A swarm raid is one of nature’s great spectacles. In tropical rainforests around the world, army ants march in groups by the thousands to overwhelm large solitary invertebrates, along with nests of termites, wasps, and other ants. They kill and dismember their prey and carry it back to their nest, where their hungry brood devours it. They are the ultimate social hunters, demonstrating the most fascinating collective behavior.
In Army Ants: Nature’s Ultimate Social Hunters we see how these insects play a crucial role in promoting and sustaining the biodiversity of tropical ecosystems. Through observations, stories, and stunning images, Daniel Kronauer brings these fascinating creatures to life. Army ants may be small, but their collective intelligence and impact on their environment are anything but. Here is an excerpt from the book about the traveling circus — one of the lesser known amazing things that army ants do is travel with a menagerie.
While either pretending to be one of the ants or being well protected against attacks certainly helps when crashing the party, army ant nest intruders face a challenge that the guests of other ants are hardly confronted with: joining the caravan when the circus hits the road. To mites, a group that is ripe with phoretic species and ectoparasites that attach to their host, this usually comes naturally. Social parasites that are derived from free-living ancestors, on the other hand, require novel adaptations to accomplish the feat.
Springtails and many beetles, especially the tiny featherwing beetles, employ the same general trick as the mites: they hitch a ride (e.g., Wasmann 1904; Wilson et al. 1954; Kistner 1966a, 1979; Witte et al. 2008). One example is Cephaloplectus mus, which is phoretic on host ants and prey items, both during emigrations and raids. The beetles often run back and forth across short distances in the center of ant columns, attempting to climb onto passersby. Phoresis is also common in clown beetles; one example is the peculiar strategy of the great bareback rider Nymphister kronaueri, as previously described. Nymphister kronaueri prefers workers of intermediate size; several other clown beetles ride along on Eciton soldiers, comfortably sitting on the underside of the ant’s head (Akre 1968; Tishechkin et al. 2017; reviewed in von Beeren and Tishechkin 2017).
Some inquilines are even actively picked up and carried by the ants. These species often have grasping notches or other modifications that serve as handlebars (Parker 2016). Trichotobia gracilis, for example, has abdominal trichomes that the ants hold on to when carrying the beetle in emigrations (Maruyama et al. 2009). When the aleocharine beetle Mimaenictus wilsoni — a Wasmannian mimic of its host, the Asian army ant Aenictus laeviceps — arrives at an obstacle in the emigration trail that it cannot pass, it makes its predicament known to the passing ants via touching them with its mouthparts. One of the alerted ants will soon pick up the beetle by its thick first antennal segments and carry it to the new bivouac (Kistner and Jacobson 1975; Maruyama et al. 2009). Finally, diapriid wasps of the genus Notoxopria have hornlike projections on the thorax that the ants can conveniently clutch (Kistner and Davis 1989). It almost seems like the ants have an easily exploited grasp reflex.
If you cannot catch an army ant taxi, you have to walk — or slither — on your own. One of my most memorable army ant–related encounters occurred during fieldwork at Mount Kenya with my colleague Caspar Schöning. We had been watching an emigration of the driver ant Dorylus molestus for a while when, suddenly, a blind snake, possibly Afrotyphlops lineolatus, passed by in the middle of the column (see also Loveridge 1944; Gotwald 1982, 1995; Schöning et al. 2005b; Broadley and Wallach 2009). The snake, which was about 30 centimeters long, simply seemed enormous compared with the ants and entirely out of place, giving the scene an almost surreal appearance (Figure 6.12). The blind snakes occasionally occurring with Dorylus driver ants have hardly been studied, but a similar, facultative association is found between North American Neivamyrmex army ants and the blind snake Leptotyphlops dulcis, which has repeatedly been observed in raiding columns (Watkins et al. 1967b). Under controlled laboratory conditions, the snakes even follow Neivamyrmex pheromone trails in the absence of ants (Watkins et al. 1967b, 1972; Kroll et al. 1971). When unnerved, however, the sudden movements of the snakes elicit aggression, and the snakes respond by discharging feces and a cloacal liquid to repel the ants (Gehlbach et al. 1968; Watkins et al. 1969). When I picked up the blind snake at Mount Kenya to collect it, it coiled up, and the ants, alerted by the frantic movements, immediately attacked it, easily cutting into its flesh with their mandibles. I had pulled back the myrmecophile’s cloak of invisibility.
The ability to follow army ant trails has also been suggested for amphisbaenians, fossorial, wormlike reptiles that facultatively feed on subterranean army ants (de Araújo Esteves et al. 2008). Many arthropod inquilines of African driver ants are likewise able to follow their hosts’ emigration trails. David Kistner, for example, collected an impressive 4,000 plus specimens of various types of myrmecophiles from a trail over which a Dorylus wilverthi colony had emigrated. These guests were able to pinpoint the path of the colony, even though ant traffic had ceased at least fourteen hours earlier. Because African driver ants alter the substrate over which they travel by clearing the path and building tunnels, their guests might use visual or tactile cues in addition to chemical cues when following abandoned trails (Kistner 1979).
That most army ant inquilines indeed rely on chemosensory cues, possibly the trail pheromones themselves, to follow or retrace the ants’ emigrations has been established in the guests of New World army ants (Rettenmeyer 1962a, 1963b; Akre and Rettenmeyer 1966, 1968; Torgerson and Akre 1969, 1970). A study that exposed a wide range of myrmecophiles to army ant scent trails in the laboratory, without the ants themselves being present, found that almost all of them followed the odorous signposts. Among others, these experiments included rove beetles of the genera Ecitophya, Ecitomorpha, Tetradonia, and Vatesus, clown beetles, featherwing beetles, millipedes, and the bristletail Trichatelura manni (Akre and Rettenmeyer 1968). Some myrmecophiles, such as Ecitomorpha and Vatesus beetles, even distinguished between trails of their host species and trails of other army ants (Akre and Rettenmeyer 1968). Inquiline scuttle flies, on the other hand, did not follow trails in the laboratory assay. They do, however, follow army ant trails in the field, and hundreds of flies can often still be seen hopping along long after ant traffic has ceased, suggesting that they do indeed use scent marks for orientation (Rettenmeyer and Akre 1968).
Some army ant guests employ a single strategy to keep up with their hosts. Tetradonia and Vatesus beetles, for example, always undertake the journey from bivouac to bivouac on foot, and Nymphister kronaueri has only ever been found riding on the ants. Other species, however, prefer to combine approaches. The clown beetle Euxenister caroli with its long legs often strides along, but will also climb on the ants or their cargo for a brief respite, while the featherwing beetle Cephaloplectus mus for the most part hitches a ride but sometimes walks for short distances (Akre 1968; von Beeren and Tishechkin 2017).
The adult army ant inquilines travel with their hosts, but their immature stages usually do not. In fact, for the vast majority of army ant guests, we do not know what their immature stages look like, let alone where they live. But here again, DNA barcoding is starting to shed some light. For example, using this technique, some unusual looking grubs found in the refuse deposits of Eciton burchellii were identified as the larvae of myrmecophilic clown beetles, and it seems likely that the immature stages of other inquilines might live in the refuse or soil around army ant bivouacs as well (Caterino and Tishechkin 2006). The only known case where immature stages regularly move with the ants are the larvae of the rove beetle genus Vatesus, which walk in the emigration columns of New World army ants just like the adults (Akre and Torgerson 1969; von Beeren et al. 2016b). In fact, the lifecycle of Vatesus beetles is synchronized with that of their phasic hosts. Whereas females have quiescent ovaries early in the nomadic phase, they carry large and fully developed eggs toward the end of that phase. These eggs are then laid during the early statary phase, and the Vatesus larvae hatch shortly before the colony becomes nomadic again, just in time to accompany their hosts on the next emigrations. As the nomadic phase proceeds, however, the larvae grow and, about halfway into the nomadic phase, disappear, probably to pupate and undergo metamorphosis in the soil (Akre and Torgerson 1969; von Beeren et al. 2016b). This means that even Vatesus beetles do not complete their entire lifecycle associated with the ants, and that the young adults have to find new host colonies, probably guided by olfactory cues.
Dispersal to new host colonies each generation is probably the case for the majority of army ant guests, even though a few might also be able to complete their development between subsequent emigrations (Akre and Rettenmeyer 1968; Rettenmeyer and Akre 1968; Torgerson and Akre 1969, 1970). Especially hemimetabolous inquilines, whose immature stages resemble the adults behaviorally, might not require a lot of downtime before they are ready to move. In the bristletail Trichatelura manni, for example, even the early instars join the ants on their emigrations (Torgerson and Akre 1969). Nevertheless, horizontal transmission — the dispersal of parasites between unrelated colonies — is certainly the most common strategy among army ant inquilines.