Tuesday, 30 August 2016

Variation and form ...

Common Blue (Polyommatus icarus)

Surely one of our most beautiful insects ...





P. icarus is the most widespread member of the Polyommatinae found in the British Isles but has significantly declined with the destruction of much semi-natural herb-rich grassland through agricultural intensification. It now lives in discrete colonies on remaining herb-rich grassland and disturbed sites where its larval foodplants can be found. The primary foodplant is Bird's-foot Trefoil (Lotus corniculatus). Black Medick (Medicago lupulina), Common Rest-harrow (Ononis repens), Greater Bird’s-foot Trefoil (Lotus pedunculatus), Lesser Trefoil (Trifolium dubium) and White Clover (Trifolium repens) are also used.

Two broods are typical in the southern counties of England and one brood further north. Occasionally, in favourable seasons, there may be a third brood. In good years, adults may be seen as early as the middle of May on more southerly sites. These peak at the end of May, giving rise to a second generation that emerges in the second half of July, peaking in the middle of August. Colonies in northern England and Scotland typically have a single brood that emerges in June, reaching its peak in July. I am currently seeing freshly emerged individuals of both sexes in local Surrey and Sussex locations. Whilst the male has bright blue uppersides, the female is primarily brown, with an extremely variable amount of blue and extent of spotting. Not only can this be noted in distinct geographical populations but also within the same seasonal population. P. icarus is therefore the subject of a vast number of named and unnamed aberrations. The brown form of the female is sometimes mistaken for the Brown Argus (Aricia agestis) and vice versa.

The above images, in descending order, show (i) a male from 2015, (ii and iii) two females from 2016, and (iv) an extremely blue female referable to ab. supra-caerulea, Oberthür (1896) from St Mary’s, Isles of Scilly, 2012.

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