when the need arises

: necessary duty : obligation

: a lack of something requisite, desirable, or useful

: a physiological or psychological requirement for the well-being of an organism

: to be needful or necessary

: to be in want

: to be in need of : require

: to begin to occur or to exist : to come into being or to attention

: to originate from a source

: to get up or stand up : rise