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Hi, in for example Stubbs & Falk, they mention that "Others, such as E.balteatusm show a range of colour forms from very light to very dark; observations suggest that pale forms are most often seen in hot summers and dark ones under colder conditions, as if temperature is playing a role. Dusek & Láska (1974) have shown that the temperature during pupal development does indeed affect the extent of dark markings in E.balteatus and other Syrphini.


Dusek & Laska, 1974: Influence of temperature during pupal development on the colour of syrphid adults (Syrphidae, Diptera). Folia Prirod. Fak. Univ. Purkyne. 15: 77-81