10 Fascinating Facts about Monarch Butterflies

1. The surface of a Monarch's wing is covered with thousands of tiny, colorful scales. Loss of these scales is what causes Monarchs to lose their color!

Have you seen a tattered and faded Monarch? After flying for a long period of time, these scales will fall off and the Monarch's wings will appear faded or even clear. You can generally tell the age of a Monarch this way!
​The tiny scales on the wings of Monarchs have been studied by biologists and aerospace engineers alike to better understand how they affect flight. (Source)

Faded Monarch after a long journey

2. The Monarch caterpillar is a voracious eater and they can gain about 2700 times their original weight!

Monarchs must consume A LOT of food in a short amount of time in order to have enough food stored  to go through metamorphosis. Monarchs literally outgrow their skin FIVE times. These sheds or molts are called "instars". Once the caterpillar has reached their fifth instar, they will find a place to pupate. 

3. Two black spots on the inside surface of their hind wings distinguish male Monarch butterflies from the females.

It is helpful to know how to identify a male from a female Monarch when gardening to attract butterflies. If you notice a female on a milkweed plant, they are most likely laying their eggs there. You can then bring those leaves inside to raise your own Monarchs! Male and female Monarch butterflies are easily distinguishable. The females generally have thicker veins on their wings and they do not have black spots on their hind wings (which are actually specialized scales). In the case of most butterflies, these scales release hormones to attract females but they aren't sure that this is the case with Monarchs as well. 

It can be a little more difficult to tell the Monarch's sex when their wings are closed but if you look VERY closely you can still see the black dot that indicates a male even when the wings are closed. 

4. Monarch and Viceroy butterflies utilize Müllerian mimicry in order to mutually co-mimic each other and warn predators of their toxicity. 

MThe main visual difference between the viceroy and monarch butterfly is the black line drawn across the viceroy's hind wings, which monarch butterflies do not have. The Viceroy is also a bit smaller than the Monarch. The caterpillars of Monarchs and Viceroys look very different as well. 

This is not merely a coincidence but a means of survival for these butterflies. There is a name for this phenomenon called Müllerian mimicry- “a form of mimicry in which two or more noxious animals develop similar appearances as a shared protective device”.

Viceroy caterpillar

Monarch caterpillar

It was long believed that the Viceroy used Batesian mimicry- “mimicry in which an edible animal is protected by its resemblance to a noxious one that is avoided by predators.” However, it was found that the Viceroy feeds on Willow species (cottonwood, willow, poplar trees) which contain salicylic acid, the active ingredient in aspirin and make them taste bitter to birds and other prey. ​

5. One female Monarch butterfly can lay an average of 300 and 500 eggs in the wild. Captive monarch butterflies average about 700 eggs per female over 2 to 5 weeks of egg laying, with a record of 1179 eggs in captivity! 

According to a study done by Karen Oberhauser (1997), the average amount of eggs laid in 1994 was 715 (range 290–1179). It is difficult to know exactly how many eggs Monarch butterflies lay during their lives in the wild, but the average is estimated to be between 300 and 500. 
Monarch females usually lay a single egg on a milkweed plant, often on the bottom of a leaf near the top of the plant. Eggs are only about the size of a pencil tip and are off-white or yellow, characterized by longitudinal ridges that run from the tip to the base. The eggs hatch about four days after they are laid (University of Minnesota, Monarch Lab).

6. Monarchs that emerge in the warmer months will become sexually mature within 5 days. However, migrating monarchs will emerge in reproductive diapause (suspension of reproductive development) and will not reproduce until Spring.

Environmental factors play a role in triggering this physiological response. These factors include day length, temperature, and host plant quality (University of Minnesota, Monarch Lab). Monarchs that overwinter in Florida and California, however, may not emerge in reproductive diapause because they do not migrate to Mexico. Migrating monarchs emerging in cooler temperatures will pause their reproductive duties until they return to warmer, more favorable conditions. 

7. The Monarch's studded gold chrysalises are created by the coupling of a carotenoid pigment and a hill-like structure that reflects light from the peaks. They get the carotenoids from their diet of milkweed.

Monarch chrysalises shimmer in the sunlight like golden studded enclosures. In fact, word ¨chrysalis¨is derived from the Greek word ¨chrysos¨ meaning ¨gold.¨ What is the purpose of this beautiful feat of nature? Fred Urquhart was the first to study the gold spots on Monarchs in the 1970s. Interestingly enough, all danaine butterflies (monarchs and their relatives) have metallic spots on their chrysalises but how do they do this? 
In monarch butterflies and a few others the gold is created by a coupling of a carotenoid pigment and a hill-like structure that reflects light from the peaks. They get the carotenoids from their diet of milkweed. If the caterpillars are fed an artificial diet lacking carotenoids then the would-be golden crown develops as silver in the chrysalis.

Here are some hypotheses for the reasons that Monarchs have metallic-looking spots on their pupae:

1. Camouflage- the golden studs might increase their camouflage by looking like dew droplets on leaves and reflecting leaves on the surface
2. Warning- to deter predators they might be showing aposematic or warning coloration 
3. Protecting from harmful sun rays (unlikely but who knows?)
4. They might not have any function but just be the result of something else in the cuticle of the insect. (Source)

8. Monarch butterflies smell and taste with their antennae and legs which are covered with sensory cells called chemoreceptors. These chemoreceptors help Monarchs find milkweed to lay their eggs on. 

Like Monarchs, when we smell and taste we are actually sensing chemicals in our environment. We also have chemoreceptors, which are concentrated on our tongue as tastebuds and in our nose. Monarchs use sensing chemicals in order to find their host plant, milkweed, quickly and accurately. 

Dr. Bill Calvert describes what he observes in the field: "The female flits over the field and, when she stops on a plant, she drums the surface with her forelegs." She is basically putting tiny cuts into the plant in order for the chemicals to be released and for her to tell if it is  indeed milkweed.
"Monarchs use a combination of visual and chemical cues to find milkweed," says monarch scientist Dr. Karen Oberhauser. "Once they land on a plant, they use sensory organs on their feet and heads to tell them if it is a milkweed, and probably the quality of the milkweed." (Journey North)

9. An overwintering Monarch typically lives 7-8 months compared to other generations that only live 2-6 weeks. 

During the summer breeding season, monarchs live for only 2-6 weeks. But the monarchs that migrate to Mexico in the fall are different: They are born in late summer, stay alive all winter, and migrate north the following spring.
​

10. Milkweed contains toxins called cardiac glycosides and Monarch caterpillars will ingest and store these toxins to make themselves toxic to predators such as birds. 

The bright colors on a monarch caterpillar serve as a warning sign to predators that they may be poisonous. The toxic effect on vertebrates depends on the level of intake and most birds will throw up these toxins but will learn not to eat them again. 

References

Journey North. How Do Monarchs Find Milkweed? 
 https://journeynorth.org/tm/monarch/sl/30/text.html​

Oberhauser, KS. 1997. Fecundity, lifespan and egg mass in butterflies: effects of male-derived nutrients and female size. Functional Ecology 11: 166-175                                                                                                                             https://besjournals.onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-2435.1997.00074.x

​Rothschild M, Gardnier B, Mummery R. 1978. The role of carotenoids in the “golden glance” of danaid pupae (Insecta: Lepidoptera).  Journal of Zoology 186: 351-358.

​University of Minnesota Monarch Lab 
​https://monarchlab.org/biology-and-research/ask-the-expert/faq/