3So Moses and Aaron went in to Pharaoh and said to him, “Thus says the LORD, the God of the Hebrews, ‘How long will you refuse to humble yourself before me? Let my people go, that they may serve me. 4For if you refuse to let my people go, behold, tomorrow I will bring locusts into your country, 5and they shall cover the face of the land, so that no one can see the land.
In the bible God brings a mental plague of locusts on to the Egyptians oppressing his chosen people, the Israelites. But in instances where God isn’t directing locusts in order to help ‘liberate his chosen people’, the mass movements of insects are rather poorly understood by scientists. I guess the example of locusts is probably a bad one, as we know their swarming is mainly influenced by the position of food sources. When it comes our other insect friends though, the question becomes interesting. Moths and butterflies are known to migrate to (pretty much) the same locations each year. Most spend the winter in mediterranean regions, returning here in spring for a good bit of moth-on-moth action (doubtless helped along by a bit of bombykol), and complete the cycle back to Spain (or wherever) only in the autumn.
What puzzles some scientists is that butterflies (for example) can fly at about 5 meters per second (mps) , but at the altitudes at which they fly when migrating (often 150 – 1,200 m up), the wind speeds can often greatly exceed this. On the day I write, here in Bristol the ground level wind speed is a lazy 6 mph which equates to 2.68 mps . Even this meagre side wind would undoubtably be enough to send an unsuspecting moth off course, and winds are much stronger a kilometre up in the sky. That means migrating insects must have to coordinate their travels very carefully with the changing winds, otherwise they would be blown wildly off course.
A team of researchers from all over the UK, led by Jason Chapman has been wondering how exactly it is they manage this feat for some time now. Armed with modified radar equipment positioned the over the breadth of the country they have tracked the progress of 569 separate groups of migrating moths and butterflies over the past decade to investigate the question.
It is widely believed that insects have some sort of ‘inherited’ knowledge of roughly where they need to migrate to from birth. Supporting this, the researchers found that insects always took off when the winds were blowing in vaguely the direction they were headed, that is north in spring and south in autumn.
But the winds don’t always blow in exactly the right direction. The researchers found that when the wind is blowing pretty much in the right direction the insects are happy to just go with it. If the wind direction is more than about 20 degrees off course though, the insects can’t tolerate it, so they take up a bearing which is at a slight angle to the prevailing wind in order to correct for wind drift. The more the wind differs from the direction they want, the more the creatures increase their correction angle. If things become too crazy, they will stop for a rest and pick up a favourable current later. Generally the Chapman’s team found the insects had to make about four separate flights to complete their entire journey.
To do all this, and do it accurately seems impossible unless the insects posses some kind of internal compass. The tactic of correcting for wind drift is already known in birds, but since insects have tiny brains, I wonder if this built-in compass would have to be an innate, rather than a thought-through ability? Whilst I could – at a push – believe that birds could use the position of the sun, say, to navigate, I’d have a hard time accepting insects could match this intellect.
However they do it, this tactic allows the butterflies and moths migrate accurately and increase the distances they can travel by around 40%. For the second time in as many months  I find myself admiring the Lepidoptera.
1. Chapman et al., Curr. Biol., 2008, 18, 514 – 5-4. (.pdf here).
2. My working: 1 mile is 1609 meters x 6 = 9654 m. Divide this by the number of seconds in an hour (3600) to give 2.68 mps.
3. Chapman et al., Science, 2010, 327, 682 – 685. (.pdf here).
4. See my earlier post on Lepidoptera here.