Like most arthropods, they have compound eyes.

That means that they are made of thousands of small eyes, named ommatidia, each of which detects the light coming from a certain direction, so that all together can provide an image. Each small bump that you see on the surface of their blackberry-like eyes is one of the small eyes, i.e. one of the ommatidia.

Each of the ommatidia is a long tube, having at the interior end one or more photoreceptors. The length of the tube ensures that only the light coming from a direction parallel to the axis of the tube can reach the photoreceptors, instead of being absorbed by the walls of the tube.

In the photographs, it can be seen that even the species that are otherwise mostly transparent have black eyes, or at least eyes of a dark color. This is a requirement for any kind of eye, because in order to detect light it must absorb it. So in many animals where the eyes may not have a more obvious structure they can still be recognized by being black or at least dark spots.

In most compound eyes each of the ommatidia corresponds with 1 pixel of the image that they see, so the number of pixels in an image is only of a few thousands, thus they have poor angular resolution in comparison with vertebrates or cephalopods.

There are compound eyes where several ommatidia correspond to a single pixel, trading off angular resolution for a greater sensitivity in low light, or where each of the ommatidia corresponds to several pixels, because it contains a small lens that can separate the light coming from different directions, projecting it on distinct photoreceptors.