Charles Ellicott Commentary Psalms 109:23

"I am gone like the shadow when it declineth: I am tossed up and down as the locust." — Psalms 109:23 (ASV)

Shadow when it declineth. — Literally, a lengthened shade. (Compare to Psalm 102:11, and see Note. Song of Solomon 2:17.) When the day declines the shadow lengthens, it becomes longer and longer, until it vanishes in the universal darkness. Thus does the life of the suffering generation pass away.

Tossed up and down. — Better, tossed or shaken out, as from the lap. So Septuagint and Vulgate (See Nehemiah 5:13, where the same verb is three times used.) The grasshopper was an emblem of timidity (Job 39:20).