Labium is a Latin-derived term meaning "Lip". Labium and its derivatives (including labia, labial, labrum and labellum) are used to describe any lip-like structure, but in the, often specifically refers to parts of the vulva. A variant of the word, labellum (plural: labella), is used by people studying entomology and botany. The labia are anatomical structures that are part of the female genitalia; they are the major externally visible portions of the vulva. In humans, there are two pairs of labia: the outer labia, or labia majora are larger and fattier, while the labia minora are folds of skin often concealed within the outer labia. The labia surround and protect the clitoris and the openings of the vagina and urethra. When standing or with the legs together, they usually entirely or partially cover the other parts of the vulva. Protection is the main function. The labia majora are homologous to the scrotum in males. The coloration, size and general appearance of the labia can vary extensively from woman to woman. In some women the labia minora are almost non-existent, and in others they can be fleshy and protuberant. Usually, but not always, they are symmetrical. Some differences are purely personal, while others may be genetically linked; a striking example of the latter is the elongated labia minora of the Khoisan peoples, whose "khoikhoi aprons" can hang down up to 10 cm (four inches) past their labia majora when they are standing. During sexual arousal, the labia minora become engorged with blood, typically swelling slightly and darkening or reddening in color.