阅读理解： Firefly is just a name．They are actually very cool insects．And they are not even flies, they are beetles．They have a way of sending out light signals every now and then．If you see them at night on a tree, you might think it is a brightly lit Christmas tree．That is the kind of light these fireflies produce． But the light they send out does not have heat, like the bulbs we use．The firefly's light is cold。Aetually these beetles could be the most efficient bulbs if only we knew how to use them．For almost all the energy they produce gets changed into bright lightcool light． How do they do it? The back portion of their stomachs are transparent．There are cells of crystals or chemical substances inside which get broken down by enzymes．Some energy is released in the form of light． The fireflies send out 1ight signals at fixed intervals．And the purpose behind it is to attract beetles for mating．But one firefly's signal is different from another's, depending on its sex and the type of family it belongs to．If a firefly sends a signal at a fixed interval but gets a response either too soon or too late, it will ignore the signal．It is like a perfect dance sequence．If you are faster or slower than your partner, the dance fails． Fireflies come from two families of beetles．One family of fireflies is usually found in Europe．North Ameriea and Australia．They are found on trees．The other family of fireflies is found in the Pacific islands．There is another type of beetle which gives out a softer 1ight．It is called the glow worm． | 
