Stuart tells me that 'These animations attempt to show the relative frequency with which the species was recorded (i.e. attempting to correct for recording effort) over a five year period, finishing in the year shown. It uses the neighbourhoods and weights from Frescalo, i.e. a combination of the proximity and environmental similarity of a neighbouring square relative to the target square. So, for a given five year period and 10km square - the weighted total number of records in a neighbourhood is accumulated and also the weighted numbers of records of the targeted species. That leads to a calculation of the weighted proportion of the number of records of the targeted species in the neighbourhood. These calculated proportions form a 3D surface which is represented as shades of blue - darker the blue, higher the relative proportion of the species. Although the proportions are not scaled the same between species, they are between years within a given species. So if there are years when the species was recorded less frequently, the overall colour will be paler. Of course the opposite is also true - a spike in the proportion will peg the darkest colour. So I think there must be such a spike early in the run for V. zonaria, and that leads to everything being pale later in the run! It does seem to work OK for the other two species.'
When I first started recording hoverflies, this was a very rare animal that was confined to a ver few locations. It was even given Red Data Book 2 status is the first Insect Red Data Book. Today, it is remarkably widespread!
This was formerly a largely southern species and a classic example of one associated with urban heat islands. Today, it has escaped that envelope and has spread far into Yorkshire, but quite a bit of its northern rage is within the largely urban environment; possibly because its favoured wasps nest in houses?
Until the 1940s, this was a rare vagrant but it became established at several locations on the south coast before gaining a foothold in London where it was a classic associate of urban heat islands. It has spread a long way northwards - with a slightly more northerly range than the (assumed) native V. inanis. It is, however, much more coastal in the north.