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See: Gilbert, F., The evolution of imperfect mimicry in hoverflies, 2005
http://eprints.nottingham.ac.uk/96/1/ImperfectMimicry.pdf

You can find there some information about darker speciments on autumn in this publication:
1. Darker specimens could be the result of thermoregulatory abilities developed on the evolution
2. Developing yellow pigments requires more nitrogen and it is the limiting factor for most
animals.



"The development of perfect mimicry might
compromise thermoregulatory abilities, placing constraints on the evolution of
colour patterns: Heal (1981, 1989) put this forward as an explanation of Eristalis
colour patterns, subsequently followed by others. In both sexes, individual and
particularly seasonal variation in the pattern generated by the way the pattern
responds to rearing temperature has usually also been interpreted as adaptations
to thermal balance (e.g. in Holloway, 1993; Ottenheim et al., 1998), since darker
insects are active in cooler weather."

"It may be difficult in
resource terms to make the appropriate pigments for creating mimetic patterns.
The black colours of syrphids are presumably created from eumelanin, a
nitrogen-containing compound present as granules in the exocuticle (Chapman,
1998: 660). The yellows are probably xanthopterins (heavily nitrogenized
compounds made from the nucleotide guanosine) synthesized in the epidermis,
as in the wasp models. For there to be a cost of producing a mimetic pattern, it
must be more costly to produce xanthopterins than melanins (since an all-black
hoverfly is not mimetic); while a single molecule of xanthopterin has similar
numbers of carbon, hydrogen and oxygen atoms to a single molecule of the
quinone monomer of melanin, it has five times the number of nitrogen atoms.
Thus it is possible that the yellow colours of hoverfly mimics are costly to the
nitrogen budget; according to White (1993) nitrogen is the limiting factor for most
animals. However, it is not the case that good-quality mimics have more yellow
than poor-quality ones – it is the distribution of the yellow that matters."

Best Regards
Robert