firefly

tulikärpänen Any beetle of the family Lampyridae, which exhibit bioluminescence during twilight.
2000, Exploring Life Science, 2: Bats—Cave Life, Marshall Cavendish, http://books.google.com.au/books?id=gqcX6n4rW0EC&pg=PA96&dq=%22firefly%22%7C%22fireflies%22&hl=en&sa=X&ei=YJsZUsSEIMqhkwXtnIGYDw&redir_esc=yv=onepage&q=%22firefly%22%7C%22fireflies%22&f=false page 96,

Fireflies are the best-known insects that produce light.(..)Many fireflies use their light to communicate, particularly at mating time when males and females have to get together.

The luciferins are different in different species and today both the luciferin and luciferase in the firefly are available in the crystalline form.

Various other luciferase genes, especially the firefly luciferase gene, are also used for the study of gene regulation.(..)The classic use of bioluminescence, however, is the use of a luciferin-luciferase mixture from fireflies as an assay for ATP.