Evolution of nest-weaving behaviour in arboreal nesting ants of the genus Polyrhachis Fr. Smith (Hymenoptera: Formicidae)

Polyrhachis ants represent one of the most taxonomically and ecologically diverse ant genera, with over 500 described species organised into 12 subgenera. Nesting habits range from subterranean localities to arboreal nests incorporating silk produced by the ants' own larvae (nest weaving). In this article, we combine scanning electron microscopy and gross observations of the nests of 35 species representing nine subgenera thought to contain individuals that nest above the ground (Cyrtomyrma, Hagiomyrma, Hedomyrma, Hemioptica, Myrma, Myrmatopa, Myrmhopla, Myrmothrinax, Polyrhachis) to revaluate the relationship between nest locale, the type of nest material used and the use of larval silk for nest construction. Nesting habits are highly diverse, ranging from truly arboreal nests on or between leaves and branches, to lignicolous nests inside hollow stems or bamboo internodes and lithocolous nests on the sides of rock walls. Flat sheets of larval silk are used only by arboreal nesting species within the subgenera Cyrtomyrma, Hemioptica, Myrma, Myrmatopa, Myrmothrinax and Polyrhachis. Lignicolous nesting habits were demonstrated predominantly by Hedomyrma spp., but these habits also occur in Myrma and Myrmhopla. Lithicolous nesting habits occur within Hagiomyrma and Hedomyrma, though the actual nesting material used can be either carton or dense masses of spider silk. Based on existing phylogenetic hypotheses, the use of larval silk for nest construction has evolved independently within the genus, and has evolved independently of the construction of silk nests per se. Further examination of the exact type of silk found in the colonies of 'nest-weaving' Polyrhachis is warranted

Comparative nesting biology of two species of Australian lithocolous ants: Polyrhachis (Hedomyrma) turneri Forel and P. (Hagiomyrma) thusnelda Forel (Hymenoptera: Formicidae: Formicinae)

The nests of both Polyrhachis (Hagiomyrma) thusnelda and P. (Hedomyrma) turneri are restricted to the sides of rock walls (lithocoly), but, despite the similar nesting localities, the nests differ in the types of construction material used. The walls of nests of P. thusnelda are constructed of carton material without any silk, while those of P. turneri are comprised of a thick and diffuse mass of fluffy silk with very little carton. However, unlike other species of Polyrhachis that live in silk nests, the silk material in nests of P. turneri is produced entirely by spiders and not their own larvae. Scanning electron microscopy indicated that the silk strands found in natural P. turneri nests resembled those of nearby spider egg sacs rather than the strands found in the cocoons of their own larvae, and captive brood-right colonies of P. turneri failed to produce silk nests in the absence of an external silk source. These results: confirm the existence of an additional nesting habit within the genus, lithocoly; suggest there are at least two ways by which lithocolous nesting habits can evolve, carton vs. silk; and indicate that the presence of silk within a nest need not imply that ant larvae are the source. Studies of the distribution and evolution of nest-weaving within the genus Polyrhachis may need to be reassessed, now that the presence of silk nests could actually represent the origin of two independent behavioural traits (silk from larvae vs. silk from external sources such as spiders)