Although mammals use retinal exclusively as the opsin chromophore, other groups of animals additionally use four chromophores closely related to retinal: 3,4-didehydroretinal (vitamin A2), (3"R")-3-hydroxyretinal, (3"S")-3-hydroxyretinal (both vitamin A3), and (4"R")-4-hydroxyretinal (vitamin A4). Many fish and amphibians use 3,4-didehydroretinal, also called dehydroretinal. With the exception of the dipteran suborder Cyclorrhapha (the so-called higher flies), all insects examined use the ("R")-enantiomer of 3-hydroxyretinal. The ("R")-enantiomer is to be expected if 3-hydroxyretinal is produced directly from xanthophyll carotenoids. Cyclorrhaphans, including "Drosophila", use (3"S")-3-hydroxyretinal.