Ibacus peronii, the Balmain bug or butterfly fan lobster, is a species of slipper lobster. It lives in shallow waters around Australia and is the subject of small-scale fishery. It is a flattened, reddish brown animal, up to 23 cm (9 in) long and 14 cm (6 in) wide, with flattened antennae and no claws.In common with other slipper lobsters, Ibacus peronii has a broad, flattened body and a large carapace. The carapace is reddish brown, and reaches lengths of 2–8 centimetres (0.8–3.1 in), with the whole animal able to reach a length of 23 cm (9 in), and a width of 10–14 cm (3.9–5.5 in). The antennae are also short and broad, and the flattened form of the whole animal allows it to partly bury itself in soft substrates. There are no claws on the five pairs of legs.