When you hear the phrase “love spot”, I’m sure the last thing you think of is the common housefly. However, did you know that 15 different families of fly, including the mayfly, have a male-specific region of the eye called “love spots”? This region of the eye is highly specialized for motion detection and small-area targeting, and is most heavily utilized by males as they aerially pursue females during mating rituals. Talk about romantic. In some species the love spots are visible to the naked eye. The males have large eyes that are connected, whereas females have eyes that remain separated by other tissues.
Also, there are some stark differences in pigment between the male and females in some species, such as the horsefly. How do we know that the male love spots make them more adept at motion sensing? Measurements of the photoreceptors of the eye using small electrodes measures a difference in speed of 60% between male love spots and the corresponding female region of the eye! It’s thought that the structure of these love spots was then used in other fly families to become what are known as “Killer Spots”, which are found in both male and female and are used in predation.