Rhopalomastix is only the second ant genus known to live with armoured scale insects (Diaspididae)

Abstract

Rhopalomastix is a myrmicine ant distributed throughout tropical parts of Asia, with almost nothing known about its biology. Its closest relative Melissotarsus in Africa is the only ant genus known to live with diaspidid scale insects inside their nests, and to rely on these for food. Twelve colonies of four species of Rhopalomastix were sampled from seven tree genera in Singapore. Ants chewed tunnels under live bark, and these were occupied by many diaspidids belonging to five genera; at least two species are known to also live freely on the outside of trees. Inside ant nests, only a few diaspidids secreted their trademark shields. The association with the pupillarial Fiorinia is unexpected, since they have fewer secretions that can be used as food by the ants. Rhopalomastix and Melissotarsus workers share several morphological adaptations, including powerful mandibles and silk glands, but while the legs of Melissotarsus are modified for traction during tunnelling, those of Rhopalomastix are not. Consequently, Rhopalomastix workers are able to walk outside their tunnels, but they behaved timidly except in R. murphyi. Silk was used to repair damaged tunnels, and the spinning behaviour resembled that of Melissotarsus. Host trees do not gain protection from the ants against leaf herbivores, so they apparently get no benefits from this mutualism. We discuss the likely economic impact of this parasitic lifestyle, especially on fruit trees.

Glandular innovations for a tunnelling life: Silk and associated leg glands in Melissotarsus and Rhopalomastix queen and worker ants

“…Rhopalomastix sp. was collected throughout Singapore, with a range of host trees as well as partner diaspidid genera (details in Yong et al, 2019). Rhopalomastix janeti was collected in Cha-Om, Khaeng Khoi district, Saraburi province, Thailand.…”

“…Highly modified coxae and basitarsi of mid-and hindlegs in Melissotarsus indicate a total commitment to life inside tunnels, resulting in workers being unable to walk and forage outside (Khalife et al, 2018). In contrast, Rhopalomastix lack such specialized legs and can walk on the outside of host trees (Yong et al, 2019). The existence of the Delage-Darchen gland in the basitarsus of all legs in Melissotarsus is a further difference between these sister genera.…”

“…Melissotarsus and Rhopalomastix engineered a novel ecological niche: these minute ants (2-2.5 mm) live permanently in live wood and feed on diaspidid scale insects (Peeters et al, 2017 and references therein;Yong et al, 2019). The two genera group together on a separate branch within Tribe Crematogastrini (subfamily Myrmicinae) (Blaimer et al, 2018), and are distinguished by a unique combination of morphological features absent in other Myrmicinae (Bolton, 1982).…”

Revision of the elusive ant genus Rhopalomastix (Hymenoptera, Formicidae, Myrmicinae) in Thailand based on morphology and DNA barcodes, with descriptions of three new species

“…Ants of this genus can be distinguished by unique morphological characters also seen in their Afrotropical cousin Melissotarsus Emery, 1877, such as: 1) median section of clypeus not projecting back between antennal sockest, 2) very short antennae, each with a somewhat flattened funiculus and an expanded, robust club comprising two apical segments, 3) frontal lobes short, narrow and very closely approximated, separated by a small impressed line, 4) mesosoma short and box-like, dorsum without sutures or conspicuous deep impressions, usually finely striated, 5) middle and hind basitarsi short and stout, 6) propodeal lobes absent (Bolton 2003;Wang et al 2018b). Moreover, both genera are the only ants known to live with scale insects (Hemiptera: Diaspididae) in their nests, with which they seem to share an obligately mutualistic relationship (Peeters et al 2017;Yong et al 2019).…”

“…Overall, Rhopalomastix seems to be more 'generalized' in morphology with less conspicuously developed or specialized characters (Bolton 2003). Furthermore, while both genera have been found to exhibit silkspinning behaviour (Peeters et al 2017;Yong et al 2019), and possess silk-producing glands in their heads (Billen & Peeters 2020), Rhopalomastix appears to have silk glands with relatively fewer secretory cells and lower secretory activity (Billen & Peeters 2020).…”