These are commonly known as the Hairstreaks, Coppers and Blues. There are 20 British species. The adults are small to medium sized and often metallic-coloured. The beautiful colours are due to the microscopic structures on the scales. Their flight is short, quick and agile. In some species the caterpillars are associated with ants. Many of the caterpillars in this family have a gland on the 7th abdominal segment which can secrete a sweet fluid.
Lysandra coridon, chalkhill blue
On the left is an adult male Lysandra coridon, chalkhill blue. The female has brown wings with sometimes a trace of blue on the forewings, and is slightly larger. This butterfly is fairly common all over central and southern Europe. In the UK it is found only in south-east England. As its common name suggests it is found on chalk and limestone hills. The adult wingspan is 30 - 36 mm. Adults fly in July and August. In the UK there is one generation a year.
The green eggs are laid in August on Horse-shoe vetch, but do not hatch until the following spring.
The caterpillar feeds on vetches and trefoils, and is carried by ants to a suitable foodplant near the ant's nest. The ants "milk" a gland on the back of the caterpillar which exudes drops of a sweet liquid.
A fully grown caterpillar can be up to 16 mm long, is quite fat, but very tapered at both ends. The body is green with yellow stripes. In the UK the caterpillars feed on horseshoe vetch for about 10 weeks, then pupate on the ground in a greenish-brown cocoon at the base of the foodplant.