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A preliminary study of nest structure and composition of the weaver ant Polyrhachis ( Cyrtomyrma ) delecta (Hymenoptera: Formicidae)


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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 additional 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.
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Journal of Natural History
ISSN: 0022-2933 (Print) 1464-5262 (Online) Journal homepage:
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
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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:
C. Tranter and W. O. H Hughes
School of Life Sciences, University of Sussex, Brighton, UK
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 identiable chambers, and describe the distribution of
ants of dierent castes and life stages between them. We also
note the construction of silk girderstructures, 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.
Received 20 November 2014
Accepted 30 September 2015
Nest architecture; colony
structure; silk girder; social
insect; silk
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
© 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 ner 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)
Material and methods
Ants were identied 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
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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 eort 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.
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 ve 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
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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 nests 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.
We present here the rst 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 dierent rates of caste production (i.e. workers versus sexuals) or
the season in which brood may tend to be produced. Downes (2015) demonstrated
seasonal uctuations 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.
External dimensions Internal chamber details
(height × width × depth) Count (n) Chamber Volume (%)
1. Formed of three leaves 6 1 10
140 × 70 × 65 mm 3 10
volume: 333.53 cm
2. Formed of ve leaves 7 1 10
135 × 75 × 60 mm 3 5
volume: 318.09 cm
3. Formed of three leaves 6 1 10
110 × 85 × 80 mm 3 25
volume: 391.65 cm
Numbering detailed in Figure 1. Approximate volumes are calculated from dimensions.
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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 518% 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
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 dierentiation 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 17. 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.
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et al. 2012). In this study there were indications of some degree of compartmentalization
of nest components to dierent 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).
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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
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
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
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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 conrm 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.
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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 dierent
location, may dier signicantly 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 rst 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.
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
Disclosure Statement
We the authors have no nancial interest or benet from the applications of the research in this
This work was supported by Biotechnology and Biological Sciences Research Council.
Christopher Tranter
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... In Polyrhachis, workers of a few subgenera use larvae to weave silk pavilions on the undersides of leaves, or mix silk with vegetable fragments to construct hard walls or close off a folded leaf; a few species steal spider silk instead (Dorow and Maschwitz, 1990;Robson and Kohout, 2007). In Polyrhachis (Cyrtomyrma) delecta, 'girder' structures of silk (>1 cm long) can give extra strength to leaf nests (Tranter and Hughes, 2015). A different use of silk from ant larvae is found in Harpegnathos saltator (Ponerinae), where underground nest chambers are lined with empty cocoons, presumably to control humidity (Peeters et al., 1994). ...
As in other Hymenoptera, adult ants cannot secrete silk, unlike the larvae that spin a cocoon prior to metamorphosis. Fisher and Robertson (1999) first showed the existence of a silk gland in the head of adult Melissotarsus beccarii workers, and we confirm this with detailed histology and ultrastructural comparisons of both queens and workers. This African genus exhibits extreme morphological adaptations (legs, head shape and mandibular muscles) for tunnelling behaviour inside living trees, that underlie an obligate mutualism with scale insects. Rhopalomastix is its sister genus distributed across Asia, and we show that queens and workers also have a silk gland. This lineage of minute workers relies on silk to secure their network of tunnels against other arboreal ants. We show striking differences between these genera in the anatomy and ultrastructure of the cells that secrete silk, especially numerous vacuoles and an unexpectedly branched end apparatus in Melissotarsus. Moreover, the legs of Melissotarsus are much more specialized for tunnelling, and this includes highly expanded basitarsi. The latter house the novel ‘Delage-Darchen gland’, and we document its anatomy and ultrastructure, suggesting a proteinaceous secretion to harden roofs made of silk combined with wood fragments. The restriction of the Delage-Darchen gland to Melissotarsus, combined with a modified silk gland (an almost three-fold increase in the number of secretory cells, and ultrastructural differences suggestive of higher secretory activity), are evidence of an outstanding evolutionary divergence relative to Rhopalomastix. Synthesis of silk by adults is a significant innovation among ants, but its augmented production in Melissotarsus makes them better adapted for the hazards of arboreal life.
... To cope with these risks, social insects show individual immunity and a large array of behavioural, physiological, and organisational strategies, collectively known as social immunity (Cremer et al., 2007). Many ants build their nests with complex architectures that, together with the structure of the interaction network and division of labour, constitute the organisational component of the social immunity (Baracchi and Cini 2014;Stroeymeyt et al. 2014;Tranter and Hughes 2016). Many ant species show a much simpler nest architecture. ...
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Nest architecture is a fundamental character shaping immune strategies of social insects. The arboreal ant Temnothorax unifasciatus nests in cavities such as oak galls where the entire colony lives in a unique small chamber. In these conditions, physiological and behavioural strategies likely prevail over compartmentalisation and are presumably tuned with colony size. We designed two experiments to study chemical and behavioural immune strategies against the entomopathogenic fungus Metarhizium anisopliae in colonies of different sizes. First, we compared spore germination and length of germinal tubes inside artificial nests, designed to impede the contact between the ants and the fungus, in colonies of different size. In the absence of direct contact, Temnothorax unifasciatus colonies inhibit fungal growth inside their nests, presumably through volatile compounds. The analysis revealed a positive correlation between fungistatic activity and colony size, indicating that workers of smaller colonies do not invest a higher per capita effort in producing such substances compared to larger colonies. Second, we performed a removal experiment of contaminated and non-contaminated items introduced inside the nests of colonies of different size. Small colonies challenged with contaminated fibres showed an increased removal of all the items (both contaminated and non-contaminated) compared to small colonies challenged with non-contaminated fibres only. Conversely, larger colonies moved items regardless of the presence of the spores inside the nest. Colony size qualitatively affected removal of waste items showing a pathogen elicited reaction in small colonies to optimise the reduced workforce, while the removal behaviour in larger colonies revealed to be expressed constitutively.
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Tingkat perkembangan lahan umumnya diikuti dengan perubahan struktur vegetasi penyusun ekosistem yang ada didalamnya. Dalam perjalanannya, muncul intervensi yang dapat mempengaruhi tingkat perkembangan lahan baik mendukung ataupun menghambat. Salah satu bentuk intervensi yang dapat terjadi adalah berupa aktivitas pemanfaatan lahan, sistem agroforestri (AF). Pergeseran tingkat perkembangan lahan melalui pendekatan tingkat perkembangan AF diikuti perubahan vegetasi penyusun menuju kearah ekosistem yang mendekati ekosistem hutan. Dari sisi ekologi, bagaimana perkembangan AF berdampak terhadap struktur komunitas serangga penyusun di dalamnya, khususnya semut, penting diketahui. Penelitian ini dilakukan untuk mengetahui struktur komunitas semut yang menyusun ekosistem AF. Pengambilan data dilakukan di tiga tingkat AF (awal, tengah, dan lanjut) di musim kemarau (April, Mei, Juni, Agustus, dan September). Metode koleksi semut yang digunakan adalah pit-fall trap dan penangkapan langsung. Total diperoleh 7 jenis semut terdiri dari Odontoponera denticulate (Ponerinae), Odontomachus sp. (Ponerinae), Anaplolepis gracilipes (Formicinae), Pheidologeton sp. (Myrmicinae), Camponotus sp. (Formicinae), Polyrachis sp. (Forminae), dan Crematogaster sp. (Myrmicinae). Tingkat perkembangan AF tidak diikuti dengan perbedaan jenis semut namun berdampak pada kemelimpahan masing-masing jenis. Keanekaragaman jenis tertinggi serta jumlah individu semut terbesar berturut-turut ada pada AF tengah, awal, dan lanjut. Selanjutnya, terdapat 5 kelompok fungsional dengan variasi kemelimpahan berdasarkan perkembangan AF. Tingkat AF awal lebih sesuai untuk kelompok Generalized myrmicinae (GM),AF tengah untuk kelompok Dominant opportunist, dan AF lanjut untuk Forest opportunist (FO)
Many species of the large ant genus Polyrhachis F.R. Smith make nests using silk from their own larvae or from spiders. This paper reports on 400 nests of Polyrhachis australis Mayr dissected at Townsville, Queensland, between 2009 and 2015. Details are given of host plants, nest structure, materials used (carton and silk), placement of brood within nests, resistance to disturbances such as rain and nest longevity. Carton nests are typically lined with flat-sheet silk. Complex internal structure gave more internal surface area, so absolute nest size is not a reliable indicator of ant numbers. Occasional use of fluffy spider-silk in outer walls led to more flaccid nest structure. Use of a sticky, ductile form of silk, probably derived from moths, was also identified. Dedicated brood chambers were not noted, but brood clumping was usual, possibly representing offspring of different queens. Brood was attached to the nest substrate by diffuse silk strands. Individual nests could persist for 15 months and grew little after an initial period of expansion.
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Parasites are an important force in evolution, driving the need for costly resistance mechanisms. The threat from disease is potentially high in group-living species such as social insects, which have accordingly evolved behavioural and chemical defences that vary between species depending on their life histories. Several ant genera have lost a key exocrine antimicrobial defence, the metapleural gland, and yet are still able to thrive in environments abundant with parasites. We investigate, in species lacking the metapleural gland, how the production of antimicrobial venom, grooming behaviours and the use of potentially antimicrobial larval silk may have evolved as alternative antiparasite defences. We focus on the Australasian weaver ant Oecophylla smaragdina and compare this to Polyrhachis weaver ants. We show that the production of venom by O. smaragdina workers is important for disease resistance but that the presence of larval silk is not, and that workers use their acidic venom to maintain nest hygiene. The grooming defences of O. smaragdina differ between castes, with minor workers allogrooming more and major workers showing greater upregulation of grooming in response to parasites. Chemical and behavioural defences differ interspecifically between O. smaragdina and Polyrhachis, with O. smaragdina appearing to rely primarily on its venom while Polyrhachis use higher rates of grooming. The results show how alternative investment strategies can evolve for disease defence, notably the highly effective application of acidic venom by O. smaragdina, and highlight the need for targeted comparative studies to understand how organisms respond to the ubiquitous threat from parasites.
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Understanding the maintenance and evolution of complex group behavioural systems has broad significance to our understanding of social evolution, yet we have little insight into how these systems have evolved. Nest-weaving, a rare group behaviour considered a pinnacle of cooperative action in social insects, involves the coordination of workers and larvae by incorporating larval silk into the nest structure. To investigate the evolution of this complex behaviour in the ant genus Polyrhachis, we used comparative analysis and an inferred molecular phylogeny based on three mitochondrial genes COI, COII and CytB, and three nuclear genes EF1 a-F2, Wg and Tf. Our results showed that arboreality and nest-weaving are closely associated, but in contrast to the previous hypotheses, represent the ancestral state in the monophyletic genus. Nest-weaving within the genus, moreover, is remarkably labile. Arboreality and nest-weaving have been lost and partially regained on at least two occasions: two non-weaving subterranean species (sister taxa likely reflecting a single evolutionary event) have reverted to arboreal nesting habits without regaining the use of silk nests, while a third subterranean species has transitioned to nesting in silk nests on the sides of rocks, obtaining silk from spiders and not their own larvae. The loss of larval cocoons, which is correlated with the most complex form of nest-weaving behavior as typified in Oecophylla, has occurred independently on at least two occasions within Polyrhachis. The repeated loss of nest-weaving behaviour and its partial regaining within the genus provides the first example of a complex group-level trait that did not arise through behavioural progression from simple to complex states. The evolution and loss of complex group-level traits may be more evolutionarily labile than previously appreciated.
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Complex insect societies are formed by thousands to millions of individuals which incessantly interact in the sheltered nest space under the constant pressure of different challenges, such as infectious disease and group coordination. Understanding how colony members interact in space and time may therefore help unravelling how different evolutionary pressures have moulded insect societies. We investigated the spatial positions and the association network of workers and the queen in the honeybee (Apis mellifera) society using high-resolution recordings of spatial behaviour and ‘network connections’ at the individual level. Our socio-spatial combined approach highlighted a highly compartmentalised structure inside honeybee colony. Connectivity and spatial overlap are high among same-age workers but low among different-age workers. Foragers are at the periphery of the social network, while the colony core is formed by young bees and the queen. Our study presents the first empirical quantitative description of socio-spatial organisation of a honeybee hive, confirming a highly compartmentalised structure and highlighting how the pressures exerted by ergonomics requirements could have shaped insect societies.
Ninety-one species of the genus Polyrhachis are recorded from the Queensland Wet Tropics. Fifty-nine of these represent previously described taxa and thirty-two are recognised as new. Polyrhachis doddi Donisthorpe is considered a junior synonym of P. australis Mayr and P. yarrabahensis Forel a junior synonym of P. lombokensis Emery. The genus Echinopla is represented by two species. Notes on synonymy and distribution are included.
Australian Polyrhachis species of the subgenera Myrma, Myrmatopa, Myrmothrinax and Polyrhachis are reviewed. A total of ten Australian species are recognised; four in the subgenus Myrma, three in Myrmatopa, two in Myrmothrinax and a single species in Polyrhachis. Polyrhachis inusitata Kohout and P. yarrabahensis are reinstated as valid species. Polyrhachis sericeopubescens Donisthorpe and P. lombokensis are considered extralimital and removed from the list of Australian species. Polyrhachis alphea Fr. Smith and Polyrhachis menozzii Karavaiev are reported from Australia for the first time. The extralimital species Polyrhachis dolomedes Fr. Smith is considered a senior synonym of Polyrhachis schang var. amboinae Santschi. The former subspecies, P. relucens var. breviorspinosa Donisthorpe is raised to specific status. A replacement name, Polyrhachis luteogaster, is proposed for the former subspecies and junior primary homonym P. alpheus var. rufiventris Emery. A lectotype of P. semitestacea Emery is designated. All species are illustrated and their known distributions and nesting habits summarised. Keys to the Australian species of the subgenera Myrma, Myrmatopa, Myrmothrinax are included.
Thirty new species of the subgenus Cyrtomyrma are described, including nine from Australia: P. abbreviata sp. nov., P brevinoda sp. nov., P. decumbens sp. nov., P. delecta sp. nov., P expressa sp. nov., P hoelldobleri sp. nov., P monteithi sp. nov., P robsoni sp. nov. and P. rutila sp. nov.; six from Borneo: P. achterbergi sp. nov., P. bruehli sp. nov., P. danum sp. nov., P. lepida sp. nov., P. sulang sp. nov. and P. widodoi sp. nov.; twelve from New Guinea: P. aporema sp. nov., P. barryi sp. nov., P. conspicua sp. nov., P. dorsena sp. nov., P hybosa sp. nov., P. indueta sp. nov., P. inflata sp. nov., P. integra sp. nov., P. kyawthani sp. nov., P. sedlaceki sp. nov., P strumosa sp. nov. and P tuberosa sp. nov., and three from the Solomon Islands: P. pacifica sp. nov., P. setosa sp. nov., and P. undulata sp. nov. Seven subspecies, P. laevissima aruensis Viehmeyer, P. rastellata celebensis Viehmeyer, P. rastellata corporaali Santschi, P. rastellata fulakora Mann, P. rastellata johnsoni Mann, P. rastellata nomo Donisthorpe and P. rastellata semiinermis Donisthorpe are raised to specific status. A neotype of P. australis is designated. Two species, P. rastellata (Latreille) and P. debilis Emery, previously listed from Australia, apparently do not occur there. Checklists and identification keys to the Cyrtomyrma species of Australia, Borneo, New Guinea and the Solomon Islands are provided. All the new species are illustrated and notes on their distribution and nesting habits provided.
The entire distribution of P. mayri seems limited to desert or very dry forest in NE Colombia. Colonies are of moderate size for the genus, averaging 603 workers per nest. Nest density, however, is high, resulting in a biomass comparable to that of Pogonomyrmex with much larger colonies. Nests have hidden entrances and consist of only 2 levels and a few chambers. Nest relocation in the area studied was frequent and executed within 4-5 hr. P. mayri is not granivorous, but relies mostly on dead arthropods for food. -from Authors
The Australian spiny weaver ant Polyrhachis australis Mayr, 1870 is an arboreal formicine ant that constructs nests using silk and particulate matter (carton). Between September 2009 and December 2012, the contents of 181 P. australis nests were recorded. Of these, 14% were queenless, 21% were monogynous and 65% were polygynous. Monogynous nests had more than double the number of brood per queen, but were outnumbered nine to one by polygynous nests in January, just after the reproductive phase and before the heaviest rain of the wet season. This proportion fell close to equal before the start of the next reproductive phase in September, suggesting attrition of queens by competition during the ergonomic phase of the annual cycle that peaks in May. The reproductive phase of the annual cycle started with the appearance of new gynes in September through November. Male production increased in October, peaked in November and December, and declined in January. Hence, gynes were produced before males (protogyny). The pupal sex ratio was heavily male-biased during this time. Males continued to be produced in relatively low numbers throughout the year, supporting (along with other evidence including nest-size limiting habitat structure) the view that P. australis colonies are polygynous and polydomous. The polygynous status of most nests may have arisen more from retention of queens in the mother nest than from adoption. Queen size was bimodal and the microgyne–macrogyne dimorphism may point to a mixed strategy of nest/colony founding whereby macrogynes typically fly and establish colonies independently, while microgynes typically walk to new nest sites and found nests dependently under the protection of a retinue of workers. In support of this inference, all of 49 queens which displayed brachyptery or other wing malformations were microgynes.