These recognisable insects are one of the UK’s most widespread butterflies, appearing across the British Isles.

Despite its cheerful looks, the small tortoiseshell is facing a sinister threat. Populations of a parasitic fly species called Sturmia bella have recently been on the rise in Britain – global warming means that the UK’s climate has become similar to that of other countries in Europe where the flies thrive.

These parasites intentionally lay their eggs on plants where small tortoiseshell caterpillars feed. Butterfly larvae unknowingly eat the eggs, which later hatch and slowly eat the caterpillar from the inside, eventually killing it. It’s a gruesome scenario for one of Britain’s best loved insects. The species has recovered somewhat since plummeting between 2003 and 2008, but the flies still pose a threat.