Two types of positive phototaxis are observed in prokaryotes. "Tenebrio molitor" by comparison is one species which carries its negative phototaxis into adulthood. This asymmetric positioning of the eyespot in the cell is essential for proper phototaxis. Other eyed nematodes have a negative phototaxis, moving away from light. When phototaxis and gravitaxis have leveled out, the larvae have found their preferred depth. "Platynereis dumerilii" nectochaete larvae can switch between positive and negative phototaxis. The juvenile "M. nigrescens" also has a weak negative phototaxis before its eye has developed. Phototaxis is mediated by the rhabdomeric eyes. In contrast, "Chlamydomonas" phototaxis is mediated by archaeal-type rhodopsins. Signal transduction in phototaxis involves depolarization of the cell membrane. The gravitaxis forms with phototaxis a ratio-chromatic depth-gauge. Their functions range from bacterial photosynthesis (bacteriorhodopsin) to driving phototaxis (channelrhodopsins in flagellates). "S. seticornis" have displayed notable negative phototaxis, meaning the organism prefers to avoid sunlight. Phototaxis there is mediated by two pairs of more complex pigment cup eyes. Caterpillars also use phototaxis to locate fruits to feed on.