Barnacle larvae are sensitive to many environmental factors. Many species attach only to particular types of surfaces. The cyprid larvae of Balanus amphitrite, B. improvisus, and Chthamalus fragilis are sensitive to light at the time of attachment and react negatively to it. These larvae do not attach at random, but "test out" the surface before attaching, in which process they have been observed to spend a very considerable period of time. The antennae are the "feelers" as well as the final "holdfasts" during these reactions.