Speckled Woods: The very hungry caterpillars
My lab-mate Callum informed us all at lunch one day that he simply had far too many caterpillars spare. He was raising Speckled Wood butterflies (Pararge aegeria) for a genetics experiment, but they had produced far more caterpillars than anticipated. ‘What will you do with them?’ I asked.
‘Probably just leave them – I don’t have time to look after them at the minute,’ he replied, somewhat callously to my ears. Immediately I flashed back to my misspent childhood hunting caterpillars and raising them in jars.
‘Can I take them off your hands?’ I asked excitedly. Callum agreed, and helped me set up their home for the next few weeks. Before long, I had 12 tiny green caterpillars in a Tupperware, happily munching enormous quantities of grass every day. I watched them grow, pupate, and eventually break out of their chrysalises, at which point I released them into the nearby forest. I loved every minute of being a Speckled Wood mother.
The Life Cycle of the Speckled Wood
Speckled Wood butterflies are making an extraordinary comeback around the U.K. as a result of climate change, spreading northwards. These butterflies are often found in woods and forests, like their name would suggest. They’re brown or orange in colour, with cream, yellow or white eyespots. Due to a reduction in coppicing, which has increased the amount of shaded woodland, they’ve actually increased their range in the U.K., unlike around 75% of other U.K. species. This species is unique in another way, as they’re the only species of butterfly that can hibernate both as a larva (caterpillar stage) or pupa (chrysalis stage). This can cause a very complex flight pattern, with several adult flight events happening per year.
Females are monandrous, meaning they only mate with a single male in their lifespan. Males by contrast, can mate several times. In order to find and mate with a female, they employ one of two strategies: territorial or patrolling. Territorial males perch on a sunny patch, and wait for a receptive female to come by. These types of males tend to be of higher genetic quality, as they have to defend their patch of land against invading males. However, a choosy female has to spend a lot of her limited time and resources finding a male. Meanwhile, patrolling males actively search for females – they may not be of as high genetic quality, but especially later in the female’s life when she may have exhausted her resources, a patrolling male may seem like a good choice!
Once a female has chosen a mate, she lays eggs. The heavier the female is, the more eggs she can lay. After around a week, they hatch into bright yellow-green caterpillars, with a pronged tail. Depending on environmental cues, like temperature and light, the caterpillars can pupate at various times during the larval stage. The climb a nearby grass stalk (or the side of a plastic container), form a J-shape, and pupate. The pupae, like many other butterfly species, have a special defence mechanism – when they sense they’re being touched, they wiggle from side to side, in an effort to be dropped. This worked very well against me: I dropped one immediately in surprise the first time this happened to me! Callum originally recommended putting the chrysalises in the fridge, but mine hatched so quickly, they still had a chance of mating and laying eggs that would be able to hibernate as caterpillars, with a good chance of making it alive to spring. Come March and April, when they come out of hibernation, I’ll be keeping an eye out for my grand-butterflies, displaying their velvety brown wings on sunny patches of woodland, drinking honeydew, or munching on grasses.
Speckled Woods are one of the species that are doing better as a result of climate change: warmer weather means they’ve been able to spread up north, increasing their range significantly in the last half-century. But it’s one of the fortunate species: the vast majority of other U.K. butterfly species are suffering from severe contractions in range and abundance as time goes on. Butterfly Conservation groups are asking that people go out and record Speckled Woods. Hopefully, by tracking and understanding how the Speckled Wood has spread, conservationists can begin to understand why other species of butterfly are faltering while the Speckled Wood thrives. Keep an eye out for a Speckled Wood butterfly flitting through the sunshine in your local forests!
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