Formicidae, Hymenoptera, Insecta, Arthropoda, Animalia
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Range
Currently known only from Costa Rica. Collections are known from midelevation wet forest sites in the Cordillera de Tilaran (Monteverde, Penas Blancas Valley, Laguna Arenal), Cordillera Volcanica Central (north of Volcan Barba), and Cordillera de Talamanca (Hitoy Cerere Biological Reserve).
Identification
Mesosomal dorsum with erect hairs; opening of propodeal spiracle viewed perpendicularly slit-shaped, more than twice as long as wide; mandible with approximately 13 teeth; dorsal outline of mesosoma interrupted by a distinct, impressed propodeal groove, so that the mesonotum forms a convexity separate from the more or less convex propodeal dorsum; in full-face view, a line drawn through the centers of the eyes falls near head mid-head, crossing near posterior limits of frontal carinae; cheeks with a distinct carina reaching from clypeal wing to eye margin; anterior face of petiolar node perpendicular, forming an angular anterodorsal summit; sides and posterior face of petiole smooth, not rugulose; anteromedian portion of clypeus transversely rugose.
Natural History
This species is a specialist inhabitant of Cecropia saplings. It is relatively common in Cecropia saplings in mature forest gaps.
Most Cecropia species are specialized ant-plants with a suite of adaptations associated with being inhabited by ants. These include hollow stems (internodes) that provide a nest site, thin spots in the stem wall (prostomata) that facilitate access to the nesting space, and dense pads of hairs (trichilia) that produce food bodies (muellerian bodies) on which the ants feed. The dominant plant-ants associated with Cecropia are members of the genus Azteca, but a number of species of Pachycondyla are also known to depend on Cecropia. For example, P. luteola is a specialized Cecropia ant known from Peru (Davidson et al. 1989, 1991, Davidson and Fisher 1991).
The following observations support the claim that P. insignis is an obligate plant-ant on Cecropia: (1) I have never collected or seen workers away from Cecropia; (2) I have collected founding queens in Cecropia internodes; (3) all nests I have observed have been inside Cecropia stems; (4) I have seen workers harvesting muellerian bodies, (5) I have found muellerian body caches inside the nests; and (6) workers and queens use the prostomata, rather than some other part of the stem wall, to gain entrance to the internodes.
Cecropia occupied by P. insignis are fairly easy to discern in the field, because the entrance hole is very large and circular, much larger than entrance holes made by Azteca. Other non-specialized species of Pachycondyla may be found nesting opportunistically in Cecropia stems (e.g. crenata, striatinodis), but their entrance holes are often more irregularly shaped, and often enter through an irregular hole or wound in the plant, rather than through the prostoma.
Among plant-ant species that are obligate inhabitants of Cecropia trees, all are not equal with respect to behavior and effect on the hostplant. Some species, such as Azteca constructor, Azteca xanthochroa, and Pachycondyla luteola, are highly aggressive defenders of hostplants. They swarm out at the slightest disturbance, readily attacking perceived enemies. In contrast, Azteca alfari, another obligate Cecropia ant, is less aggressive. Workers do not emerge as readily on disturbance, and their trees are often smaller, more vine covered, and more herbivore damaged than trees inhabited by the more aggressive species. However, large A. alfari colonies can be found in reproductively mature Cecropia trees. Pachycondyla insignis represents an extreme in the direction of reduced investment in the defense of a host tree. Colonies are small and exclusively inhabit small saplings. Workers are timid and run inside or flee on disturbance. Presumably the colonies are short-lived, because I have never seen a large colony in a mature Cecropia tree. Either the colonies are being replaced by Azteca colonies as the sapling grows, or the saplings are dying young, before reaching reproductive maturity. I think the latter is more likely, since the presence of a Pachycondyla colony probably inhibits colonization by Azteca queens.
Most ant-plant systems do not represent cases of co-cladogenesis, in which one lineage of ants becomes associated with a lineage of plants and speciates in tandem with plant speciation. Instead, ant-plant systems often appear to be the result of multiple independent colonizations of a plant lineage by diverse ant lineages. Pachycondyla insignis may represent an independent colonization of Cecropia. Pachycondyla insignis is nearly indistinguishable from P. villosa and P. bugabensis, species that are generalized foragers and opportunistic cavity nesters. Pachycondyla insignis differs only in having transverse rugae on the clypeus. Perhaps the rugae function in the excavation of the prostoma; alternatively they could be involved in the gripping of muellerian bodies. Pachycondyla insignis is a local endemic which could have evolved through a sympatric speciation mechanism from P. bugabensis, P. villosa, or some ancestral version of them.
I have made the following collections/observations of P. insignis:
Hitoy Cerere Biological Reserve
JTL0974: lone founding queen in an internode of a Cecropia insignis sapling.
16km N Volcan Barba
JTL1372: nest in internode of Cecropia insignis sapling; entire nest contained 4 workers, 1 dealate queen.
North side Laguna Arenal
JTL1713, JTL1722: two nests observed in Cecropia saplings.
Rio Penas Blancas
JTL1591: Nest in 10 internodes of Cecropia insignis, most of colony collected; 33 adult workers and 1 dealate queen.
JTL1604-s: Lone foundresses in Cecropia insignis internodes.
JTL1619: Nest in internodes of Cecropia insignis. Two saplings in a clump had fallen over where the bank had slumped into a small creek. Parts of the saplings were dead or dying. Ants were in live portions.
JTL2030: Lone queen in C. insignis sapling internode.
JTL2524: a small Cecropia insignis tree on the riverbank had multiple stems sprouting from a thick base. A nest occupied a contiguous series of 22 internodes. The nest contained 15 adult workers, 2 adult males, 13 cocoons, and a few larvae and eggs. I did not find an adult queen. I opened the cocoons and found 2 with adult males, 5 with male pupae/pharate adults, 3 with prepupae, and 3 with unrecognizable goop. There were no muellerian body caches. There was a single large opening at the top, through the prostoma. Every other internode had a scar over the prostoma, of the same size as the opening at the top. It appears that the Pacycondyla continually open a new entrance in each successive internode. The septa each had a single large circular hole 6-7mm wide. The entrance hole was elliptical, 5.5x3mm. The colony was concentrated in the third and fourth internodes down. The lower internodes contained frass and old cocoon shells.
Carrillo, Braulio Carrillo National Park
JTL2098: Lone founding queen with brood; in single internode of Cecropia insignis sapling. No muellerian bodies in internode, although there was a large open entry hole and abundant mullerian bodies on trichilia.
Casa Plastico, near Rara Avis
JTL2243-s: In C. insignis sapling along road through tall forest.
JTL2293: Lone queen in internode of Cecropia insignis sapling.
JTL2295: Nest in Cecropia obtusifolia sapling. No cache of food bodies present; ants occupying area of former scarab damage, very wet and messy inside.
Monteverde
JTL2570: colony in 237cm tall Cecropia obtusifolia sapling; in scrubby second growth vegetation. I first observed this colony on 28 May 1989. At 0700hrs I passed the sapling, tapped it, and noted no response. At 1945hrs, I passed it again, and observed a worker take a muellerian body. It kept working at it, leaving and then returning. It finally worked it free and took it into the entrance hole. Two other workers were out on the shoot tip, walking from trichilium to trichilium. On 11 June I collected the top 147cm of the sapling. When collected, several workers were out on the shoot tip. Their response to the disturbance was to immediately go into the stem. They did not emerge while I walked to the house with the entire cut stem. Brood and muellerian bodies were intermixed in the active nest space. The entire adult population was 35 workers and 1 dealate queen. There was a single large entrance hole near top, over the prostoma, and numerous abandoned, grown-over entrance holes below. The internal septa, which separate internodes, were also perforated in the active nest space. Each septum had a central hole similar in size and shape to the entrance hole.
Detailed internode contents, starting at top:
1. unoccupied, still sealed.
2. workers, 1 cocoon.
3. workers.
4. workers, 4mm diameter round hole over prostoma (entrance to nest).
5. 1 egg, 2 larvae, muellerian bodies.
6. 1 larva, 1 egg, muellerian bodies.
7. 8 eggs, 2 larvae, muellerian bodies.
8. empty, but part of active nest space.
9. pile of brown, chewed pith with large larva just spinning a cocoon; larva entirely embedded in pith pile.
10. 6 eggs, 3 larvae, muellerian bodies; possible woodpecker damage; there was a hole, fairly fresh, which had been plugged from the inside with a wet, slimy mass of the chewed pith material.
11. 25 eggs, 6 larvae, muellerian bodies.
12. 1 egg.
13. similar damage to internode 10.
14. similar damage.
15. 10 eggs, 1 larva, 1 cocoon, muellerian bodies; similar damage.
16. 1 larva, 1 cocoon; similar damage.
17. 3 larvae (one spinning cocoon in pith pile), 2 cocoons; similar damage.
18. 1 egg, 2 larvae, 4 cocoons; wet pith pile.
19. 4 larvae, 3 cocoons; similar damage.
20. 1 larva.
21. 2 larvae.
22-26. empty but with perforated septa, part of active nest space.
27-32. with perforated septa; filled with old chewed pith, frass, old cocoon shells, old insect parts, including dead Azteca xanthochroa queens. This appeared to be the bottom of the active nest space.
33. similar damage; water-filled; septa entire, without holes.
34. sealed; 2 dead Azteca constructor queens.
35. sealed; remains of a black Azteca queen.
36. completely water-filled; looks like a former Pachycondyla entrance hole, but now nearly completely filled in, with only 1mm diameter hole remaining.
37. weevil larvae; frass.
38. frass.
Internodes 12, 16, 17, 19, 20, 21, 22, 23, 26, 32, 34, 36, 38 had former exit holes over prostomata, grown closed with callous tissue.
The internodes below 38 showed no signs of previous Pachycondyla occupation. There were some sealed internodes with dead Azteca constructor and xanthochroa queens. Some contained weevil larvae and frass, and some had round weevil exit holes.
Internodes 1-26, the main active space of the nest, together were 61cm long. Internodes 27-38 were 46cm long. Internodes 39-56 were 40cm long. The bottom 90cm of stem was left in the field; not dissected. Thus the sapling was 237cm long. The sapling had 6 leaves total.
Comments
Pachycondyla insignis was a Bill Brown manuscript name, based on a full nest series I sent him in 1988. Mackay and Mackay (2010) formally described it, from material Brian Fisher collected in Pe–as Blancas.
Literature Cited
Davidson, D.W., R.R. Snelling and J. T. Longino. 1989. Competition among ants for myrmecophytes and the significance of plant trichomes. Biotropica 21(1): 64-73.
Davidson, D. W., R. Foster, R. R. Snelling and P. W. Lozada (1991) Variable composition of some tropical ant-plant symbioses. In P. W. Price et al. (eds.), Herbivory: Tropical and Temperate Perspectives. John Wiley and Sons, New York, pp. 145-162.
Davidson, D. W. and B. L. Fisher (1991) Symbiosis of ants with Cecropia as a function of light regime. In C.R. Huxley and D. Cutler (eds.), Ant- Plant Interactions, Oxford Univ. Press, pp. 289-309.
Mackay, W. P., and E. E. Mackay 2010. The Systematics and Biology of the New World Ants of the Genus Pachycondyla (Hymenoptera: Formicidae). Edwin Mellon Press, Lewiston.
Page author:
John T. Longino, The Evergreen State College, Olympia WA 98505 USA.longinoj@evergreen.edu
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