A group of aërial demons, primarily foes of Indra, are the Panis. The proper meaning of the word is "niggard," especially in regard to sacrificial gifts. From this signification it developed the mythological sense of demons resembling those originally conceived as withholding the treasures of heaven. The term dasa or dasyu, properly the designation of the dark aborigines of India contrasted with their fair Aryan conquerors, is frequently used in the sense of demons or fiends. By far the most conspicuous of the individual aërial demons of the Rigveda, is Vritra, who has the form of a serpent, and whose name means "encompasser." Another demon mentioned with some frequency is Vala, the personification of the mythical cave in which the celestial cows are confined. In post-Vedic literature these two demons are frequently mentioned together and are regarded as brothers slain by Indra. The most often named among the remaining adversaries of Indra is Çushna, the "hisser" or "scorcher." A rarely-mentioned demon is Svarbhanu, who is described as eclipsing the sun with darkness. His successor in Sanskrit literature was Rahu, regarded as causing eclipses by swallowing the sun or moon. The second class of demons consists of goblins supposed to infest the earth, enemies of mankind as the Asuras are of the gods. By far the most common generic name for this class is Rakshas. They are hardly ever mentioned except in connection with some god who is invoked to destroy or is praised for having destroyed them. These goblins are conceived as having the shapes of various animals as well as of men. Their appearance is more fully described by the Atharvaveda, in which they are also spoken of as deformed or as being blue, yellow, or green in colour. According to the Rigveda they are fond of the flesh of men and horses, whom they attack by entering into them in order to satisfy their greed. They are supposed to prowl about at night and to make the sacrifice the special object of their attacks. The belief that the Rakshases actively interfere with the performance of sacrificial rites remains familiar in the post-Vedic period. A species of goblin scarcely referred to in the Rigveda, but often mentioned in the later Vedas, are the Piçachas, described as devouring corpses and closely connected with the dead.