Manna is believed to fallen from heaven to feed the Israelites in the Sinai during the Exodus from Egypt.
In 1927, an expedition set out from Jerusalem into the Sinai to find the origin of manna. They found two sources of the sweet manna: the excretions of plant-lice (Aphidae) and scale-insects (Coccidae). These and other manna-producing insects live on Tamarix trees and the manna gets blown off into the winds. Chemical analyses indicate glucose, fructose and saccharose with a trace of pectin [Nature 124:1003-1004. 1929].
Wind blown manna from the tamarisks of Oman has been traced across Arabia and the Persian Gulf and into Iran. [Nature55: 440. 1897]. Tamarisks now provide the same manna along most of the arid waterways of the American arid lands.
Insect excretions, algae and lichen –- three stories of the meaning of manna. Call them manna-hypotheses. Perhaps anything that falls from the sky that you can eat is manna.