For our next topic we cross the Atlantic. There is a huge range of butterflies in the world. Every butterfly starts off as an egg, which hatches into a caterpillar. After a while this turns into a chrysalis, which in turn hatches out into a flying insect that feeds on the nectar of flowers, and eventually mates and lays eggs. Many are beautiful to look at, and some have amazing camouflage. But few are as impressive as the Monarch butterfly.

The Monarch is found across a wide band of the United States and over the border into Canada. It is about 10 cm (4 inches) across, and gold in colour with black markings. The caterpillar feeds exclu­sively on milkweed plants. The sap of this weed is poisonous to other creatures, and the toxins persist after the caterpillar has turned into a butterfly, so that the Monarch’s bright colours warn birds not to eat it. The most impressive feature of this but­terfly is that it migrates. Every spring vast clouds of Monarchs spread northwards up the east side of the USA, drinking nectar from the flowers of milkweed and other plants. After one or two months they mate and lay eggs on milkweeds which hatch into stripy grubs. These feed and grow, and after a fortnight turn into a chrysalis which hangs from a leaf. After another fortnight this breaks open and a new butterfly emerges, which continues to fly north, seeking more milk‑weeds. This cycle is repeated several times through the summer, each new generation pressing on northwards until the last hatching may reach the Canadian border. But as the temperature begins to drop, this last generation turns south, and begins to fly unerringly back down the continent, not stopping to breed, but heading down and down, eventually crossing the border into Mexico. In the case of those Monarchs flying south from areas on the east side of the States, their flight path actually takes them across the Gulf of Mexico, and they fly across the ocean, out of sight of land.

The reason for this migration is that the Monarch cannot survive a frost. It has to find somewhere warm to spend the winter. So all the Monarch butterflies from the east side of America (millions of them) gather together into one small area of about 200 square miles (52,000 hectares) in Mexico. They do not breed there, but spend the winter months congregating on the leaves of oyamel fir trees, which grow as mountain forests in this area. But once the spring returns, the urge to migrate returns, and the winter population, now up to seven months old, begins the return flight north. The extraordinary feature of this migration is that it is not the same butterfly that travels north and then comes south again. It is the third or fourth generation from the original brood that decides it must hurry south for the winter. The distance flown by the butterflies from the far north is up to 2000 miles (3300 km).

The Great Migration

Once more, we must seek some answers. How could such an enormous migration begin in the first place? It is hard to imagine that it would start by butterflies exploring northwards from Mexico, because milkweed is not abundant in Mexico itself. The Monarchs do not feed while they are down there. On the other hand it is difficult to see butterflies from the USA seeking to escape the frost by flying to Mexico, a huge distance away, without knowing it was warmer down there. And why does this particular butterfly migrate, when others hibernate in the place where they grew up? And how does it navigate over such enormous distances? The answer to this last question seems to be – by the sun. In experiments, Monarchs in migration phase have been tricked by mirrors to see the sun from an unnatural direction, in which case they adjust their angle of flight to suit the new ‘position’ of the sun. They even take into account the apparent daily movement of the sun across the sky, and if the wind blows from one side, they adjust their track to compensate, just like the pilot of an airliner. But how does the butterfly in the north know which direction to take? And why do the first and sec­ond generations to hatch fly north, and only the third or fourth generation turns south? The answer must be that the DNA code in their chromosomes commands the generation experiencing shorter or cooler days to set off south, and that the ‘map’ for the long journey to Mexico is already imprinted into their brains. The brain of a Monarch is smaller than a pin head. They are like robots, following a computer programme. But this analogy cannot hold, if evolution is true, because there was no one to write the programme. Here is the real dilemma. It is an accepted principle of biology that acquired characteristics cannot be inherited. If one more adventurous Monarch began to explore outside its normal territory, found better supplies of milkweed, and then learned to fly back to where it started off, this new knowledge would not be inherited by the next generation. And in any case, there is a three generation gap between the start and the finish of the Monarch’s migration. It is easier to postulate a super-intelligent Designer who devised this beautifully coloured creature to keep milkweeds in check, and at the same time give pleasure to so many people each spring.

Footnote

Reproduced by kind permission of the Christadelphian Auxiliary Lecturing Society (CALS)