John Calvin 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)

I walk about as a shadow. These are two very appropriate comparisons. I previously referred to the first of them in Psalm 102:12: namely, that the afflicted person, and one who is almost lifeless, is very fittingly compared to the evening shadow. At sunrise, or when the sun is shining with midday brightness, the constant shifting of the shadow is not as noticeable; but towards sunset, the shadow flits before us with every passing moment.

The other comparison points out the transitory nature of all earthly things. For just as locusts constantly skip from one place to another, so David complains that his life was constantly made uneasy by incessant persecution, allowing him no space for rest. This is similar to what he says in Psalm 11:1: that he was compelled to flee like a sparrow, for which the fowler lays snares in all directions. In short, he mourns his forlorn situation: that he could find no place of safety and no dwelling even among people. And, since in this psalm he presents us with a picture of the whole Church, we should not be surprised if God tests us and rouses us from our lethargy through countless different events. Accordingly, Paul, in 1 Corinthians 4:11, speaking of himself and others, says that they have no certain dwelling-place; a description that is more or less applicable to all the children of God.