What is the marsh fritillary?
The marsh fritillary is a rare butterfly, which was once abundant in the British Isles. Its favoured habitats are damp grasslands and shorter coastal grasslands.
Marsh fritillary lifecycle
To survive the winter, the marsh fritillary (Euphydryas aurinia) builds itself a life-support bubble. This rare butterfly, as its name suggests, often frequents damp meadows and quivering bogs and marshes, where its foodplant, devil’s-bit scabious, grows. Here in summer, clusters of eggs hatch and the tiny caterpillars start their gregarious onslaught on the downy scabious leaves.
Their larval lives are very much a co-operative existence. Playing a central part in their plan is a silky web, to which every caterpillar contributes strands of silk from a silk gland-and-spinneret combo in its lower lip. The famous fibre creates a fine framework of criss-crossing strands that enshrouds the foodplant. This network keeps the gang together, provides a sunbed and offers shelter from bad weather and predators.
The latter function becomes even more crucial in late summer, when the caterpillars moult from light brown creatures to almost-black, bristly beasts – still only a few millimetres long. Coinciding with this change in appearance is one in their behaviour.
Now the caterpillars change the fibre of their fabric, starting to spin a denser web. They head down deep to the base of their foodplant to do this, just a couple of centimetres above the soil, and here begin the collective craft of creating a winter survival capsule. This web is a smaller, tighter, less nebulous affair than the previous feeding web, and its purpose is subtly different. The structure itself is more parchment like and tends to be roughly spherical.
Inside this chamber, they do – for caterpillars – very little. They shut down their bodily processes, stop feeding and wait out the winter. Not much is known about the rest of their story – it really is a chamber of secrets. But we can assume that the structure is more or less sealed, and there are many anecdotes of larval webs on waterlogged sites surviving complete submersion for several weeks.
The low metabolism of the larvae means that the little air trapped within the winter web is enough for their requirements. The drier environment inside might also lower the incidence of mould and bacterial attack, and minimise damage from sub-zero temperatures, while the trapped air acts as a further thermal buffer.
Here they stay, out of sight, sitting there safe, awaiting the very first warm rays of spring sunshine. When, sometimes as early as February, they will crawl out of their slumber. Forming a dense black mass, they bask, harness the sun’s energy to kickstart their appetites, and pick up their life-cycle where they left off.
Main image: © Westend61/Getty Images
