Monarch butterflies headed this way

M stands for ... more marvelous, mysterious monarchs migrating — northward, this time. You may remember being buried under an avalanche of monarchs back in October when they were heading south for the winter. Reports from Mexico are that they have now begun their trek back to northern environs and should be coming through here again any day — perhaps even before this column makes it into print. But don’t necessarily expect the same staggering numbers — the journey south took its toll in terms of predators, disease, collisions with automobiles (I can’t count the numbers that were sacrificed on our windshield as we drove along the Bluewater highway) and simple exhaustion. So there are usually fewer returning in the spring.

If you were to ask 10 people to name the first butterfly that came to their minds, 11 of them would probably say monarch. That’s because the monarch can be found in just about every nook and cranny of the United States. But, although his presence is so obvious, there are still many things about this mysterious little fly that are unknown even to the most knowledgeable entomologist.

For instance, did you know that no individual monarch makes the complete round-trip migration from Mexico northward and then back to Mexico again? It’s the descendants of those who started the journey that make their way “homeward” to the wintering grounds where they’ve never been before but from whence their predecessors always start out. How do they know where to go? Science has yet to come up with an explanation for that, but there is some evidence that a sun compass is involved and also that monarchs use the Earth’s magnetic field in their navigation.

But as for some of the lessor mysteries — such as how can you tell a male monarch from a female and how do you know if a monarch caterpillar is coming or going and which is the deadlier sex — well, we can solve them right here!

Only the male monarch has two oval scent glands — one located on each of his lower wings — with which to woo the female. You can see them clearly in the photo of the male monarch and see that they are absent in the photo of the female. You can also see that the black veins on the female are wider than on the male. Although monarch caterpillars have a set of antennae on each end of their bodies, the longer set is on the head end. And as for the “deadlier” sex — it’s the female, of course! The only plant upon which monarchs will lay their eggs (the host plant) is the milkweed (above). Milkweed plants contain a toxin and when the eggs hatch and the caterpillars chow down on the leaves they get a special protection — their milkweed diet makes them likewise toxic to hungry birds or other predators that soon learn to avoid making a meal of them. But for some reason, which is not yet fully understood, females have 30 percent more of the toxins in them than do males.