The short gray arrow points to the mesopleural flange...a small projecting ridge directly below and parallel to the tegula. It's shape is somewhat like the end of a thumbnail, or a small upwardly-oriented saucer cut along its diameter.

Note that the 2nd image in this series, though very similar in its positioning, does not show the metapleural flange as clearly as here.

[See the full-size image here.]