Thursday 3 August 2017

Spiny little farmers: the ant genus Polyrhachis in Zambia.

Of the many – very many – ants in Zambia, Polyrhachis are not only among the most conspicuous, but some of the most recogniseable – and, to my unending joy, there exist two open access, freely-downloadable keys to the genus within Africa, and – possibly because Polyrhachis are blessed with a great number of readily visible external features – these are (relatively) easy to read and easy to work through.



Polyrhachis schistacea is the most conspicuous and common Polyrhachis
in much of Zambia, and indeed much of sub-Saharan Africa


The first Polyrhachis to be encountered is almost always Polyrhachis schistacea, a very distinctive, ground-nesting species that is found almost everywhere. Although it probably absent from deep forest, it thrives even in dense, canopied woodland, and occasionally into wetter, riverine forest.

Between Bolton (1973)’s and Rigato (2016)’s reviews of the genus, five species are recorded from Zambia; a sixth species, P. schistacea, is also present throughout the country, and records from Zambia may have escaped mention simply because it is abundant almost everywhere.
With almost 500 species described globally, Polyrhachis is made more manageably by division into a number of subgenera – although all 61 species in sub-saharan Africa fall together into Polyrhachis (Myrma) – and species groups. Of the six species groups recorded from sub-saharan Africa, 2 are known to be present in Zambia, and a third occurs in a number of neighbouring countries and almost certainly extends into Zambia.

Polyrhachis epinotalis is a typical member of the militaris
species group, of which at least six species occur in Zambia.
Kundabwika, N. Province, Zambia. 








Polyrhachis weissi is the sole member of the
revoili species-group recorded from Zambia.
Chongwe, Lusaka Prov., Zambia. 


Although their (rather more speciose) cousins in the genus Camponotus often have not only major and minor workers – often requiring separate keys – but sometimes also several grades between, Polyrhachis ants have only a single worker caste. Camponotus are also rather smooth-bodied, generic-looking ants, whose visible distinctions can be easy to miss; while the various Polyrhachis can have spines and ridges almost anywhere, making them a much easier group to work with.
                















Key to Species known or likely to occur within Zambia 
(brutalised into existence from the keys produced by Bolton (1973) and Rigato (2016)'s derived key)

For those who aren't familiar with the weird and wonderful (?) jargon of ant anatomy, just beyond this list, you will find a picture of Polyrhachis epinotalis with all the body features used in the key. labelled. 


     
     1.    > Pronotum without a distinct margin, sides and dorsum joined by a smooth, uninterrupted curve (revoili group)(S., W., and Central Africa, including Zambia)………..........….Polyrhachis weissi
>          >>Pronotum at least with a partial margin, seen as a raised or projecting flange, ridge, or an acute angle separating dorsum from sides………………………………………….…......………...to 2

     2.    >Metanotal groove indistinct, represented at most by a line scoring across the dorsum of alitrunk, which may or may not interrupt the sculpturation; never impressed, and sometimes undetectable (viscosa group)……………………………………………………………………......………to 3
>    >>With broad, impressed and distinct metanotal groove, in profile this may be seen as a V or U shaped trench separating mesonotum from propodeum (militaris group)…………….….….to 6
 
     3.    >Gaster finely longitudinally striate (South Africa, Malawi)..…....……….Polyrhachis arnoldi
     >>Gaster finely reticulate-punctate……………………………………….………………….to 4
    
     4.       >Antennal scape broadened into a hood at tip, concealing base of following (first funicular) segment from above; the covered section of the first funicular segment is strongly flattened. Eyes generally flat, occasionally convex (widespread throughout Sub-Saharan Africa, from South Africa to Sudan and Ghana)…………………………………….……..........…………Polyrhachis viscosa
     >>Antennae not like this; eyes convex……………………………………………………...to 5
 
     5.       >Ridge between spines of propodeum is raised medially into a distinct tooth or tubercle; the spines themselves curving distinctly upwards (East and Southern Africa) ..Polyrhachis wilmsi
     >>Ridge between spines of propodeum at most arched medially; the spines directed behind and to the side, curved only very slightly upwards (East and Southern Africa, and Angola)……................……………..…………………………………………………….……………Polyrhachis spinicola

      6.       >Petiole with 2 spines………………………………………….........…………………….to 7
     >>Petiole with 4 spines; the second pair may be reduced to small teeth………………...….to 8. 
     >>> Petiole with 6 spines or teeth, the smallest of which often reduced to blunt tubercles behind the lateral spines; eyes placed behind the mid-length of side of head..............Polyrhachis decemdentata
     
     7.     >Spines of petiole more-or-less parallel, strongly hooked backwards at the tips. Clypeus with carina, gaster (usually) with golden pubescence (West and Central Africa, south to D.R.C. and Angola)…………………………….…………………………….........………Polyrhachis laboriosa
     >>Spines of petiole diverging, curved backwards along their length but never hooked apically. Clypeus without carina; gaster usually with grey pubescence, never golden….Polyrhachis wellmani

     8.       >Pronotum entirely without erect hairs dorsally (East and Southern Africa, from Kenya to Natal)………………………………………………………………………………..Polyrhachis schlueteri
>>Pronotum with erect hairs dorsally……………………………………………………….to 9

     9.       >Dorsal surface of thorax without erect hairs except on the pronotum. Sides of head below eyes without erect hairs. Gaster polished, shining, only with very fine reticulation; pubescence short, diluted……………………………………………………………...………….Polyrhachis gagates
     >>Dorsal surface of thorax with erect hairs on all segments; side of head below eyes with erect hairs. Gaster generally dull, with fine reticulate-punctulate sculpturing, or sculpturing hidden by pubescence………………………………………………………………………………….to 10

     10.   >Pubescence sparse, not hiding sculpturation of alitrunk or gaster, usually greyish. Relatively narrow, slender species………………………………………..…………..Polyrhachis schistacea
     >>Pubescence abundant everywhere, at least partially hiding sculpturation of dorsal alitrunk and gaster; generally golden…………………………………………………….……………..to 11

     11.   >Head in full-face view rectangular; sides weakly convex, posterior corners always with distinct, more-or-less obtuse angles. Each eye with a blunt emargination behind, separating dorsum from sides. Propodeal spines short, tooth-like, curving upwards and much shorter than the depth of the propodeal declivity……………………………………..................…………Polyrhachis militaris
     >>Head in full-face view oval, either without or with very faint corners behind; no margination of head behind eyes, sides and upperparts of head smoothly confluent. Propodeal spines long, upturned; as long as or almost as long as the depth of the declivity………………………………………………………………………Polyrhachis epinotalis


Main body parts labelled for reference; unless your eyes are much better than mine, you will have to expand it to read the labels (sorry).
Polyrhachis epinotalis, Kundabwika, Zambia. 


Bolton (1973) includes an overview of the habitats and habits of these species, as known at the time:
  • weissi constructs silk-and-debris nests under or between leaves in trees; it occupies a wide range of habitats from forest and veldt to savannah. 
  • arnoldi has been recorded nesting in tree hollows, protecting the nest with a matrix of silk and dirt. 
  • viscosa nests in sandy soil, usually in exposed sites; ants are primarily found foraging on the ground, but may also climb. It is largely restricted to savannahs and arid zones, but is also found coastally in Ghana. 
  • wilmsi nests in stem galls (in trees, I assume?)
  • spinicola has been recorded from acacia and citrus trees; its nesting habits are not clear. 
  • decemdentata makes its nests in rotten and termite-mined tree branches. 
  • laboriosa is restricted to forested environments, where it builds a nest of twigs and leaf fragments, bound with silk and fungal hyphae, adhered to the undersides of leaves or built in the fork of tree branches; they aggressively defend the nest, spraying acid under their abdomens. Individuals disturbed are far less aggressive, usually simply dropping off their branches. 
  • Boston speculates that wellmani most likely has the same habits as the (very similar) schistacea, of which it may be a synonym. 
  • schlueteri is apparently restricted to hot, moist sites. 
  • gagates is a ground-nesting species, typically found in drier, open habitats such as savannah and semi-desert, only occasionally in scrub forest. The nests have been recorded at the bases of grasses, and under rocks; the entrance may be marked by a wide crater of dirt, or with a wall of woven material which extends into the entrance. 
  • schistacea is primarily a species of savannah and scrub forest, absent from rainforest; it nests in open ground or under stones or – rarely – decaying wood. They may construct a cup-like wall of grass-blades around the nest; they are also noted as a tender of hemipteran bugs. 
  • militaris is noted as an arboreal species of forests, especially rainforest; constructing a nest inside hollows of trunks and branches. Bolton does not separate epinotalis and militaris, so it is assumed here that epinotalis lives much the same life. 

Polyrhachis decemdentata in deep spray forest beside Lumangwe Falls,
Northern Province, Zambia. October 2017.

When this post first went up, back in 2017, I had only been up to the Kalungwishi valley once. Returning in October, I happened upon a small number of a small colony of very uncooperative ants in dense spray forest abutting the magnificent Lumangwe Falls. Between the low light and my sluggish reactions, this is the best picture I managed to get, but these should be Polyrhachis decemdentata, which Rigato lists as occurring in West and Central Africa, to the western parts of Kenya and Tanzania; northeastern Zambia represents a very slight southerly expansion of that range. 




And in case you were wondering why I described them as "farmers" specifically: well, herders might be a better description. Like most ants, some Polyrhachis are fond of sweet things, and, as Homopteran bugs tend to ooze sugar water from their rear ends, they 'milk', protect and sometimes even propogate such insects; Polyrhachis schistacea in particular can reliably be found wherever nymphs of various leafhoppers are seen, such as in the picture below:


Polyrhachis schistacea tending Membracid plant-hoppers in Mpongwe, Copperbelt, Zambia




References: 

1.       Bolton, B.  (1973). The ant genus Polyrhachis F. Smith in the Ethiopian Region (Hymenoptera: Formicidae). Bulletin of the British Museum (Natural History) Entomology 28 (5) pp. 283-369
2         2.       Rigato, F. (2016). The ant genus Polyrhachis F. Smith in sub-Saharan Africa, with
d                descriptions of ten new species. (Hymenoptera: Formicidae). Zootaxa 4088 (1) pp. 1-50