Albert Barnes on Acts 28:3

Had gathered a bundle of sticks. For the purpose of making a fire.

There came a viper. A poisonous serpent. Mt 3:7.

The viper was, doubtless, in the bundle of sticks or limbs of trees which Paul had gathered, but was concealed, and was torpid. But when the bundle was laid on the fire, the viper became warmed by the heat, and ran out, and fastened on the hand of Paul.

And fastened on his hand. καθηψε. This word properly means, to join one's self to; to touch; to adhere to. It might have been by coiling around his hand and arm; or by fastening its fangs in his hand. It is not expressly affirmed that Paul was bitten by the viper, yet it is evidently implied; and it is wholly incredible that a viper, unless miraculously prevented, should fasten himself to the hand without biting.