Updated: Nov 17, 2019
Otherwise known as Denbies Hillside, Ranmore Common comprises both mixed woodland and substantial stretches of chalk downland; it is the latter which supports decent numbers of Adonis Blue butterflies which are on the wing to a limited extent in May and June, but emerge in more substantial numbers in August and September.
The Adonis Blue has an extraordinary life cycle. It is a chalk downland specialist and the habitat on the south-facing slopes of Ranmore Common suite it well. The caterpillar requires Horseshoe Vetch upon which to feed in moderately well-grazed areas; the recent increase in rabbits has helped maintain such habitats, alongside the controlled grazing of livestock. However, it is the peculiar symbiotic relationship that the Adonis Blue caterpillar (and chrysalis) has with ants which is truly remarkable. There are a numerous anthills on the common - some of which may not be far off 100 years old.
The caterpillar elects to pupate within an anthill; the obvious benefit of this strategy being that it gains protection from the elements. However, even before pupating, the caterpillar possesses an extraordinary method of self-preservation. It gains protection from the ants themselves! To prevent attacks on it by the ants, the caterpillar secretes two substances: the first is a drug-like pheromone which pacifies them; the second is a form of honeydew that the ants find irresistible. Both these substances are secreted through tiny brushes that continually emerge and disappear from the back of the caterpillar and, subsequently, the chrysalis. Indeed, such is the attraction of this second substance that fights over the caterpillar have been observed between rival species of ant. Whilst the caterpillar feeds above ground on Horseshoe Vetch, it gains vigilant protection from the ants which are constantly concerned for its welfare. When it descends into the anthill to pupate it continues to secrete the honeydew and, as such, protection from the ants is maintained.
As the adult butterfly emerges, the chrysalis continues to secrete its honeydew just long enough for the butterfly to dry its wings, fly away and begin this extraordinary life-story all over again.
Other butterfly species seen today included Chalk Hill Blue, Meadow Brown and Gatekeeper. A pair of Ravens flew westward at about 17:00 and the family of resident kestrels could be seen hunting.