This flyday, it’s time to focus on Eristalis arbustorum, a hoverfly. This species has no common English name, but the Dutch name is “kleine bijvlieg”. Turns out this species is pretty adaptable!
Based on the markings and the placement of the eyes, the particular model in this picture is male. Males have more yellow/orange markings and their eyes touch (I wonder how that must feel…). However, the colorations of the abdomen and the length of the wings can differ and this depends on the temperature during the larval stage. Especially females could be entirely dark! This is a good example of phenotypic plasticity, which is the variation in looks (phenotype) because of differences in the environment (e.g. temperature, nutrition).
According to a study, the yellow/orange patches in adults are larger when the temperature was higher in the larval phase. So, the adults emerging in the summer are brighter than the adults emerging in the spring. It is thought that this variation in appearance has a function. Two theories exist: 1) The brighter adults can handle the higher temperatures of the summer better than darker adults, by not overheating. Conversely, during spring, darker adults heat up faster. 2) As these flies mimic bees, and bees themselves display phenotypic plasticity, it would make sense for the flies to mirror this behaviour as well. Also, darker adults mimic solitary bees, which are more common during spring and lighter adults mimic wasps and lighter bees, which are more abundant during summer. Of course, the theories are not mutually exclusive. The idea behind both theories is that the flies are more succesful (live longer and/or have more offspring) in their respective seasons. I encourage you to read the study for more details!