What type of front fangs, hinged and hollow, characterizes vipers?

The hinged, hollow front fangs characteristic of vipers are solenoglyphous.

Solenoglyphous fangs are long, hollow teeth mounted on short, rotating maxillary bones. When a viper’s mouth is closed, the fangs fold backward against the roof of the mouth. As the snake opens wide to strike, linked bones rotate the fangs forward into an erect position, allowing them to penetrate prey deeply and inject venom efficiently.

This arrangement is distinctive among the major snake-fang categories. Aglyphous snakes lack specialized venom-injecting fangs; opisthoglyphous snakes have grooved rear fangs; and proteroglyphous elapids, including cobras, have shorter fixed front fangs. Vipers therefore combine a front-fanged position with a hinged, tubular delivery system.

The system lets vipers store unusually long fangs inside a relatively small mouth. Their venom glands lie toward the rear of the upper jaw, and muscles surrounding the glands help force venom through ducts and into the hollow fangs. The term solenoglyphous comes from roots meaning “pipe-grooved,” referring to the enclosed channel rather than an open groove.

Atractaspis is a notable exception to the usual pattern: its fangs are solenoglyphous but can swing sideways, enabling a different striking method.