Size and Mass
by Paula Donham (Frontier Trail Junior High School, Olathe, KS) and
Orley R. Taylor (Department of Entomology, University of Kansas)

Do all Monarchs survive the migration to Mexico or is the trip more hazardous for some?

The Monarch butterflies' migration can be quite long, sometimes more than 2500 miles, and hazardous. Unfortunately, many of the butterflies do not survive the journey. Some of the deaths are accidental but others may be due to the inability of certain individuals to survive when conditions are somewhat stressful. Little is known about which Monarchs survive the journey and which do not. Is size and/or mass related to survival? Maybe individuals representing all size and mass combinations survive equally from their origin in the north to the roosts in Mexico or perhaps individuals of some size and mass groups are more apt to die along the way. If this occurs, samples of Monarchs obtained along the migration should reflect these changes in survivorship. For example, if most of the smallest Monarchs died during the migration, we would see fewer and fewer small individuals as the migration moved southward and the average size of the individuals in each sample would increase. But it may not be this simple, nature seldom is. Mass, in particular the fat body the butterfly accumulates from consuming nectar, may be as important or even more important than size. So, our question is: how is size and/or mass related to survival during the migration.

What determines size and mass?

After an adult butterfly emerges from the chrysalis, the wings and body stay the same size throughout the life of the butterfly. But not all butterflies are the same, some are small while others are much larger. What determines size? The three most important factors determining size are sex (males are usually larger than females), genetics and nutrition (butterflies raised on old plants are usually smaller). Mass is determined by these three factors as well but is also determined by the ability of the butterfly to obtain nectar from flowers. The nectar is rich in carbohydrates which are converted into fat tissue (fatbody) stored in the abdomen. The fatbody is an important reserve, and during bad weather when the butterflies are unable to visit flowers, breakdown of the fats produces the energy and water needed to survive. Thus, it is possible for the butterfly to gain or lose mass depending on the availability of flowers and on the weather conditions. Actually, the differences in mass can be quite large and two butterflies which are identical in all size measurements can differ by 40% in mass. Although these differences in size and mass seem small to us, they could have significant effects on the success of butterflies making the long trip.

How can we learn more about the relationship between migration and Monarch size and mass?

We need help from students throughout the country to find out more about which Monarchs are best equipped to make the journey. The challenge is to collect butterflies in September and October, make size and mass measurements, record the data, and send it to us. Once you have the data there are lots of questions you can ask.

  • Are there differences between males and females?
  • Are Monarchs collected earlier in the year larger and heavier than those collected later?
  • Are the Monarchs reaching your area different from those of other locations on the migration route?
  • How many Monarchs do you need to weigh and measure before you can answer any of these questions? Two are too few and thousands are too many. Samples of any size will help us, but with samples of at least 6 males and 6 females, or better 10 or 20, you can obtain averages for each group and make comparisons yourself. Also, send your data to us and we will summarize the patterns of size and weight gain or loss.

How do you capture a butterfly?

Briefly: Use a butterfly collecting net to sweep down on Monarchs as they visit flowers or rest on trees for the night. You want the butterfly in the deep end of the net. With one hand holding the handle, use the other hand to collapse the end of the net. There should be enough space at the deep end to prevent damaging the butterfly. Flatten the net bag so the wings of the butterfly are closed over its back (thorax) and place thumb and forefinger over the leading edge of the wings (from outside of net). Next, with the thumb and forefinger of your other hand, reach in to the net and firmly grasp the thorax. Remove the butterfly for measuring.

How can you tell female Monarchs apart from male Monarchs?

Because male and female Monarchs may differ in both wing measurements and mass, it is important to record the sex of each individual you measure. It's pretty easy to tell the difference between male and female Monarchs. Females have a distinctive ventral (underside) notch at the end of the abdomen that is absent in male abdomens. Males, on the other hand, have an obvious, swollen black pouch along a vein that is parallel with the abdomen on the upper side of the hindwing. This pouch is absent in females.

What parts of the butterfly do you measure? Two measurements should be made on the butterfly (in millimeters): Monarch Diagram

1 - Forewing length - To make the first measurement, find the white spot on the thorax that is closest to the base of the forewing. Then measure from the center of that spot out to the apex (end) of the forewing.

2 - Hindwing length - To make the second measurement, find the discal cell (the mitten-shaped cell on the hindwing). Next, find the point where the vein leading away from the cell meets the black margin of the wing. Finally, measure the distance between the beginning of the discal cell and the point where the vein meets the hindwing margin.

How do you keep a butterfly still while you measure it?

The easiest and least damaging way to measure a Monarch is to place the butterfly in a small, clear plastic or acetate folder. To make a folder, take a piece of plastic like that used for overhead transparencies and cut a piece approximately 16cm x 8cm. Fold this in half the short way so you have a square. Secure one of the edges with clear tape. Now the butterfly can be carefully placed inside the plastic square where it can be immobilized by holding the square with thumb and forefinger over the wings. The measurements can be made easily through the clear plastic and by holding the plastic folder gently but firmly, the butterfly won't wiggle while you are trying to measure the wings. The measurements should be recorded as forewing length and hindwing length on the data sheet.

How do you weigh a butterfly?

If you have a scale that is accurate to 0.01 grams, you can weigh or obtain the mass of the butterfly. Not all of you will have such a scale available in your school, but if you do, after measuring a butterfly, place the whole package (butterfly in the plastic folder that you made earlier) on the scale to obtain the gross weight. After you have removed the butterfly from the plastic folder, weigh the folder alone. You can figure out the butterfly's weight by subtracting the folder-alone weight from the butterfly-in-folder or gross weight.

What are the difficulties you might encounter?

The greatest challenge here is making accurate measurements. Even though it may seem accurate at the time, there may be slight errors in measures of the wings due to butterfly movement, etc. One easy way to limit the effect of this kind of random error is to make each measurement twice, each time by a different student or at different times, and use the average of these measured values as your recorded value. To obtain accurate measures of mass, you will need to use an electronic balance accurate to 0.01 grams. Most high schools and some junior high and middle schools will have these. If, however, you are unable to measure the mass accurately enough, please send only the length measurements and stress the accuracy of these measurements in class.

What other observations can you make on Monarch butterflies?

Feeding behavior Since Monarchs get their food from the nectar that plants provide, students can observe and catalogue the flowers that Monarchs do and do not visit. This should spark lots of questions.

Effect of weather Students could also keep a log or notebook in which they record the numbers of Monarchs seen each day together with the weather conditions. A review of these notes will help students appreciate the relationship between the butterflies' activity levels and the weather conditions.

Scales Insects have some amazing structures. Have you ever looked at the wing and body scales of butterflies or moths under a microscope? You can do this with living Monarchs by gently rubbing the sticky side of a piece of clear tape (not the magic transparent kind - it's too cloudy) on the body or the wing. Place this tape on a microscope slide and look at it under a microscope. If the butterfly was collected from a flower, you may even get lucky and find some pollen grains amongst the scales!

*This is a modified version of an article entitled "The Great Dragonfly Challenge: Measuring Monarchs" which appeared in Dragonfly Magazine vol. 1, September/October 1996.

All material on this site © Monarch Watch unless otherwise noted. Terms of use.
Monarch Watch (888) TAGGING - or - (785) 864-4441