The antibiotic-producing metapleural gland has been shown
experimentally to have a major role in the resistance of leafcutting
ants to fungal parasites (Poulsen et al. 2002) and is
generally thought to be important to the disease resistance
of ants in general (Boomsma et al. 2005; Schlüns and
Crozier 2009). However, we found that the lack of a
metapleural gland did not mean that the weaver ant P. dives
is more susceptible to parasites. In fact, P. dives workers
were considerably more resistant to the Metarhizium fungal
parasite than A. echinatior leaf-cutting ants. As the parasite
used is a generalist entomopathogen that all the ant species
tested will encounter, and, as the strain used was exotic to
all species, the species differences found cannot be due to
host–parasite coevolution. The results therefore do not
support the hypothesis that weaver ants have lost the
metapleural gland as part of an evolution of reduced
investment in disease resistance in response to a lower
parasite threat associated with an arboreal or weaving
lifestyle (Johnson et al. 2003; Boomsma et al. 2005).
An alternative explanation for the loss of the metapleural
gland in weaver ants is that they have evolved to rely on
alternative resistance mechanisms. The self-grooming
results provide support for this. In all species, selfgrooming
and allogrooming occurred to a similar extent
after application of Triton-X control solution and Metarhizium
suspension, indicating that the response was to the
presence of a foreign substance on the cuticle rather than
recognition of the parasite.Workers of P. dives self-groomed
substantially more than any of the other ant species when
kept individually in Experiment 1 and also self-groomed
significantly more than A. echinatior workers when kept in
groups in Experiment 2. Self-grooming has been shown to
be effective at removing fungal spores from the cuticles of
ants and other social insects (Hughes et al. 2002; Yanagawa
et al. 2008). It is possible that the ant species may differ in
the efficiency of conidia removal by grooming, but the
higher survival of P. dives suggests that its more frequent
self-grooming does improve its disease resistance. It is
notable that of the four ant species examined, A. echinatior
had the lowest frequency of self-grooming and also had the
lowest resistance to Metarhizium when ants were kept
individually. There was much less difference in resistance
between P. dives weaver ants and A. echinatior leaf-cutting
ants when ants were kept in groups. In contrast to the selfgrooming
frequencies, A. echinatior had higher rates of