Tuesday 20 August 2019

Country Matters: Blue notes of the singing caterpillars

COMMON BLUE: Polyommatus Icarus goes in for the nectar
COMMON BLUE: Polyommatus Icarus goes in for the nectar

Joe Kennedy

Tiny blue butterflies carefully exploring undergrowth, low tree branches, wild grasses and the leaves of fruit bushes that will give summer bounty, were an early warming weather sight of the family Lycaenidae.

There are at least three well-known species of blue here, and more in England, with the caterpillars having a honey-gland on their bodies which secretes a fluid ants like to drink. In return, the ants work to keep predatory bugs, flies and wasps away from them. The ants also 'herd' the caterpillars, moving them towards suitable food plants which source the honey drink - a fine example of symbiosis.

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Ants are fairly adept at 'herding', regularly enslaving others of their species to work on their farming enterprises beneath the surface, usually with a guard at every door.

The blue butterflies best known here are the Common (Polyommatus Icarus/Gorman Coiteann), Holly (Celastrina argiolus/Gorman Cuilinn) and Small (Cupido minimus/Gorman Beag). Some of those in southern England may have migrated from France.

I am not a lepidopterist but in school days chased and pinned the bodies of beautiful insects on cards, writing their names to show off to classmates. Duplicates were swapped like football cards.

This pastime was probably a fragmentary hangover from the Victoriana of some comic books in which schoolboys and eager clerics went on netting forays and had magnificent glass-covered displays of beautiful insects in their studies. The innocent pleasures of summer days - except that the short lives of the poor butterflies were terminated with straight pins.

From now on, blues may be found everywhere as the caterpillars feed on plants of the pea family such as clover, bird's-foot trefoil and rest-harrow. The male Common has the showy metallic markings which make the wings glitter in the sun - the bright colour is produced by the diffraction of sunlight by thousands of corrugated scales on the wings which absorb all colours of the spectrum except blue. There is no blue pigment on the wings. Two generations are produced each year with short lifespans of about three weeks.

The Holly caterpillars have different food plants at different times - holly in spring, ivy in autumn, caterpillars preferring the flower buds.

The Small Blue flies from late May, the male with grey-brown uppers and a scattering of blue scales on the wings. This tiny, delicate creature is of a fragmented population but is well-known to seasoned walkers of the Burren.

Naturalists Dr Stephen McCormack and Dr Eugenie Ryan tell us the fully grown caterpillars and chrysalises of the Small can produce 'songs' to attract ants to help defend defenceless pupae.

The bright green caterpillar of the Common also chirrups a bar or two which experts say is probably caused by rhythmic compression of abdominal air. There is another blue-type butterfly out there called Purple Hairstreak (Quercusia quercus/Stiallach Corca) which flies from mid-July and may be seen in woodland with mature oaks. It spends most of its time high up in the canopy enjoying the sunshine and drinking the honeydew from the leaves.

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