Dejean, A. et al. Arboreal ants build a trap to ambush and capture prey
Brief communication to Nature
Supplementary information
Methods
This study was conducted between 2001 and 2003 during both the rainy and dry seasons in the forest around the field station of Petit Saut, Sinnamary, French Guiana (5° 03' 39" N; 53° 02' 36" W) on Hirtella physophora (Chrysobalanaceae) individuals taller than 30 cm (small saplings frequently do not yet have domatia, or only a few). We cut one branch per plant whose domatia were decorticated later in the laboratory in order to harvest the ants. Voucher specimens of the ants were deposited in the Laboratório de Mirmecologia, Itabuna, Brazil.
We observed the construction of galleries on 12 H. physophora. We then sampled pieces of stems supporting the galleries at different stages of construction and analyzed them using scanning electron microscopy. As previously noted1 (p. 249), we recorded the existence of fungi in the A. decemarticulatus galleries. We sent voucher samples of these fungi to the Natural History Museum of Los Angeles County, and Don R. Reynolds confirmed our hypothesis that they belong to sooty moulds as described by Hughes2.
For the study of prey capture, we conducted field experiments on 15 H. physophora where, using forceps, we carefully planted prey so that the extremities of their legs just barely touched the galleries and then we released them, permitting us to verify the process of prey capture and to evaluate its duration according to the prey species and size.
Ant manipulation of the fungal mycelium
In order to verify the influence of A. decemarticulatus on the development of the fungi, we (a) observed 34 young seedlings, (b) raised nine H. physophora from seeds and (c) transplanted and raised in a greenhouse 15 others sheltering ants, and (d) we eliminated their associated ant colonies on five of the latter. These plants, safe from defoliators, then developed.
We also compared the A. decemarticulatus galleries to the very similar spongy-looking structures built by Allomerus octoarticulatus3 on 65.1% of 212 Cordia nodosa (Boraginaceae), another myrmecophyte of the Guianian understorey. Among the three H. physophora individuals that did not host A. decemarticulatus in the field (out of the 280 monitored) only the one that hosted A. octoarticulatus bore the mycelium of a fungus on the structures built by these ants. Finally, an identical fungal community was noted for the spongy-looking structures built by A. octoarticulatus on C. nodosa (N = 138) and never noted on C. nodosa that hosted other ant species (N = 74). These structures also serve as traps to capture flying prey (AD, pers. obs.).
For both A. decemarticulatus galleries and A. octoarticulatus structures, the end result is a composite material that is stiff and adheres well to the supporting stems thanks to the uncut trichomes (Supplementary fig. S1). These ant-built structures, wherein the cut trichomes are randomly intermingled and bound together by the mycelium of sooty moulds, are reminiscent of the case of the ant Lasius fugilinosus that grows an ascomycete in the walls of its carton nests4. Sooty moulds, common in the tropics, are frequently associated with plant trichomes, EFNs and honeydew-secreting Hemiptera2; in the present case, they develop from insect debris and probably the plant's EFN.
Weight of prey captured
The monogynous colonies harvested from ten 1.20 to 1.70m-tall H. physophora contained 1185±104 workers (m±SE) for a mean weight of 459±27 mg (brood included). Using a microbalance, we also weighed eight series of 50 A. decemarticulatus workers originating from different colonies, recording 0.225±0.021 mg per worker.
The weight of most prey captured by A. decemarticulatus was much heavier than that of the workers themselves. For example, the weight of 6-8 mm long winged termites, of 9 mg, was 40 times this of a single worker, while the heaviest prey, a 45 mm-long hairy caterpillar, weighted 405.2 mg or 1801 times the weight of a single worker and almost the weight of an entire A. decemarticulatus colony. Nevertheless, numerous fragile small insects were very light and so dismembered by the workers that they were unidentifiable, but mostly belonged to the Diptera.
1. Dumpert, K. The Social Biology of Ants (Pitman Advanced Publishing Program, Boston, London, Melbourne, 1981)
2. Hughes, S. J. Sooty moulds. Mycologia 68, 693-820 (1976)
3. Benson, W. W. in Amazonia (eds Prance, G. & Lovejoy, T. E.) 239-266 (Pergamon Press, New York, 1985)
4. Maschwitz, U. & Hölldobler, B. Der Kartonnestbau bei Lasius fugilinosus Latr. (Hym. Formicidae). Zeits. Vergl. Physiol. 66, 176-189 (1970)
Supplementary figure legend
Supplementary figure S1 Scanning electron microscopy of the different steps in the construction of the galleries. (a) The workers first cut a section of trichomes along the plant stem, creating a pathway. (b) Then, they use the cut trichomes to build the gallery’s vault (1); uncut, adjacent trichomes serve as pillars (2). (c) A dense network of the mycelium of sooty moulds (1) first develops around the holes punched throughout the gallery. (d) Close-up of this mycelium. (e) Structure of a gallery during construction: cut trichomes, randomly intermingled, are bound together with mycelium (scale bars: 0.2 mm in d; 1 mm all others).
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