What does the term 'orthognathous' describe in spiders?

• A. Fangs that are parallel and oriented downward.
• B. Wings on the thorax.
• C. Leg segments fused into a tagma.
• D. Pincer-like fangs of araneomorph spiders.

Answer: (D)

Orthognathous describes how the spider’s fangs (chelicerae) are oriented. When a spider is orthognathous, the fangs point straight downward and run parallel to each other, so they strike in a vertical, downward motion. This arrangement is typical of spiders in the older, more primitive lineage (the mygalomorphs, like tarantulas and trapdoor spiders).

In contrast, araneomorph spiders have fangs that face toward each other and operate like pincers; this is called a labidognathous arrangement. That configuration allows a cross or pinching action as the fangs come together.

So, orthognathous specifically refers to the downward, parallel fang orientation, whereas the “pincer-like” description belongs to labidognathous.