What are the and how effective are photoreceptors in mammals, insects, reptiles and vertebrates?

Well first, mammals and reptiles *are* vertebrates, so that question is phrased a little oddly. (Who writes these biology assessment questions?) Photoreceptors are the light-sensitive cells in the retina. Each photoreceptor has a specific 'photopigment', which is a protein that changes shape when it is hit by a photon of light in a specific range of light wavelengths. This causes the photoreceptor to trigger a nerve impulse in the retina, which is the first stage of vision. I'm not sure what is meant by "how effective are photoreceptors". The acuity of vision is not determined by the photoreceptors themselves, but by the concentration of them in the retina. Photoreceptors also vary by the sensitivity to low or high levels of light, and in the wavelength range of light to which their photopigments react. Nocturnal animals have sensitive photoreceptors (rods) that respond to low levels of light, and diurnal animals generally also have one or more of a second class of photoreceptor (cones) that respond to higher levels of light. To the extent that the species has two or more cones that respond to different ranges of light wavelengths, the animal has color vision. Most mammals have limited color vision. Sometime during their long existence as nocturnal animals they lost the cones that enable red-green color vision. Primates seem to have regained the cones that enable red-green color vision about 23 million years ago, but this was after the split between Old World primates (African primates, including the great apes, which includes humans), and the New World primates (S. and Central American monkeys) ... a split that occurred about 35 million years ago. So most mammals have a single type of rod photoreceptor, and two cone photoreceptors. But New World primates have a rod photoreceptor, and three types of cone receptors. Insects, fish, reptiles, and especially birds all have multiple types of cone photoreceptors giving them very color vision. Again, exclusively nocturnal species generally have fewer photoreceptors. Insects having photoreceptors with sensitivity into the ultraviolet range invisible to us, while reptiles generally have a narrower range, but many have special organs that can detect light into the infrared region (making them specially adapted to finding warm-blooded animals that emit their own infrared radiation as body heat). Hope that helps.