Digging small hands: Ants in order to gnaw wood to do what adaptive changes | BMC Journal

Title: Skeletomuscular adapts of head and legs of Melissotarsus ants for tunnelling via living wood
Journal:
Adam Khalife, Roberto A. Keller, Johan Billen, Francisco Hita Garcia, Evan P. Economo and Christian Peeters
Published: 2018/08/14
Digital ID: 10.1186/s12983-018-0277-6
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Melissotarsus ants and crustaceans have a wonderful symbiotic relationship that has led to special adaptive changes in these ants The altered ants were able to dig tunnels through gnawing in the standing wood.
's new article on Frontiers in Zoology takes us through the extraordinary morphological specializations of these ants.
thousands of species of ants make their homes from trees, they usually nest in existing holes, which may have been naturally formed or excavated by beetle larvae. Only a few ants can bite dry wood or build nests outside the tree. Merissotarsus worker ants, on the other hand, have created a tunnel in the trees in an extremely clever way by biting, which has led them to establish a symbiotic relationship with the Diaspididae. These small ants, which are about 2 mm long, have enlargement heads, which are wrapped in a large number of muscles and are used to control the opening and opening of the upper jaw during bites. The endoskin protrusion helps to secure muscle fibers that extend from all angles, allowing them to fill up the free space available to the head.
for insects, opening the upper jaw requires little force, so there is a clear asymmetry in the muscles that control the opening and closing of the upper jaw. In Merissotarsus, however, the muscles that open the upper jaw are particularly strong because the worker ants need to push away the gnawed wood as they dig forward. In addition, there have been some changes in the upper jaw shape of melissotarsus ants, allowing the strength of the muscles to be further amplified using the principle of leverage. At the same time, they are combined with nanoscale zinc clusters in a few butyl substates at the end of their upper jaws, which further enhances their digging capacity.
this powerful gnawing ability is also associated with the specialization of the four hind legs and the increase of the muscle-filled base joint (hip joint), which holds the side wall of the tunnel while excavating. The adaptive changes in these limbs are so extreme that the worker ants are unable to walk out of the tunnel. This also basically confirms that these worker ants need to rely entirely on the shield crustaceans in the tunnel for food. In addition, their front legs are covered with hair, which is used to pull the silk from the head glands out. These wires and wood chips are used to repair damaged parts of the tunnel. During evolution, Melissotarsus ants lost their needles, so they could only defend them against this silky structure, especially against other tree-dwelling ants invading dug tunnels to snatch eggs. As soon as we cut through the tunnel dug by the Merissotarsus ants with a knife, they begin to repair immediately, so there is no way we can directly observe other behaviors such as biting wood.most species of ants live in tunnels of wood or dirt, but the adaptability of Merissotarsus is particularly novel. We infer that the leg changes are intended to act as a support during the excavation process. It is worth mentioning that the Melissotarsus queen is in perfectly normal shape, with more typical wings and legs and larger eyes. By contrast, workers have spent their lives digging holes in trees, leading to a high degree of degeneration of their eyes (a phenomenon not yet known among tree-dwelling ants) and extremely simplified brains (especially the visual leaves) to make room for more developed upper jaw muscles.
While thousands of ant species are arboreal, very few are able to chew and tunnel through living wood. Ants of the genus Melissotarsus (subfamily Myrmicinae) inhabit tunnel systems excavated under the bark of living trees, where they keep large numbers of symbiotic armoured scale insects (family Diaspididae). Construction of these tunnels by chewing through healthy wood requires tremendous power, but the adaptations that give Melissotarsus these abilities are unclear. Here, we investigate the morphology of the musculoskeletal system of Melissotarsus using histology, scanning electron microscopy, X-ray spectrometry, X-ray microcomputed tomography (micro-CT), and 3D modelling.Both the head and legs of Melissotarsus workers contain novel skeletomuscular adaptations to increase their ability to tunnel through living wood. The head is greatly enlarged dorsoventrally, with large mandibular closer muscles occupying most of the dorsal half of the head cavity, while ventrally-located opener muscles are also exceptionally large. This differs from the strong closing: opening asymmetry typical of most mandibulated animals, where closing the mandibles requires more force than opening. Furthermore, the mandibles are short and cone-shaped with a wide articulatory base that concentrates the force generated by the muscles towards the tips. The increased distance between the axis of mandibular rotation and the points of muscle insertion provides a mechanical advantage that amplifies the force from the closer and opener muscles. We suggest that the uncommonly strong opening action is required to move away crushed plant tissues during tunnelling and allow a steady forward motion. X-ray spectrometry showed that the tip of the mandibles is reinforced with zinc. Workers in this genus have aberrant legs, including mid- and hindlegs with hypertrophied coxae and stout basitarsi equipped with peg-like setae, and midleg femura pointed upward and close to the body. This unusual design famously prevents them from standing and walking on a normal two-dimensional surface. We reinterpret these unique traits as modifications to brace the body during tunnelling rather than locomotion per se.Melissotarsus represents an extraordinary case study of how the adaptation to – and indeed engineering of – a novel ecological niche can lead to the evolutionary redesign of core biomechanical systems.(
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(Source: Science.com)