More arboreal black weaver ants – Polyrhachis dives and others

A guest post by Dr Mike Downes; his first, about a different species of black weaver ants, is here.

The subject of ant nests came up recently after an object of interest was handed in at the Museum of Tropical Queensland:

ant nest
Exhibit A

ant nest
The internal structure revealed by sectioning it, and a closer look at its outer texture.

The object was a nest of Polyrhachis dives, an arboreal weaver ant not known to occur south of Halifax. The great pity is that the donor’s name wasn’t recorded, nor the place of collection. Perhaps it came from up north, but maybe it was collected locally – wouldn’t we love to know, but I guess we never will.

Without collection data it is merely a curio in the eyes of science, hence dispensible. So, since the identity of the erstwhile occupants couldn’t be confirmed without evidence in the form of the occupants themselves or, fingers crossed, their identifiable remains, the nest was sectioned giving a look at its internal structure. Luckily this yielded three ants which tracked nicely to P. dives through Kohout‘s (2006) identification key.

black ant
Intact worker from the nest
black ant
Virgin queen, born and died in the nest

Unlike most Polyrhachis species which have little to say for themselves apart from their names, even those being almost invariably in pseudo-latin, P. dives enjoys at least ten pages of print about its biology, courtesy of Katsusuke Yamauchi and co-workers (Yamauchi et al. 1987). This account includes much about the nest, which can be just one brick in a long wall, so to speak, because a P. dives colony, in Japan at least, can extend immensely, boast thousands of nests and comprise a million workers.

The common green ant Oecophylla smaragdina, familiar to Queenslanders, structures its colonies in the same way, albeit on a more modest scale. As for the Polyrhachis ants of Queensland, their colony structures are unknown. It is not unlikely that the concept of a (queen-centred) colony becomes meaningless in most cases, with each individual nest having its own queen(s) and thus being a ‘colony’ within a ‘supercolony.’ Read ‘metacolony’ for that: ‘supercolony’ has all the wrong connotations.

P. dives also enjoys a thriving amateur enthusiast culture world-wide, and there are several videos of it on You Tube. One of them shows its nest with an internal structure similar to the nest pictured above:

Uploaded on Jan 15, 2012.
Nest von Polyrhachis dives. Erst kurz nachdem die Kolonie umgezogen ist. Irgendwann ist die Königin gestorben und darauf auch die Kolonie zu Grund gegangen. Das 2te Teil des Videos zeigt dann das leere Nest von innen. For those inclined, it is fun using the sequence of events in the video to get a rough translation of the German. Some keywords: die Königin (queen, as in der König for king); sterben (to die); der Grund (ground); leer (empty).

The nests of other species

Naturally, the Museum’s nest was compared with those of other species of arboreal weaver ants in the genus Polyrhachis, of which there are about 17 or 18 in Queensland and northern New South Wales.

The most studied and commonly observed of these is P. australis (nests pictured here), distributed from Townsville south to the border and widely known as the rattle ant, which seems to be an unfortunate misnomer because here in Townsville it doesn’t rattle (drum on its nest wall to frighten off intruders). No, the real rattlers are P. queenslandica and P. micans, once heard never forgotten, neither of which are likely to be seen south of the wet tropics (except for an isolated population of P. queenslandica in the Mount Elliot area). How P. australis got the gong as the rattle ant is a mystery. Either it rattles in Brisbane but not in Townsville, or there is another culprit, P. mackayi perhaps, that unknown to us is a closet rattler and has framed its congeneric cousin, or some other explanation. All suspects look alike among these Polyrhachis weaver ants, so an ID parade won’t solve it: this really is a plum job for a naturalist sleuth

The form of the P. dives nest is unique and would rightly be called an ‘extended phenotype,’ i.e. a trait as characteristic of the ant as any of its anatomical features. Not that such a thing would cut much ice with museum taxonomists who view with suspicion, if not disdain, any taxonomic pretensions from things like bird nests or beaver dams. As for behavioural traits, those are liable to cause apoplexy in museums, perhaps less so these days than in olden times when being dead was a condition of entry (human visitors reluctantly excepted).

One other Polyrhachis weaver ant has had its nest examined in detail. Three nests of P. delecta, a wet tropics species, came under scrutiny from Chris Tranter and Bill Hughes of Sussex University, UK, while they (or at least one of them) was in Cairns for a conference in 2014. Their account (Tranter and Hughes 2015) contains remarkable photos of the ‘girders’ the ants construct to support the nest internally: concentric silk circles in cross section, as you would see looking end-on at a rolled-up newspaper with only six or seven pages. The images can’t be posted here for copyright reasons but can be provided for viewing on request to interested readers.

The nest of our all-too-familiar green ant, Oecophylla smaragdina is famously constructed with the binding help of silk squeezed out of the ant’s larvae like toothpaste from a tube. The same child-abuse is employed by P. australis and some other Polyrhachis weaver ants whose larvae pupate naked, presumably because the silk that most ant larvae use to cocoon themselves in preparation for metamorphosis has all been commandeered for infrastructure.

At least arboreal nests are comparatively accessible. A Nobel Prize for earthwork awaits anyone who would examine the nest of Polyrhachis sokolova, the only saltwater-inhabiting ant. Its nest lies in the thick mud beneath mangroves, entwined in the tough roots of those hardy trees. It is a marine version of the nests of most ant species whose homes are excavated in the ground. The shape and extent of these labyrinths are typically revealed by pouring plaster of paris or its more ghastly but effective equal, down into the chambers and passageways where it sets forming a solid cast which can then be dug out without everything collapsing. The process can be seen on this Youtube video and has caught on both artistically and commercially.

So much for ants’ nests or, rather, so little: there’s far more to be said than we could possibly say here.

The nests of other hymenopterans – mud-dauber wasps, paper wasps and bees – are even more various, and often as distinctive as those of ants. Green Path has already described some of them in posts about Bee Heaven and Brown Paper Wasps. Native Bees (stingless bees, sweat bees) are described here, but without any nest photos. There are more to come (Yellow Paper Wasps are next), when time permits. Meanwhile, here is a collection of my wasp and bee photos on flickr, with nests of some of them. – Malcolm

References

Kohout RJ. 2006. Review of Polyrhachis (Cyrtomyrma) Forel (Hymenoptera: Formicidae: Formicinae) of Australia, Borneo, New Guinea and the Solomon Islands with descriptions of new species. Memoirs of the Queensland Museum 52: 87-109.

Tranter C, Hughes WHO. 2015. A preliminary study of nest structure and composition of the weaver ant Polyrhachis (Cyrtomyrma) delecta (Hymenoptera: Formicidae). Journal of Natural History. doi 10.1080/00222933.2015.1103912

Yamauchi K, Ito Y, Kinomura K, Takamine H. 1987. Polycalic colonies of the weaver ant Polyrhachis dives. Kontyu (Japanese Journal of Entomology) 55: 410-420.

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