Abstract
Mutualisms between ants and sap-sucking insects generally involve clear benefits for both partners: the ants provide protection in exchange for honeydew. However, a single ant genus associates with armoured scale insects (Diaspididae) that do not excrete honeydew. We studied three colonies of Melissotarsus emeryi ants from two localities in Mozambique. Vast numbers of the diaspidid Morganella conspicua occupied galleries dug by the ants under the bark of living trees. Unlike free-living M. conspicua and other diaspidids, M. conspicua living with ants are known to lack shields, likely because they gain protection against enemies and desiccation. Nevertheless, we documented the occurrence of rare individuals with shields inside ant galleries, indicating that their glands continue to secrete wax and proteins as building material. This is likely to constitute a significant portion of the ants’ diet, in addition to diaspidid exuviae and excretions from the Malpighian tubules. Indeed, Melissotarsus workers cannot walk outside the galleries due to modified middle legs, forcing them to obtain all nourishment within the tree. Melissotarsus founding queens, however, must locate a suitable host tree while flying, and acquire diaspidid crawlers. This mutualism involves ants that are highly specialised to chew through living wood, and diaspidids that can also live freely outside the bark. It is extremely widespread in Africa and Madagascar, recorded from 20 tree families, and harmful effects on plant hosts require rapid study.
Highlights
Mutualisms with sap-sucking insects are one of the many fascinating adaptations that have elevated ants to pre-eminent ecological importance
We identified the diaspidids as Morganella conspicua (Brain), making it the same association that was studied in South Africa (Prins, Ben-Dov & Rust, 1975; Schneider et al, 2013)
This genus is able to chew through living wood, and, uniquely among ants, adult workers can secrete and spin silk to seal off gaps in their galleries (Fisher & Robertson, 1999; Figure 5 Various shields of Morganella conspicua inside ant galleries. (A) Nymphal exuviae are incorporated in center of shield; (B) Shield lifted up, revealing two crawlers inside; (C) Shield with exuvia, several shield-less adults and an ant larva; (D) Two shields next to adults and one ant egg
Summary
Mutualisms with sap-sucking insects are one of the many fascinating adaptations that have elevated ants to pre-eminent ecological importance. Scale insects are inherently vulnerable to predators and parasitoids because they cannot quickly withdraw their elongated sucking stylets to escape. For this reason, they can benefit greatly from ant protection, with much variation. How to cite this article Peeters et al (2017), A mutualism without honeydew: what benefits for Melissotarsus emeryi ants and armored scale insects (Diaspididae)? When the mealybugs require better feeding sites, the entire colony relocates In contrast to such nomadic habits, many other ants keep scale insects permanently in their domatia (i.e., specialized plant structures that house ant colonies) (Grimaldi & Engel, 2005). The singular association with Melissotarsus ants is a puzzle since diaspidids excrete no honeydew
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