The Lord sent fiery serpents among the people— Bochart takes these serpents to have been of that kind which is called Hydrus, or Chersydrus, whose bite dries up the skin, and occasions a violent heat; whence the Hebrew calls them fiery: their poison is more inflammatory in the hot months, as this was, being the month of August. The same author shews, that some species of them were flying serpents, of which Isaiah speaks, ch. Numbers 14:29 and Numbers 30:6 and with such Arabia particularly is said to be infested. Accordingly Herodotus tells us, that he had seen these winged or flying serpents in Egypt, and that there are such prodigious numbers of them in Arabia, that, if they were to increase according to the usual course of nature, men could not exist for them; see Herodotus, lib. 2: cap. 75, &c. Bochart Hieroz. par. 2: lib. 3: cap. 13 and Calmet on serpents. These serpents might have been called שׂרפים seraphim, burning; either from the heat and burning pain occasioned by their bite, or, as is more likely, from their vivid, fiery colour: accordingly, Strabo, Geog. lib. 16: has taken notice of a kind of serpents produced near the parts where the Israelites journeyed, which might be called fiery from their colour οφεις φοινικοι την χροαν; and both he and Diodorus were of opinion that the bite of these was incurable.
See commentary on Numbers 21:9