There are many proposed explanations, fast moving insects, tricks of light etc, they have even been labelled as spirits, but are they a life form? Are there insects that grow up to a metre long? Surely if they were living Insects then we would have found evidence in the form of corpses or remnants from dead or consumed sources.

They have appeared in a variety different lighting, locations, time, day and night and weathers, so the argument that they are tricks of the light must surely also be out of the question.
Take a look at the videos below, see what you think !

The Cave of Swallows, also called (Spanish: Sótano de las Golondrinas), is a pit cave situated in San Luis Potosí, Mexico. The entrance lies along a sloped section of karst, is elliptical in shape, and measures approximately 160 by 205 feet (49 by 62 m).[1] The entrance is undercut around its entire perimeter, and the pit opens up to a room that measures approximately 995 by 440 feet (303 by 134 m).[1] The low point of the entrance provides a 1094 feet (333 m) freefall drop to the bottom of the cave while the high point provides a 1220 feet (372 m) freefall drop. [1] These measures make it the largest cave shaft in the world, second deepest pit in Mexico and the 11th deepest in the world

from Wikipedia