyssey that must be precisely timed and executed in order to succeed. We should expect to find an array of physiological and behavioral mechanisms that enable the young queens simultaneously to avoid enemies, get to the right habitat on time in order to build a secure nest, and mate with males of the same species. Field studies have shown that such specialized traits exist in abundance.

As also expected from the evolutionary argument, mating patterns vary greatly from one species to the next. However, most of the patterns thus far studied fall into one or the other of two broad classes, or "syndromes" (Hölldobler and Bartz, 1985). In the first, the female-calling syndrome, the females, which are often wingless and sometimes just fertile workers, do not travel far from the nest. Standing on the ground or low vegetation, they release sex pheromones to "call" the winged males to them (Figure 3-5). This pattern is displayed by Amblyopone and Rhytidoponera, which are members of the phylogenetically primitive subfamily Ponerinae (Haskins, 1978); presumably also by the very primitive Nothomyrmecia macrops (Hölldobler and Taylor, 1983); at least one pseudomyrmecine, the Neotropical acacia ant Pseudomyrmex ferrugineus (Janzen, 1967); and the socially parasitic species of the myrmicine genera Doronomyrmex, Formicoxenus, Harpagoxenus, and Leptothorax (Buschinger, 1968a,b, 1971a,b, 1975b; see Figure 3-6). In general, the colonies of female-calling species are typically small at maturity, with 20 to 1,000 workers, and produce relatively few reproductives. So far as known the females mate only once. An unusual variation on this pattern is followed by the Florida harvester ant Pogonomyrmex badius. Females gather on the surface of their home nest and are inseminated by males; afterward they fly off to start new colonies. Van Pelt (1953) thought that the males came from the same nest as the females with whom they copulate, but S. D. Porter (personal communication) observed that they usually fly for about a quarter-hour first before settling on a nest different from their own. Porter observed one case in which a male mated with two females after alighting.

The second combination of traits during mating is the male-aggregation syndrome. Males from many colonies gather at specific mating sites, usually prominent features of the landscape such as sunflecked clearings, forest borders, hilltops, the crowns of trees, and even the tops of tall buildings. Sometimes, as in some species of Lasius and Solenopsis, the males cruise in large numbers at characteristic heights above the ground. The females fly into the swarms, often from long distances, in order to mate (see Figures 3-7 through 3-9 and Plate 2), and afterward they typically disperse widely before shedding their wings and excavating a nest. The winged queens and males of the fire ant Solenopsis invicta, for example, fly up to heights of 250 meters or more; 99 percent then descend to the ground within a 2-kilometer radius of their origin, while a very few travel as far as 10 kilometers. The ability of a single mature colony to disseminate fertile queens in many directions over long distances is one of the reasons the fire ant is so difficult to eradicate (Markin et al., 1971). Male-aggregation species typically differ from those utilizing female calling in two other key respects: the mature colonies are large, containing from several thousand to over a million workers and producing hundreds to thousands of reproductive adults yearly, and multiple insemination is common. An unusual reversal of the usual swarming procedure was recently discovered in some Pheidole species of the southwestern United States: the winged queens gather in aerial swarms, where they maintain a more or less uniform distance from each other while attracting males with pheromones. The males fly into the female swarms and mate with individual females (Hölldobler, unpublished). Swarms of variable composition, some predominantly male and others predominantly female (occasionally exclusively female), have been reported by Eberhard (1978) in the coccid-tending formicine Acropyga paramaribensis of northern South America.

Ant species can be classified another way into two broad types. When the males alight on the surface of the mating site, either in response to female calling or in swarms to compete directly with one another, they are often typically large and robust in form and possess well-developed mandibles. In contrast, males that gather in aerial swarms are usually (but not invariably) smaller relative to the queen than are males of the first type. Also, their mandibles are reduced in size and dentition, sometimes consisting of nothing more than vestigial lobate or strap-shaped organs. An example of this type is the small myrmicine Pheidole sitarches of the southwestern United States. Up to 50 males form circular swarms that hover from a few centimeters to two meters above the surface of woodland clearings. The virgin queens fly in slow, even circles through the aggregations until mounted in midair by a male, whereupon the pair cease flying and spiral to the ground together to complete the copulation (Wilson, 1957b).

The swarms of some ant species are among the more dramatic spectacles of the insect world. W. W. Froggatt (in Wheeler, 1916c) describes the flight of the giant Australian bulldog ant Myrmecia sanguinea as follows:

"On January 30th, after some very hot, stormy weather, while I was at Chevy Chase, near Armidale, N.S.W., I crossed the paddock and climbed to the top of Mt. Roul, an isolated, flat-topped, basaltic hill, which rises about 300 feet above the surrounding open, cleared country. The summit, about half an acre in extent, is covered with low "black-thorn" bushes (Bursaria spinifera).  I saw no signs of bull-dog ant nests till I reached the summit.  Then I was enveloped in a regular cloud of the great winged ants.  They were out in thousands and thousands, resting on the rocks and grass.  The air was full of them, but they were chiefly flying in great numbers about the bushes where the males were copulating with the females.  As soon as a male (and there were hundreds of males to every female) captured a female on a bush, other males surrounded the couple till there was a struggling mass of ants forming a ball as large as one's fist. Then something seemed to give way, the ball would fall to the ground and the ants would scatter. As many as half a dozen of these balls would keep forming on every little bush and this went on throughout the morning. I was a bit frightened at first but the ants took no notice of me, as the males were all so eager in their endeavors to seize the females."

Donisthorpe (1915) tells of the mass flights of the abundant Myrmica rubra from the distinctively British viewpoint of an earlier observer:

"Farren-White in 1876 observed a swarm of ants near Stonehouse rising and falling over a small beech tree. The effect of those in the air--gyrating and meeting each other in their course, as seen against the deep blue sky--reminded him of the little dodder, with its tiny clustered blossoms and its network of ramifying scarlet threads, over the gorse or heather at Bournemouth.  He noticed the swarm about thirty paces off, and it began to assume the appearance of curling smoke; at forty paces he could quite imagine the tree to be on fire.  At fifty paces the smoke had nearly vanished into thin air."

A still different mating pattern was described in the Australian formicine species Notoncus ectatommoides by W. L. Brown (1955a):

"In a cropped lawn at Montville, numerous small holes appeared, each opened by workers and accompanied by a minute pile of dark earthen particles. From these holes, males began to issue almost immediately in numbers, until within a few minutes there had accumulated on the surface a surprisingly large number of this sex and also a few workers.  The males travelled aimlessly over the sward in low, flitting flight from one blade of grass to another, never rising more than a foot or so from the ground.  Movement seemed to take place at random in all directions.  Suddenly, however, the males of one area all rushed simultaneously to a single focal point, which proved to be a winged female emerging from a small hole.  In a few seconds, the female was surrounded by a dense swarm of males in the form of a ball, which at times must have exceeded 2 cm in diameter.  This ball moved in a half-tumbling, half-dragging motion over and among the densely packed grass blades, and held together for perhaps 20 seconds, after which the female escaped, flying straight upward. She appeared not to be encumbered by a male, and no males were seen to follow her for more than a foot above the ground; she flew steadily, and soon passed out of sight.

Meanwhile, the lawn had become dotted with similar balls of frenzied males, each surrounding a female in a fashion similar to the first. Obviously, many more males than females were involved in this particular flight. On each occasion, the female left the ball after 20-30 seconds and flew straight upward."

In a similar fashion males and females of Formica obscuripes conduct nuptial swarms on the ground. Talbot (1972) observed them flying to "swarming grounds" near their nests which were maintained throughout the nuptial flight season and perhaps even from year to year. The males fly back and forth above the ground searching for females which "stand on grasses, forbs or bushes," and apparently signal their presence to the males by pheromones.

No encompassing theory exists to explain the extreme variation in the patterns of mating behavior so far observed. However, a close examination of individual species reveals details that clearly contribute to the greater success of the sexual castes. For example, flying queens of the formicine Lasius neoniger stay strictly within open fields, the exclusive habitat of the earthbound colonies. Fewer than one percent make the mistake of venturing into adjacent woodland, a habitat dominated by the otherwise closely similar Lasius alienus. In one experimental study (Wilson and Hunt, 1966), newly inseminated and flightless queens were labeled with radioactive material for easy tracking and displaced to woodland sites. They attempted to crawl out but were unable to do so. In other words the Lasius queens depend on controlled flight patterns to survive.

Like orientation, the timing of the flights is important for successful mating and colony foundation. Flights conducted as part of the female-calling syndrome do not appear to be well synchronized at the level of either the colony or the population of colonies. The search by airborne males for solitary calling females in fact resembles that of many solitary wasps (Buschinger, 1975; Haskins, 1978). In contrast, flights leading to male aggregation are tightly synchronized within the colony as well as among colonies of the same species.

The manner in which this coordination is achieved is typified by Pogonomyrmex harvester ants of the southwestern United States. The process has been described by Hölldobler (1976b). Just prior to take-off, males and females move restlessly in and out of the sandy crater nests or gather in clusters around the entrance, as shown in Figure 3-7. This preflight activity is especially pronounced in Pogonomyrmex maricopa, a morning flyer, the queens and males of which evidently need more time to warm up before taking wing. As the time of departure approaches, the reproductives run back and forth in mounting intensity. Now, in a frenzy, they climb up and down on grass leaves or small bushes around the nest. At this point many more workers pour out of the nest, running excitedly around the nest and attacking any moving object encountered (including the careless myrmecologist). When the first reproductives try to take flight, the workers at first delay many of them by pulling or carrying them back to the nest. However, once the flight is in full progress, workers cease to interfere. Although the timing of the take-off overlaps considerably between the two sexes, the males generally fly from the nest first. Once aloft both sexes appear at first to drift with the wind, but after a few seconds they take a course upwind or across the wind. Soon afterward they arrive at the swarm sites, centered on conspicuous landmarks such as tree crowns and the tops of hills or (in the case of Pogonomyrmex rugosus) merely flat local areas in the desert.

A similar marching order is observed by the carpenter ant Camponotus herculeanus, which nests in the trunks of both living and dead trees in the boreal forests of Eurasia and North America. Males leave before the queens, although the periods broadly overlap. The early departure of the winged forms is inhibited by the workers, who drag or carry many back to the nest entrance (Figure 3-10). However, when the males do succeed in taking flight, they discharge a pheromone from their mandibular gland. The concentration of this substance is highest at the peak of male activity--the gland emission can now be smelled readily by humans--enough to trigger the mass take-off of the females (Figure 3-11). Blum (1981b) reports methyl 6-methylsalicylate and mellein as two of the three components of the secretion. This pleasantly aromatic combination is shared by most other species of Camponotus, but considerable differentiation nevertheless is achieved by the addition of other substances, such as octanoic acid and methyl anthranilate, according to species (see also Lloyd et al., 1984). A similar function may be accomplished by vibrational signals rather than pheromones in Pogonomyrmex harvester ants. Both males and virgin queens stridulate just before and during take-off, running the sharp posterior rim of their postpetiole over the actively moving, striated file on the first gastric tergite (Markl et al., 1977).

Many entomologists, including especially Kannowski (1959a, 1963) and Weber (1972), have observed that each ant species, at least those displaying the male-aggregation syndrome, swarms at a precise time in the 24-hour diel cycle; and the time differs among species. Under controlled laboratory conditions, McCluskey (1958, 1965, 1967, 1974) and McCluskey and Soong (1979) demonstrated in fact that the rhythms of males are generally if not universally circadian and endogenous. Once set in a laboratory regime of 12 hours light alternating with 12 hours dark, the rhythms persist for up to a week in total darkness. They are also quite precise. McCluskey found that males of the harvester ant Messor (= Veromessor) andrei increase in movement during the last hour of darkness, then peak during the first hour of light. Throughout the remainder of the 24-hour cycle they are quiescent, usually stirring themselves only to groom, solicit food from the workers, or walk sluggishly about the nest. Males of the Argentine ant Iridomyrmex humilis, in contrast, are most active at the very end of the light period. Similarly distinctive rhythms, each spanning only one or two hours, have been documented by McCluskey and his co-workers across a wide diversity of species from four subfamilies (Ponerinae, Myrmicinae, Dolichoderinae, and Formicinae), including some that are wholly nocturnal.

Queens of at least two species, Pogonomyrmex californicus and Mesor (= Veromessor) pergandei, also display circadian rhythms, and these are more or less synchronous with those of the males (McCluskey, 1967; McCluskey and Carter, 1969). In the case of P. californicus at least, the rhythm persists even after the female has flown and lost her wings. But it ceases when she is mated.

In summary, the time of day in which flights occur is programmed by a species-specific diel rhythm. But what determines the particular day on which the flights occur? Several studies, including that by Boomsma and Leusink (1981), have shown that weather conditions play a major role in the timing of nuptial flights. One of the commonest triggering stimuli is rain, especially in species that occur in dry habitats such as deserts, grassland, and forest clearings. A typical species in this respect is Lasius neoniger, one of the most abundant ants in abandoned fields and other open environments in eastern North America. This small formicine emerges in immense swarms in the late afternoon in the second half of August or early September. The flights almost always occur within 24 hours after moderate or heavy rainfall on warm, humid days with little wind. For an hour or so the air seems filled with the winged ants. They rise from the ground like snowfall in reverse. After mating, the queens find themselves on moistened soil that is easier to excavate. They are also protected from desiccation due to overheating (Wilson, 1955a). A very similar pattern is followed by the North American leafcutter ant Atta texana, except that the flights occur well before dawn, between 0300 and 0415 hours (Moser, 1967a).

Because there are relatively few "best days" in which the young queens can be successfully launched, species belonging to the same genus are likely to swarm at the same time and location. In one respect this is a favorable result, since an apparent function of mass emergence and swarming in cicadas, termites, and other insects is the reduction of mortality by overloading predators (Wilson, 1975b). But in another respect it can be detrimental. In the tumult of the swarms, with males struggling to copulate with each female encountered, there is a strong likelihood of interspecific hybridization resulting in either sterility or the production of less viable hybrids. Applying the standard argument from natural selection theory, this circumstance favors the evolution of premating isolating mechanisms. The conventional explanation does seem compatible with a great deal of evidence. Species belonging to genera as phylogenetically diverse as Myrmecia, Pheidole, Solenopsis, and Lasius have been observed to conduct their nuptial flights within the major habitats occupied by the colonies, thus automatically avoiding sexual contact with closely related species limited to other major habitats. How widespread and efficient this isolating mechanism is among ants in general has not been determined. But it cannot be the sole device in deserts, savannas, and tropical moist forests, where large numbers of congeneric species nest closely together. To take an extreme case, in many forest localities in the Amazon Basin, thirty or more species of Pheidole can be found within a single plot of a few square kilometers. Another intrinsic isolating mechanism is differentiation in the preferred mating site within the major habitat. Among the sympatric species of Pogonomyrmex of Arizona, Pogonomyrmex desertorum and Pogonomyrmex maricopa congregate on bushes and trees, while Pogonomyrmex barbatus and Pogonomyrmex rugosus gather at different sites on the ground. In addition, males mark the sites with secretions from their mandibular glands, and apparently the females and other males are attracted by volatile pheromones contained in the material (Hölldobler, 1976b). It is possible (but not yet experimentally verified) that the pheromones are species-specific and serve as an additional isolating device.

Many congeneric species are further separated by the timing of their mating flight, either the season of the year or the hour of the day. In Figures 3-12 and 3-13 we have presented two sets of data from army ants that suggest just such a mutually repulsing spread of flight times across the seasons and the daily cycle respectively. The males of army ants, on which the data were based, fly for an unknown distance before entering the columns or bivouacs of alien colonies belonging to the same species. If the receptiveness of the workers is synchronized by the same circadian rhythm, even the hours of flight can serve as an effective barrier to "mistakes" and interspecific hybridization. Such staggering in the diel flight schedule appears to be common among ants. In Michigan, for example, Myrmica emeryana flies between 0600 and 0800 hours, Myrmica americana between 1230 and 1630 hours, and Myrmica fracticornis between 1800 and 1930 hours (Kannowski, 1959a). Similarly, in Arizona Pogonomyrmex maricopa flies between 1000 and 1130 hours, Pogonomyrmex barbatus between 1530 and 1700 hours, and Pogonomyrmex rugosus between 1630 and 1800 hours. As morning flyers, the Pogonomyrmex maricopa queens appear to be at some disadvantage. The heat of midday prevents them from beginning nest excavation for three or four hours, during which time they are subject to higher predation than the other species (Hölldobler, 1976b). Some of the most closely related European species of Leptothorax swarm at different times of the day; others come into contact, and occasionally hybridize (Plateaux, 1978, 1987).

Another potential advantage of synchronous nuptial swarming is the increase in the numbers of colonies participating and hence the degree of outbreeding. The sparse data on allozyme variation in ants collected so far indicates that outbreeding is indeed nearly total (Craig and Crozier, 1979; Pamilo and Varvio-Aho, 1979; Pearson, 1983; Ward, 1983a). Hence mating is either effectively at random, as demonstrated in experimental choice tests with Pogonomyrmex californicus by Mintzer (1982a), or disassortative, that is, directed away from nestmates.

The glandular sources of sex pheromones produced by female ants have been identified only for a few species. The reproductive females of Rhytidoponera metallica call males with a sex attractant from the pygidial gland, an intersegmental structure between the VIth and VIIth abdominal tergites (Hölldobler and Haskins, 1977). Although some of the contents of this gland have been chemically identified (Meinwald et al., 1983), the specific behavior-releasing components have not yet been established experimentally. In several myrmicine species glands associated with the sting apparatus have been pinpointed as the sources of female sex pheromones. Virgin queens release a male attracting pheromone from the poison gland in the myrmicines Xenomyrmex floridanus (Hölldobler, 1971); Harpagoxenus sublaevis (Buschinger, 1972a); Doronomyrmex kutteri and Doronomyrmex pacis (Buschinger, 1975b; see Figure 3-6); and Formicoxenus nitidulus (Buschinger, 1976a,b).

Buschinger (1972b) was also able to demonstrate that males of Doronomyrmex kutteri and D. pacis react to the other species' female sex pheromones, and that hybridization is possible in laboratory experiments. In the field, however, both species, which occur sympatrically, appear to be sexually isolated by different diel rhythms in mating activity. In general, specificity in sexual communication is consistent with phylogenetic relationships among the leptothoracines. The Canadian slavemaker Harpagoxenus canadensis shows the same mating behavior as the European H. sublaevis, and males of both species respond to the other species' female sex pheromones. Very similar sexual behavior and responses to sex pheromones have been described in several other social parasites of the "subgenus Mychothorax" of Leptothorax, whose hosts, like those of H. canadensis and H. sublaevis, also belong to the "subgenus Mychothorax." The same is true of at least some non-parasitic members of the subgenus. In fact, there appears to be no pheromone specificity among the Leptothorax species. In contrast, Protomognathus americanus males do not respond to H. canadensis or H. sublaevis pheromones. This anomaly suggests that P. americanus may be more closely related to its host of the "subgenus Leptothorax" than to the other genus of Harpagoxenus or the "subgenus Mychothorax" (Buschinger, 1975b, 1981; Buschinger and Alloway, 1979).

Poison gland secretions of Pogonomyrmex females also elicit attraction in males (Hölldobler, 1976b). In Monomorium pharaonis, on the other hand, the female sex pheromone is derived from the Dufour's gland and the bursa pouches (Hölldobler and Wüst, 1973).

Male ants are richly endowed with exocrine glands (Hölldobler and Engel-Siegel, 1982), but little is known about their function. One important fact, noted earlier, is that Camponotus herculeanus males discharge mandibular gland contents when departing from the nest that stimulate the virgin reproductive females to launch as well the nuptial flight. A variety of compounds of the mandibular gland secretions of several Camponotus species have been identified (for review see Blum, 1981b), but it is not yet clear which substance or combination of compounds elicits the behavior. Similarly, the males of Lasius neoniger discharge their mandibular gland contents sometime during the nuptial flight (Law et al., 1965), but the precise timing and function remain unknown.

Males of Pogonomyrmex discharge mandibular gland secretions when arriving at the mating sites. The collectively discharged pheromone appears to attract the virgin females to the lek (Hölldobler, 1976b). It is possible that in other species where males have well-developed mandibular glands and distinct blends of compounds, the secretions also function in promoting aggregation and competition. Examples include Lasius' and Acanthomyops (Law et al., 1965), Camponotus (Brand et al., 1973b,c; review by Blum, 1981), Calomyrmex (Brown and Moore, 1979), Myrmecocystus (review by Blum, 1981b), Tetramorium caespitum (Pasteels et al., 1980), and Polyrhachis doddi'' (Bellas and Hölldobler, 1985).

A hypothesis concerning a possible novel role of male pheromones in sexual selection in army ants has recently been proposed by Franks and Hölldobler (1987). A detailed morphological examination of the reproductives has shown a close resemblance of conspecific males and females. Males are remarkably queen-like. They are large and robust, and their long, cylindrical abdomens are partially filled with an impressive battery of exocrine glands similar in form and location to those of females. Because queens are flightless and never leave their colony, males must fly between colonies and run the gauntlet of the workers before they approach the queen. For this reason, the workers can choose which males will be admitted and which virgin queens will be inseminated by the males. Army ant workers might therefore be involved in a unique form of sexual selection in which they choose both the matriarch and patriarch of new colonies. If this interpretation is correct, males resemble queens not because they are deceitful mimics; instead, under the influence of sexual selection they have come to use the same channels of communication to demonstrate their potential fitness to the workers as those used by queens.

Worker involvement in sexual selection might not be restricted to the army ants. Wheeler (1910a) noted that males of Leptogenys elongata are also accepted into alien colonies to mate with the wingless ergatoid females, and Maschwitz and Mühlenberg (1975) observed that males run along permanent foraging trails of Leptogenys ocellifera, apparently in an attempt to find access to ergatoid females. It may therefore be significant that Hölldobler and Engel-Siegel (1982) discovered very large exocrine sternal glands in Leptogenys males. Some other ponerines have ergatoid queens and therefore are not likely to engage in ordinary nuptial swarms, including species of Diacamma, Dinoponera, Megaponera, and Ophthalmopone. Longhurst and Howse (1979a) observed that males of Megaponera foetens enter the nests of alien colonies, after utilizing recruitment pheromone trails laid by workers to guide them to the nest. No information is available, however, on the exocrine glandular system of Megaponera or for that matter most other ponerine genera. Males of Ophthalmopone berthoudi also enter strange nests after dispersal flights, but so far as known do not follow odor trails--O. berthoudi workers in fact forage in an exclusively solitary manner and hence are less likely to lay recruitment trails of any kind (Peeters and Crewe, 1986a, 1987).