Hybrid robot systems that cooperate with live organisms is an active area of research, in part to leverage biological advantages such as adaptivity, resilience, and sustainability. Formica Forma explores new possibilities of codesigning and cofabricating in partnership with Western Harvester ants to build forms that would be challenging with industrial techniques.Using a robotically controlled UV light (350-405 nm) as an environmental stimulus to bias digging behavior, we guide 600 ants to dig ~141 cm of tunnels in transparent ant gel over 646 hours. Predictability, fidelity to the UV source, repeatability, dig efficiency, amount of ant activity, and tunnel preference were studied. The resulting branching tunnels were cast in silicone to demonstrate the ability to harness this in subtractive fabrication with inexpensive, self-maintaining biological fabricators.Results showed that ants can follow the UV light as a path guide (when the light is moving) or target it as a goal (when the light is both moving and static), with longer digging effect from the moving UV stimulus. Ants showed high fidelity to the light path, aligning their tunnel direction exactly with changes in the UV position, tuning the fabrication in real-time with environmental alterations. Population size did not seem to affect digging speed or efficiency, and the ants' preexisting preferences factored into which tunnels were dug out.The research develops a hybrid biodigital way of working with biological swarms where the individual agency and the intrinsic stochasticity of the system offer possibilities in real-time adaptability and programmability through environmental templating.