Sexing | Wing Length | Mass | Wing Condition |Wing Damage | Spore Load | Behavior | Vital Statistics

Sex:
Is the monarch a male or female?

Male and female monarchs can be distinguished easily. Males have a black spot (indicated by red arrow) on a vein on each hind wing that is not present on the female. The ends of the abdomens are also shaped differently in males and females, and females often look darker than males and have wider veins on their wings.

Wing Length:

Wing length is interesting because it doesn’t change from the time that the butterfly emerges, and is thus determined by the size of the larva when it pupated. So it actually gives us information about the larval stage: Did the monarch get enough to eat when it was a larva?

We measure forewing length from where it attaches to the thorax to the tip, or apex, of the wing. If you have them, calipers are the most accurate way to do this, but it is fine to use a small clear ruler that measures in millimeters. Average monarch forewings are about 50 mm long.

Mass:

While the mass of a newly-emerged adult is determined by its life as a larva or pupa, the mass of older butterflies can change over the course of a day, as they do things like fly, eat, and mate. Mass will also change over the course of the adult life as butterflies use up the lipid reserves built up as larvae. Thus the mass of a butterfly, unlike its winglength, can provide information about what has happened to it as an adult.

To weigh live monarchs, you will need a balance that weighs things to the nearest 0.01 gram, or preferably, 0.001 gram. We use glassine envelopes, available from biological supply companies, to hold the butterflies as we weigh them. It is also possible to use a folded piece of paper as an envelope. Weigh (tare) the empty envelope, then place the butterfly inside and weigh them both together. If you don’t have a balance that automatically does this, you will need to subtract the mass of the envelope from the total mass to get the mass of the butterfly. Adult monarchs weigh, on average, about 500 mg, or 0.5 g.

If you don't have a balance available, you can estimate how fat the butterfly is. We use a scale based on the appearance of the abdomen. The abdomens of normal butterflies look convex when you look at them from the bottom (see butterfly on left below). Thin butterflies look more concave.

The male monarch on the right is thinner than the male on the left.
Notice that the white stripes on the abdomen look concave, while those on the fatter left male are convex.

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