Why did I find the peak of covered in ladybirds but devoid of other visible flora or fauna?
There are three plausible answers to this one. Take your pick 鈥� Ed
What your correspondent is witnessing here is almost certainly a consequence of a migration event. Many species of insect migrate en masse between north Africa and Europe, leaving Europe in the autumn to avoid the winter frosts, with a new generation arriving in spring for the summer months. Mount Etna lies on the migration route between Tunisia and the toe of Italy. This route and the Strait of Gibraltar are the shortest Mediterranean sea crossings.
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In contrast to birds, insects are pretty much at the mercy of wind and weather; they go where the air currents take them. There have been many reports of huge numbers of migrating insects descending on lighthouses and ships, particularly in poor visibility. Migrating insects can be found at altitudes in excess of Mount Etna鈥檚 3300 metres, so a swarm of ladybirds there is not surprising.
Terence Hollingworth, Blagnac, France
The beetles were probably , the seven-spot ladybird, which hibernates in the winter months. They build up large fat and glycogen reserves, preying on other insects when food is plentiful in the summer months, before migrating to their winter quarters. In many areas these winter quarters are in regions of high altitude where their metabolic rate can remain low and the reserves they built up in summer can last until their dispersal in the spring.
During the winter they form small aggregations on the ground, in leaf litter, at the base of plants or under stones on mountain slopes or in dry river beds. These aggregations have many benefits, such as an increased chance of mating prior to dispersal. They also enhance coloration and emit a stronger combined scent to deter predators. Similar behaviour is found in other ladybird species all over the world.
Linda Losito, Oxford, UK
The ladybirds probably got there through a process known as hilltopping. During the day the air further down the slopes of Etna begins to warm and flows upwards, taking ladybirds and other flying insects with it. The unlucky ones will take a trip to the top and be dropped there. We ran some experiments to determine how fast the air had to move to drag a ladybird up a mountainside 鈥� using a rotund man in a skydiving simulator. We found a wind speed of not much more than a puff of breath would do the job.
鈥淎 wind speed of not much more than a puff of breath will drag a ladybird up a mountainside鈥�
I filmed the phenomenon on the peak of for the TV programme Animal Planet. The wekiu bug has evolved to live there and prey on the insects deposited each day. The wekiu bug is itself a hilltopping victim; its nearest relatives live in the forests below and suck oils from seeds to survive. The wekiu somehow evolved to survive at the top, existing despite the cold and sucking the juices from other hilltoppers.
On a larger scale, this kind of process explains how so many insects colonised many of the Pacific islands. Hilltopping is not limited to volcanoes; they just happen to be barren enough that you notice the poor wayward ladybirds.
Craig Meade, Natural History New Zealand, Dunedin, New Zealand