Interesting symbolism and metaphors here in the Psalms as it points to how God closely protects His children: 

Psa 17:8  Keep me as the apple of the eye; Hide me in the shadow of Your wings

If we think about our eyes, that is the first thing that we protect, the eye is one of our most important organs of our body; if we are attacked, that is usually what we shield first, our eyes even have their own defense mechanisms built in between eye lids and eye lashes, our instincts cherish our eyes, giving more attention and protection that virtually any other parts of our bodies.   You could say that our human instincts love our eyes, they guard them more than any other parts of our anatomy, they eyes are essentially flying first class in the grand scale of the human anatomy.  In the same way we guard our eyes and give them first priority, so does God guard His children; not only through the metaphor of the apple of the eye, but also the allegory of the shadow of Your wings, which suggest that the wings belong to a mighty bird of prey like falcon or an eagle who watches over their young.  That same protection that the body provides the eye, and the raptor provides her young is what God gives those who declare Christ as King.