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Ants and sap-sucking insects have been known to coexist in the presence of sap. The ant colony would provide a safe location for the insect, which would suck the tree sap and produce honeydew, which can constitute a significant part of the colony’s diet. However, researchers have discovered that one particular species, Melissotarsus emeryi, can coexist with armored scale insects that do not produce honeydew.
The species in question, Morganella conspicua, can occur in large quantities within ant galleries. Members of the species that live within colonies tend to lack shields, but rare individuals would still grow them, feeding the ants with the proteins and wax used to construct the layer. Both the unique species of ants and the behavior of the M. conspicua are of interest, mainly due to the harmful influence of mutualism on plants.
The Dependence of the Ants
The Melissotarsus emeryi species represents an unusual mutation, as the ants are unable to survive without relying on other insects. According to Peeters et al. (2017), the workers cannot walk on flat surfaces and have to stay confined within the galleries they dig. To compensate for the weakness, members of the genus can chew through living wood as well as spin silk to seal gaps. As such, the species tends to construct galleries within trees and inhabit them. This tendency can create significant issues for the plants in question, as Melissotarsus emeryi do not provide their host trees with the benefits observed in other species (Peeters et al. 2017). Further research is necessary to identify the potential danger represented by the species.
The Nature of the Mutualism
M. conspicua can be found within Melissotarsus emeryi colonies in large numbers. Unusually for armored scale insects, most inhabitants of the tunnels do not possess the shields that characterize their free-living counterparts. However, the few individuals that display shields and were discovered by the researchers provided a clue as to the nature of the coexistence. The ants harvest and consume the materials that the M. conspicua secrete to construct their shells, as the latter mostly stop building efforts despite still producing wax and proteins.
In return, they provide their partners with the safety of the galleries as well as easy access to plant tissue. Previous theories suggested that the reason the species stopped growing shields was solely the lack of necessity due to the safe environment provided by the tunnels.
The ants do not attempt to concentrate their eggs and larvae, dispersing them throughout the colonies. Furthermore, Peeters et al. (2017) observe that they make no effort to protect the brood when disturbed. As ant larvae tend to be immobile, they would require food to be brought to them. Furthermore, they would be unable to consume an M. conspicua within a single feeding session. As such, the fact that the researchers never observed any larvae near a partially eaten crawler’s body indicates that the ants do not treat the bugs themselves as food. However, no dead specimens of M. conspicua have been observed, suggesting that the ants remove the bodies and consume them immediately after death.
Diaspidids as Cattle
M. conspicua are capable of living on their own, albeit with considerably higher mortality than in ant tunnels. Melissotarsus emeryi, on the other hand, are only able to coexist with several diaspidid species, with only M. conspicua having ever been a subject of study. As such, the ants have adapted to support the relationship while the armored scale insects have not. According to Peeters et al. (2017), the ants take care of the diaspidids’ hygiene and may apply antimicrobial secretions. They also immediately remove and consume dead individuals but appear to rarely or never predate on healthy specimens, contributing to the survival rate of the M. conspicua and further reinforcing the cattle parallel.
Shields in Colony-Living Specimens
Some M. conspicua have grown shields despite being located within the colonies. The most likely cause is confusion caused by some individuals’ distance from dense aggregations of same-species insects. The ants tend to organize M. conspicua in close gatherings, where they never construct shields, but the females still give birth. On the other hand, relatively isolated specimens sometimes built a shell, albeit a thinner one than usual (Peeters et al. 2017). The findings are supported by earlier research that notes the disturbances in the construction of shields due to physical and biological factors.
Melissotarsus emeryi are among the rare ant species that can chew through wood. The ability allows them to disassociate themselves from the honeydew-reliant mutualisms that are employed by other species and form relationships with diaspidids. The ants have adapted for the purpose and become unable to survive outside of their colonies, while M. conspicua can live on their own. Nevertheless, the partnership significantly benefits both parties, although it can be harmful to the trees they inhabit. While the research is relatively new and requires further investigation, the topic of mutualism in the ants has been the subject of considerable interest, and many articles are available on the matter.
Peeters C, Foldi I, Matile-Ferrero D, Fisher BL. 2017. A mutualism without honeydew: what benefits for Melissotarsus emeryi ants and armored scale insects (Diaspididae)? Peer J. Web.