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Journal of Natural History
ISSN: 0022-2933 (Print) 1464-5262 (Online) Journal homepage: http://www.tandfonline.com/loi/tnah20
A preliminary study of nest structure and
composition of the weaver ant Polyrhachis
(Cyrtomyrma) delecta (Hymenoptera: Formicidae)
C. Tranter & W. O. H Hughes
To cite this article: C. Tranter & W. O. H Hughes (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
To link to this article: http://dx.doi.org/10.1080/00222933.2015.1103912
Published online: 23 Nov 2015.
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A preliminary study of nest structure and composition of the
weaver ant Polyrhachis (Cyrtomyrma)delecta (Hymenoptera:
Formicidae)
C. Tranter and W. O. H Hughes
School of Life Sciences, University of Sussex, Brighton, UK
ABSTRACT
Polyrhachis weaver ants build their nests from vegetation bound
together using silk produced by their larvae. Here we provide a
pilot study of the composition and the physical structure of three
arboreal silk nests of Polyrhachis (Cyrtomyrma)delecta based on
examination of three colonies. We found broadly similar nest
architecture and size of the nests with each containing six or
seven identifiable chambers, and describe the distribution of
ants of different castes and life stages between them. We also
note the construction of silk ‘girder’structures, which spanned
larger chambers, and we hypothesize that these provide addi-
tional strength to the internal nest structure. This study highlights
the importance of more detailed investigation of the internal nest
structure and composition in Polyrhachis, and other weaver ant
species, which will help to develop our understanding of this
specialized form of nest construction and nesting habits in a
diverse group of ants.
ARTICLE HISTORY
Received 20 November 2014
Accepted 30 September 2015
KEYWORDS
Nest architecture; colony
structure; silk girder; social
insect; silk
Introduction
The ability of social insects to locate suitable nesting sites and, through manipulation of
the environment, to construct often highly complex nests is key to the success of the
colony and of social insects in general (Hölldobler and Wilson 1990). In ants, nests can
range in size and complexity from the vast underground networks of Atta leaf-cutting
ants, to a whole colony of Temnothorax contained within a single acorn (Hölldobler and
Wilson 1990). The nesting habits of organisms are an important factor in their life history
and a powerful driver of their morphology and ecology (Jeanne 1975; Mikheyev and
Tschinkel 2004). The architecture of the nests themselves is believed to be key in the
evolution of division of labour, which has contributed to the ecological success of ant
societies (Hölldobler and Wilson 1990). The internal structure of a nest and the internal
arrangement of ants and the brood within it can potentially also have important
implications for the spread of infectious diseases within colonies (Schmid-Hempel
1998; Naug and Camazine 2002). Social insects may be particularly vulnerable to para-
sites because of the very high population densities, homeostatic environmental
CONTACT C. Tranter c.tranter@sussex.ac.uk
JOURNAL OF NATURAL HISTORY, 2015
http://dx.doi.org/10.1080/00222933.2015.1103912
© 2015 Taylor & Francis
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conditions and low genetic diversity within colonies, which contribute to an increased
risk of parasite transmission. In leaf-cutting ants, waste management tasks are parti-
tioned spatially and between castes, which helps to isolate the main colony from the
increased risk of contamination (Bot et al. 2001; Waddington and Hughes 2010).
Similarly, compartmentalization between chambers within the nest may help to prevent
transmission of parasites to vulnerable aspects of the colony such as the queen or brood
(Pie et al. 2004; Boomsma et al. 2005).
Polyrhachis Fr. Smith is a diverse genus of ants within the subfamily Formicinae with
over 600 species widely distributed across Africa, Asia and Australasia. Commonly
termed ‘weaver ants’, many species form arboreal nests constructed from vegetation
bound together with silk produced by their larvae, but may vary widely from species
with subterranean nests formed from intertidal mangrove mud to lignicolous and
lithocolous species (Robson and Kohout 2007). There is also an extreme range of colony
sizes and compositions from very small colonies with just a few tens of individuals,
through to colonies with almost a million workers (Liefke et al. 1998; Dornhaus et al.
2012). Colonies tend to be polydomous, and in some species may also be polygynous
with multiple de-alate queens within a nest (Liefke et al. 1998; van Zweden et al. 2007).
Like other formicines, weaver ants produce acidic venom from their venom gland, which
they can use to disinfect themselves and their brood (Graystock and Hughes 2011;
Tragust et al. 2013; Tranter et al. 2014). Additionally, Polyrhachis and Oecophylla weaver
ants use this venom to maintain acidic conditions of their nest silk (Tranter et al. 2014;
Tranter and Hughes 2015). The general nesting habits of these ants have been well
documented (Kohout 2000,2012; Robson et al. 2015) and were recently set into a
phylogenetic framework, broadly describing patterns of weaving behaviour and basic
nest composition (Robson and Kohout 2005,2007). There has been a comprehensive
study of the unusual nesting habits of the estuarine species Polyrhachis (Chariomyrma)
sokolova, as well as brief details of mainly external nest architecture, and records of
colony composition for a few other Polyrhachis species (Nielsen 1997; Jinfu and Jue
1996; Liefke et al. 1998; Downes 2015), However, detailed observation of the finer scale
structure and colony composition of Polyrhachis nests is less well documented, espe-
cially considering the large number of species in the genus and the diversity of nesting
habits. Here we provide some preliminary information on this from three nests of the
arboreal and silk weaving species of the Australian weaver ant Polyrhachis (Cyrtomyrma)
delecta.
Material and methods
Ants were identified using keys available in Kohout (2006). Three nests of P. delecta were
collected from around Centenary Lakes (16.902°S, 145.749°E), Cairns, QLD, Australia in
July 2014. The nests were externally formed from interwoven leaves hanging in vegeta-
tion approximately 1.5 m above ground and were suspended at a single point. At each
site the nests collected were the only nests visible in the vicinity. Nests 2 and 3 were
approximately 20 m apart, and Nest 1 was located about 40 m from either of the other
nests; none of the nests was therefore found on the same plant as the others. Nests were
measured externally about three axes to give a height, breadth, width measurement
using a 30-cm ruler. Nests were all approximately ellipsoid in shape and estimated
2C. TRANTER AND W. O. H. HUGHES
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volumes were calculated using the formula V= 4/3πabc, where a= ½height,
b= ½breadth, c= ½width. The whole nest was collected by cutting the branch above
the nest and gently releasing the nest into a plastic container. Returning worker ants
were collected individually for a period of 15 min after collection of the nest and stored
separately in 95% ethanol. This collection method resulted in minimal disturbance of the
ants. The nest was then left for 6 h during which time the few ants that had emerged
during collection returned to within the nest. The whole nest was then rapidly chilled by
placing it in the freezer at −20°C, and kept there for 3 h. The nest was then removed
from the freezer, measured externally, and carefully dissected. The position and size of
chambers within the nest were recorded, and their contents –queens, gynes (alate
queens), males, workers and brood –were collected in ethanol for later counting under
a stereomicroscope. Although every effort was made to minimize disturbance of the
nest before dissection it was not possible to determine the extent to which ants within
the nest may have repositioned during procedures. Therefore any positional data of
castes within the nest should be treated cautiously. Polyrhachis delecta larvae do not
spin larval cocoons, the loss of which is thought to be restricted to the Cyrtomyrma and
Myrmatopa subgenera, and pupae are exposed within the nest (Robson and Kohout
2007; Robson et al. 2015). Downes (2015) reported that it is possible to discriminate
between incipient workers and incipient sexuals in the closely related Polyrhachis
australis using the presence of wing buds on exposed pupae, but we were not able to
reliably do so here for P. delecta. After removal with soft forceps of all ants and brood,
the internal nest structure was recorded with sketches and photographs (Canon 7D and
Canon MPE-65 mm or 100 mm f/2.8 macro) as the dissection progressed. Samples are
stored at the University of Sussex, UK.
Results
Nests 1 and 2 were similar in their external structure, dimensions and construction,
whereas Nest 3 was larger and more spherical in shape (Table 1). All three nests were
formed from five or fewer leaves at the terminus of a hanging branch, with the leaves
slightly folded and woven together with silk and carton material, and in some cases
(c. 20%) split along veins (Figure 2C). All nests had one entrance located at the bottom
of the nest. Internally, nests were composed of one or two large chambers, usually
formed as one whole section between two leaves, and a number of smaller chambers
towards the periphery (Figure 1). These chambers were lined with silk sheets, which
varied in thickness from so thin as to be almost transparent to the approximate
thickness of standard 75 gsm copy paper (100 µm). Three of the larger chambers
possessed cylindrical protuberances, which we term here ‘girders’. These girders
emerged from the inner wall and spanned the chamber, and were formed from tightly
layered silk (Figure 2A,B). A single large girder was present in Chamber 4 of Nests 1 and
2 and a smaller girder was also observed spanning the walls of Chamber 2 in Nest 2.
Nests 1 and 2 contained similar numbers of workers, alate queens and brood (Table 2;
Figure 2D,E). Nest 3 however had fewer workers and brood, no de-alate queens and very
few alate queens present, unlike the other two nests. There were some similarities
between the three nests in the locations in which most of each of the castes were
found (Figures 1 and 3). This was most evident between Nests 1 and 2, which shared a
JOURNAL OF NATURAL HISTORY 3
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more similar physical structure. The de-alate queen in Nests 1 and 2 was located in the
largest chamber of the nest, which also contained a large number of eggs and small
larvae. The males tended to be located in chambers towards the nest’s middle. Workers
were found in all chambers throughout the nest but there was some indication that they
may be more concentrated in chambers adjacent to nest entrances, and those chambers
towards the top of the nest (Table 2). These uppermost chambers also contained large
numbers of alate queens and larger larvae.
Discussion
We present here the first observations and descriptive data on the structure and
composition of three nests of the weaver ant P. delecta. There was evidence of segrega-
tion of castes within the various chambers in the nest and similarities in their spatial
position between nests. We also noted the inclusion of tightly wrapped sections of silk,
that we term here ‘girders’, which horizontally spanned a number of the larger cham-
bers. These girders were extremely rigid compared with the rest of the nest construction
and we hypothesize that they function to provide internal support to stop the lateral
compression and collapse of inner chambers and the nest as a whole.
Many ant colonies undergo seasonal variation in their size and composition, which is
often best seen in the different rates of caste production (i.e. workers versus sexuals) or
the season in which brood may tend to be produced. Downes (2015) demonstrated
seasonal fluctuations in the colony composition of P. australis. In that study, alate queens
and alate queen pupae were mostly present only from July to December. In our study,
conducted in July, we also found the presence of alate queens in quite high numbers. In
addition, we found generally similar nest sizes to Downes (2015), with the exception of
males, which were more numerous in the colonies in our study. Compared with other
Table 1. Information on external and internal colony structure of three nests of
Polyrachis delecta.
Nest
External dimensions Internal chamber details
(height × width × depth) Count (n) Chamber Volume (%)
1. Formed of three leaves 6 1 10
220
140 × 70 × 65 mm 3 10
volume: 333.53 cm
3
444
510
65
2. Formed of five leaves 7 1 10
220
135 × 75 × 60 mm 3 5
volume: 318.09 cm
3
435
510
610
710
3. Formed of three leaves 6 1 10
220
110 × 85 × 80 mm 3 25
volume: 391.65 cm
3
425
510
610
Numbering detailed in Figure 1. Approximate volumes are calculated from dimensions.
4C. TRANTER AND W. O. H. HUGHES
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Polyrhachis species these nest populations are small overall for the genus, many species
of which have thousands of workers, but are more representative of species within the
Cyrtomyrma subgenus as a whole (Dorow et al. 1990; Dorow 1995; Glaser 1997; Liefke
et al. 1998). In this study we estimated that the proportion of foragers constituted
around 5–18% of the total worker force, although returning foragers were only collected
for 15 min after nest collection. In Odontomachus brunneus the foraging population
found outside the nest was estimated to be 77% of the total workforce (Hart & Tschinkel
2012), much higher than we estimated here, but in general it is unknown quite how the
proportions of foragers varies depending on ant species or overall colony size (Tschinkel
1999).
As the size of nests and number of chambers increases, the chance of a complete
mixing of colony members decreases and so larger nests may promote colony complex-
ity through task differentiation or protection against disease through compartmentaliza-
tion (Sendova-Franks and Franks 1995; Naug and Camazine 2002; Naug 2008; Konrad
Figure 1. Lateral view schematic illustrations of the internal chamber arrangement of three nests of
Polyrhachis delecta weaver ants. Chambers are labelled 1–7. Nests were suspended from vegetation
at the top and thick black lines show the core arrangement of leaves, which were divided into
chambers through construction with larval silk (thin lines). Sections with wavy outlines in grey on
the outer surface of the nest indicate areas constructed from carton. Internal structures with dashed
lines indicate the position of internal ‘girders’. Nest openings are portrayed oriented towards the
lower right of each nest.
JOURNAL OF NATURAL HISTORY 5
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et al. 2012). In this study there were indications of some degree of compartmentalization
of nest components to different chambers within the nest, although the potential for
relocation of ants between collection and dissection, and the fact that the study was
limited to only three nests means that these data must be treated cautiously. As with
other social insect nests (Kugler and del Hincapie 1983; Longino 1991; Ito et al. 1994;
Baracchi and Cini 2014), the brood, and especially the queen, tended to be located away
from the periphery of the nest, and usually away from the nest entrance. Additionally,
nest architecture is important in producing correct internal nest microclimates and
Figure 2. Photographs detailing a silk support structure (‘girder’) spanning a chamber within a nest
(A), a cross-section of the ‘girder’(B), a view of the outside of Nest 1 showing workers on sections of
the folded leaf with an area of visible carton in the bottom right (C), examples of an alate queen
(top), male (middle) and worker (bottom) of Polyrhachis delecta weaver ants (D), and examples of the
various brood stages including a pupa (left) and variously sized larvae (middle to right) (E).
6C. TRANTER AND W. O. H. HUGHES
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Table 2. Overall details of the composition of ants in three nests of Polyrachis delecta.
Caste or life stage
Nest 1 Nest 2 Nest 3
Totals By chamber (%) Totals By chamber (%) Totals By chamber (%)
123 456 123 4567 1 23456
Adult ants
De-alate queens 1 0 0 0 100 0 0 1 0 0 0 100 0 0 0 0 0 00000
Alate queens 79 14 27 42 11 3 4 92 17 22 27 21 3 9 1 6 33 67 0 0 0 0
Males 38 63 16 21 0 0 0 37 22 41 32 3 0 3 0 15 40 0 60 0 0 0
Workers
Within colony 394 12 13 7 33 12 23 446 16 12 12 19 24 11 7 183 4 11 10 28 39 9
Outside colony 27 19 33
Total 421 465 216
Brood
Pupae 37 35 14 30 22 0 0 64 33 14 28 22 3 0 0 18 12 88 0 0 0 0
Large larvae 53 0 17 17 66 0 0 21 14 5 14 67 0 0 0 10 30 70 0 0 0 0
Small larvae 19 0 0 0 100 0 0 35 0 23 0 77 0 0 0 5 0 100 0 0 0 0
Eggs yes yes no
JOURNAL OF NATURAL HISTORY 7
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brood may be moved within colonies so as to raise them at the correct temperature,
which may be in part responsible for the location of brood observed in this study
(Sendova-Franks and Franks 1995; Tschinkel 1999). Further investigation of more
P. delecta nests is needed to confirm the descriptive data provided here.
Figure 3. Panel showing the percentage of each ant life stage within each chamber of each of three
nests of Polyrhachis delecta weaver ants.
8C. TRANTER AND W. O. H. HUGHES
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Polydomy occurs in a number of Polyrhachis species, some of which have been
described as supercolonial (Yamauchi et al. 1987; van Zweden et al. 2007), and it is possible
that the presence of the queenless, brood-bearing and otherwise healthy nest in this study
is evidence for this in this species. Hence it is hard to know if the assemblages collected
here represent distinct colonies or, as we term them here, just one of possibly many ‘nests’
that comprise the colony as a whole. As three nests were collected for this study and all
were relatively small, collected from only where nests were readily discoverable and easily
collected, and from a limited geographic locality, it is likely that there is selection bias in
these results. Larger colonies or those located higher in the vegetation, or at a different
location, may differ significantly in their structure and composition. It is likely that there will
have been some internal relocation of ants within nests between collection and dissection,
so the data on the intranidal location of ants needs to be treated cautiously. Nonetheless
Nests 1 and 2, which were similar in size and structure, seemed to also share similarities in
their spatial location of ants.
Although general nesting patterns are quite well studied across Polyrhachis species,
this study provides the first descriptive data of the interesting structure of P. delecta
nests. We hope that this work will stimulate future more detailed studies on nest
structure and composition to further explore the intricacies of this specialized form of
nest construction, which may help to elucidate evolutionary patterns of nest-building
and habitat preference in this highly diverse genus of ants.
Acknowledgements
We thank Victoria Norman for comments on this work, and are extremely grateful to three anon-
ymous reviewers for their detailed and insightful comments, which greatly contributed to this
manuscript.
Disclosure Statement
We the authors have no financial interest or benefit from the applications of the research in this
study.
Funding
This work was supported by Biotechnology and Biological Sciences Research Council.
ORCID
Christopher Tranter http://orcid.org/0000-0003-1722-672X
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