declivity

noun3

1. •
   The downward slope of a curve.

   • The Declivity was so small, that I walked near a mile before I got to the Shore, which I conjectur'd was about eight a-clock in the Evening. — 1726
2. •
   A downward bend in a path.
3. •
   An inward curve of the exoskeleton of an insect, such as between body segments; a segment of an insect's body where the exoskeleton curves inward.

   • Males of all species have more developed armature of the elytral declivity than females (Figs. S, 6, 8, 9, 11-14). — 1979