"Spotted" Brown Argus

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When an Argus with prominent white spots on both front wings was seen in August 2019 (first image), it was clear this was something out of the ordinary. While Brown Argus do often have wing-marks in this position, when present these are usually black. Consulting the guides showed that there was indeed a variant of Brown Argus with white spots - the Northern Brown Argus - but, as its name implies, it is only found in the North of England and Scotland so was most unlikely to be present in Suffolk. When it was apparently seen again several days later careful scrutiny showed it was in fact a different individual, lacking the fine white lines on the hindwings and having slightly differently-shaped spots (second image). In addition, a view showing both the upper and lower wing surfaces (third image) confirmed that the white underwing markings all had black centres, thus ruling out the Northern variant which has some "all white" markings.
Further observation revealed at least one more "fully spotted" individual, together with one with much smaller, but still distinct, spots and one with a spot on just the left wing. It is probably significant that all those seen were female (there was some doubt about the first one in this row, but doubt was dispelled when she offered to mate!) and three out of the five examples had the additional fine white line on both hindwings, suggesting that the full form of the aberration consists of both the spots and the lines, and is sex-linked.

As with the "mini" Argus the previous year we reported the observation to Suffolk Butterflies, who said that, again as with the mini Argus, they had received a number of other reports of "spotted" variants in 2019, the official term for the white spot being a halo (as the black mark is still there, in the centre of the spot). Consulting their sightings record and image gallery it would seem that, while uncommon, the halo variant is not especially rare - especially in the 2nd brood - and in fact we saw another halo female in 2022. Indeed, our image archive already held this picture, taken in 2015, which quite clearly shows a "small-spot" variant. Seems one sometimes has to have fore-knowledge of what to look for before one can see it! Reference to the British Butterfly Aberrations website indicated that this variant is in fact called "snelleni" - probably named after either Samuel Snellen van Vollenhoven or Pieter Snellen, Dutch entomologists of the late 19th Century.