How To Measure Monarchs
Sex | Wing Length | Mass |
Wing Condition | Wing Damage | Spore
Load | Behavior | Vital Statistics
Sex: Is the Monarch 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.
Wing Condition
All Lepidoptera lose scales throughout their lives, and if you touch the
wings of many butterflies or moths, you will be able to see a fine patch of these
scales on your fingers. Even though monarchs are tougher than many other species,
and don't lose many scales when you touch them, they do lose scales as they fly,
attempt to mate, and brush against plants. It is thus possible to get a rough
estimate of a monarch's age by looking at how many scales it has lost.
We look carefully at the inside of the butterfly's wings, and assess how bright
they look, and whether scales are missing. We then assign the butterfly a wing condition
score from 1 to 5.
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1: a newly emerged butterfly, with wings in perfect condition
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2: in very good condition, with very few scales lost |
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3: a few patches of missing scales, wings are slightly dull |
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4: large patches of missing scales, wings look quite dull compared
to a new monarch |
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picture coming soon! |
5: more than a third of the scales missing, wings look very dull
and even transparent in spots |
Wing Damage
In some cases, monarchs may lose pieces of their wings when birds try to eat them,
when they get caught in something, or when they bump into something. Since these
pieces tend to be removed all at once, rather than wearing off gradually like scales,
we measure wing damage separately from wing wear.
We hold the butterfly with all of its wings spread out, and look at the outer margins
of each wing. We then assign it a wing damage score from 0 to 4.
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We would assign this butterfly a wing damage score of 0, because
no pieces are missing from any of her wings. |
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We would assign this butterfly a wing damage score of 4, because
four of his wings have one or more pieces missing (even small nicks count). |
Spore Load
In a long-term study of the incidence of a protozoan parasite (Ophryocystis elektroscirrha)
that infects monarch butterflies, we have found very interesting patterns in the
degree to which different monarch populations are infected (Parasite Study).
We assess whether monarchs are infected with this disease in a way that is relatively
easy and inexpensive, and most importantly, does not harm the monarch. To do this,
first touch a 1 cm x 1 cm square of CLEAR scotch tape to the butterfly’s abdomen,
removing a small patch of scales. It is easiest to this holding the tape with a
tweezers. Place the tape on either a clear glass slide (for viewing under a compound
microscope) or a white index card (for viewing under a dissecting microscope). Be
sure to write on the slide or the card which butterfly the scales came from. View
the tape under 12 to 40 X magnification. If the butterfly is infected with this
parasite, you will see small football-shaped spots among the much larger butterfly
scales. These are spores of the parasite. Count (or approximate) the number of spores
on the tape, and assign the butterfly a score:
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Score |
# of Spores Seen |
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0 |
no spores seen |
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1 |
1 spore |
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2 |
2-20 spores |
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3 |
21-100 spores |
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4 |
101-1000 spores |
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5 |
over 1000 spores |
Very few (less than 5%) of butterflies in the eastern migratory population are infected
with this parasite. More (about 60%) are infected in the western migratory population,
and almost all of the monarchs in southern Florida are infected (except in the fall,
when migratory butterflies join the population).
Behavior
We record if the butterfly was flying in a straight line (and if so, in what direction),
nectaring, laying eggs, mating, roosting in a tree with other butterflies, flying
in a nondirectional manner, or chasing other butterflies just before we caught it.
Collecting this information in different places and at different times can tell
us a great deal about the yearly migratory cycle of monarchs.
adult monarch nectaring