The Pseudopupil and Resolution

The ommatidia that “look” in the direction of the observer appear black because no light is reflected back.

As one looks at a compound eye from different directions, different groups of ommatidia appear black.

The shape of this pseudo-pupil changes with the local radius of the eye: in a spherical eye it is approximately round, but it becomes elongated in the direction of the largest local radius in an elongated eye.

More ommatidia look into the same angle in vertical, compared to horizontal directions - as shown in this video.

The fiddler crab eye, therefore, has much better resolution in vertical, compared to horizontal directions.