With a wing-span of 80-90mm, the Swallowtail, Papilio machaon ssp britannicus, is the UK's largest, and rarest, butterfly. It has been suggested that the two antennae-like ‘tails’, together with the red ‘eyes’ on the hindwings, may appear like a second head and so confuse predators!
The range of the Swallowtail is restricted to the fenlands of eastern England due to the limited availability of Milk-parsley (Thyselium palustre), which is the primary food source for its larvae.
The actual appearance of the larvae changes as they pass through a number of moults while growing. Initially they resemble bird droppings, but by the third moult they develop typical yellow and black warning colours. A further defence mechanism is the two-pronged organ seen just behind the head. When the larva is disturbed, this organ secretes a strong-smelling acidic liquid which is wiped on the attacker. It is thought that the smell deters other insects and parasites but not birds.
