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We are interested in a time harmonic acoustic problem in a waveguidecontaining flies. The flies are modelled by small sound soft obstacles. Weexplain how they should arrange to become invisible to an observer sendingwaves from $-\infty$ and measuring the resulting scattered field at the sameposition. We assume that the flies can control their position and/or theirsize. Both monomodal and multimodal regimes are considered. On the other hand,we show that any sound soft obstacle (non necessarily small) embedded in thewaveguide always produces some non exponentially decaying scattered field at$+\infty$ for wavenumbers smaller than a constant that we explicit. As aconsequence, for such wavenumbers, the flies cannot be made completelyinvisible to an observer equipped with a measurement device located at$+\infty$.

Team organization may help swarms of flies to become invisible in closed waveguides