Aelian (Claudius Aelianus), On the Nature of Animals, 16, 27 (= BNJ 86F 21b)
“Agatharkhides claims that there is a group of people in Libya–and that they are also called the Psylli–in most ways they aren’t really different from the rest of humankind in their lifestyle except that their body has a strange and incredible quality that sets them apart. You see, creatures that sting as part of attacking don’t harm these people at all!
Indeed, they don’t even notice a snakebite or a spider-bite that might be fatal to other people, nor even a scorpion’s sting. When one of the creatures gets near them and touches them, it acts like it has taken some sleep-causing drug as soon as it smells them! They contract a kind of drowsiness or drugged state and become weak and slow until the person walks away.
Also, when they want to test whether their babies are trueborn or bastards, they leave them in the middle of snakes just as I said above, the way gold-workers test metal in fire.”
᾽Αγαθαρχίδης φησὶν εἶναι γένος ἐν τῆι Λιβύηι τινῶν ἀνθρώπων καὶ μέντοι καὶ καλεῖσθαι αὐτοὺς Ψύλλους. καὶ ὅσα μὲν κατὰ τὸν ἄλλον βίον τῶν λοιπῶν ἀνθρώπων διαφέ̣ρειν οὐδὲ ἕν, τὸ δὲ σῶμα ἔχειν ξένον τε καὶ παράδοξον ὡς πρὸς τοὺς ἑτεροφύλους ἀντικρινόμενον. τὰ γάρ τοι ζῶια τὰ δακετὰ καὶ τὰ ἐγχρίμπτοντα πάμπολλα ὄντα μηδὲν αὐτοὺς μόνους ἀδικεῖν. οὐτε γοῦν ὄφεως δακόντος ἐπαίουσιν οὐτε φαλαγγίου νύξαντος [ὡς] τοὺς ἄλλους ἐς θάνατον οὐτε μὴν σκορπίου κέντρον ἀπερείσαντος. ἐπὰν δὲ ἄρα τούτων προσπελάσηι τι καὶ παραψαύσηι τοῦ σώματος καὶ ἅμα καὶ τῆς ὀσμῆς τῆς ἐκείνων [ψαύσηι ἢ] σπάσηι, ὥσπερ οὖν φαρμάκου γευσάμενον ὑπνοποιοῦ κάρωσίν τινα ἑλκτικὴν ἐς ἀναισθησίαν ἐμποιοῦντος, ἐξασθενεῖ καὶ παρεῖται, ἔστ᾽ ἂν παραδράμηι ὁ ἄνθρωπος. ὅπως δὲ ἐλέγχουσι τὰ ἑαυτῶν βρέφη εἴτε ἐστὶ γνήσια εἴτε καὶ νόθα, ἐν τοῖς ἑρπετοῖς βασανιζοντες ὡς ἐν τῶι πυρὶ τὸν χρυσὸν οἱ βάναυσοι χρυσουργοί, ἀνωτέρω εἶπον.