Schematic of ant worker chewing a tunnel using their specialised legs to brace themselves against the walls while tunnelling.

Thousands of ant species live in trees but they often nest in existing cavities (provided by host trees, or excavated by beetle larvae). Only a few species can chew dry wood, or build nest structures outside trees.Melissotarsusworkers are exquisitely adapted for chewing tunnels in living wood, and this underlies their obligate mutualism with scale insects (Diaspididae). These minute ants (2 mm-long) have enlarged heads packed with muscles to open and close the mandibles during chewing. Cuticular structures (‘apodemes’) function to capture muscle fibres coming from a variety of angles and filling all available space in the head.

Insects normally require little power to open the mandibles, so there is a strong asymmetry in size between closer and opener muscles. However, opener muscles are exceptionally large inMelissotarsus，because workers need to move away crushed woody tissues as they tunnel forward. Moreover, the modified shape of their mandibles amplifies the force from muscles through lever effects. And their tip is reinforced with nanoscale clusters of zinc bound into the chitin matrix.

强大的咀嚼与四个后腿的新专业化有关，四肢在隧道旁进行了修饰，以靠在墙壁上，并扩大了充满肌肉的基础段（“ Coxae”）。这些腿部改编非常极端，以至于工人不能在隧道外面走路，从而为我们提供了可靠的信息，使他们完全依靠on the diaspidid scale insects inside their tunnels for food. Moreover, the forelegs are very hairy, an adaptation to spin the silk that is produced by glands inside the head. Breaches in tunnels are repaired using silk combined with wood shavings.Melissotarsusworkers have lost their sting during evolution, and silken constructions are their only defence against insect enemies (especially other arboreal ants) that readily invade damaged tunnels to pillage brood. Because workers begin repairs as soon as we open tunnels with a knife, direct observations of wood-chewing and other behaviours are impossible.

Functional morphology

The majority of ant species live in tunnels (in soil or in wood), but the leg adaptations ofMelissotarsusworkers are novel. We used our results on leg modifications to infer a bracing function during tunnelling activities. Importantly,Melissotarsusqueens have completely normal morphology (typical legs and wings, large eyes), allowing for rigorous comparisons with workers. The irreversible commitment of workers to life inside living trees has also led to highly reduced eyes (unknown in arboreal ants) and a simplified brain (optic lobe especially), making room for larger mandible muscles. Such extreme specialization in morphology is possible in a social insect, because the queen caste remains able to disperse by flight and walk outside host trees during the initial stages of colony foundation.