A Word on a Worm
Apparently, King David said about himself, "I am a worm and not a man." I don't get this. Was this some form of exaggerated humility? Is it to be understood metaphorically? What was he getting at?
You are correct, King David wrote in Psalms 22:7, "I am a worm and not a man; a reproach of man, despised by peoples." In context, King David describes how his illustrious ancestors, in their time of need, called out to G-d and were saved. Yet he is pursued by his enemies and tread down. He beseeches G-d to rescue him as He did for his forefathers.
According to this, David is simply expressing that he is being treated like a worm and not a human being.
However, this phrase is often interpreted on its own right as an expression of humility. And while it might seem exaggerated, it is compared to similar expressions uttered by other Jewish luminaries such as Abraham who exclaimed, "I am dust and ash" (Gen. 18:27); or Moses and Aaron who posited, "Of what significance are we?" (Ex. 16:7).
Interestingly, considering the spiritual digression that spans the generations, just as Moshe and Aaron describe themselves as something less than Abraham, we should expect David to describe himself as something even less than dust and ashes. This opens the possibility that David was getting at something in particular by comparing himself to a worm.
The significance of a worm may be derived from its use in another verse: "Fear not, O worm of Jacob, the number of Israel; 'I have helped you,' says the L-rd, and your Redeemer, the Holy One of Israel" (Is. 41:14). Why does G-d compare Israel to a worm? The Talmudic Sages explain that just as the strength of a worm (i.e. caterpillar) is in its mouth, so too Israel's strength lies in their prayers to G-d.
According to this, in the context of David asking G-d for salvation from his oppressors, when he asserts that he's a worm, i.e. a caterpillar, and not a man, he is expressing his complete reliance upon the power of prayer and not on his own limited human strength.
In this vein, another possible explanation is that just as a caterpillar is initially repulsive, but, upon enveloping itself in, and later emerging from, its protective cocoon, its beauty is recognized by all, David is expressing his faith that by entrusting himself in G-d's care he will be liberated and vindicated with magnificence in this world and in the next.