Monarchs, or Danaus plexippus, are easy to recognize, thanks to their orange wings with black veins and white-spotted edges. They are some of the largest butterflies in Canada, with wingspans of 93 to 105 mm.
The caterpillars have showy white, black and yellow stripes, and black filaments at either end.
Both caterpillars and adults are brightly coloured. Although you might think that being so clearly visible would be hazardous to their survival, the opposite is actually true. Their bright colours advertise their toxicity. This is called aposematism.
By feeding on milkweed plants, whose sap contains cardenolides, the caterpillars gradually become toxic. The more they eat, the more they accumulate, and the more toxic they are to predators.
Birds and small mammals are particularly sensitive to these toxins. For instance, when a bird eats a monarch, it will feel its heart rate speed up and it will experience nausea, vomiting and diarrhea. This won’t kill it, but the experience will be unpleasant enough to discourage it from ever eating another monarch! That means one less predator for the orange butterflies and brightly coloured caterpillars.