Which statement correctly describes the difference in fang orientation between mygalomorph and araneomorph spiders?

Which statement correctly describes the difference in fang orientation between mygalomorph and araneomorph spiders?

• A. Mygalomorphs have labidognathous fangs; araneomorphs have orthognathous fangs.
• B. Mygalomorphs have parallel (orthognathous) fangs; araneomorphs have labidognathous fangs.
• C. Both groups have orthognathous fangs.
• D. Both groups have labidognathous fangs.

Answer: (B)

The main idea is how the spider’s chelicerae are built to strike. Mygalomorph spiders have fangs that point downward and lie in parallel, because their chelicerae move in a vertical plane. This orientation is called orthognathous. Araneomorph spiders, on the other hand, have fangs that oppose each other and cross as they close, due to their chelicerae pivoting in a way that brings the fangs together from opposite sides. This arrangement is labidognathous. So the difference is parallel, downward fangs in mygalomorphs versus opposing, crossable fangs in araneomorphs.

The main idea is how the spider’s chelicerae are built to strike. Mygalomorph spiders have fangs that point downward and lie in parallel, because their chelicerae move in a vertical plane. This orientation is called orthognathous. Araneomorph spiders, on the other hand, have fangs that oppose each other and cross as they close, due to their chelicerae pivoting in a way that brings the fangs together from opposite sides. This arrangement is labidognathous. So the difference is parallel, downward fangs in mygalomorphs versus opposing, crossable fangs in araneomorphs.