The salpuga is found in some medieval encyclopedias. It is said to be a serpent that is invisible, or to be so small that it is difficult to see, or to be an eyeless serpent that is not easily seen. Its identification as an invisible serpent is based on Isidore of Seville's description, which was possibly taken from Lucan, where the salpuga is not described physically but is in a section on serpents; Isidore may have thought because of this that a serpent was intended. Other authorities say the salpuga is a poisonous spider or ant.

The salupa signifies the vice of a hidden grudge, which infects many without even realizing it, and while the guilt of the neighbor is not forgiven from the heart, the evil that gradually springs up in hatred is sought for revenge, not paying attention to what was said by the Lord: Unless you forgive men's sins from your hearts, neither will your heavenly Father will forgive you your sins. [Thomas of Cantimpré]