Danaus plexippus – Monarch Butterfly

Family, Nymphalidae:
Nymphalidae, commonly known as the Brush-footed Butterfly family, is highly visible and lends well to study, both in the field and in the lab. For this reason, members are often used as model systems in understanding complexities of Earth life.

Among the 6000 described species are the Monarch, the Painted Lady, the buckeye, the fritillaries, checkerspots and the electric blue morphos. Not all taxonomists agree on Nymphalidae family classification. Due to diversity in form and lifestyle some argue for the family being split up into as many as 9 different families. What does hold Nymphalidae together as a family is the single characteristic of all species having tricarinate (having 3 ridged keels) ridges on the antennae of adult butterflies.

Generally, Nymphalidae are medium to large-sized butterflies, with most species having a reduced pair of forelegs, and on some this pair has a brush-like set of hairs. Their vernacular names, “Brush-footed” and “Four-footed” respectively come from the brush-like feature adapted for cleaning their antennae, and their reputation for standing on only four legs, with the two forelegs curled up. Many species hold their wings flat when resting. The dorsal (upper) surface of most species are quite colourful, while the ventral (under) surface is quite dull, having a camouflaging effect.

Other idiosyncrasies of this family include flight behaviour that is more like flapping and gliding than fluttering, and feeding on juices of rotten plants, fruit and animals.

Danaus plexippus is a member of the subfamily Danainae (milkweed butterflies). Species in this subfamily lay their eggs on the various milkweed plants that their larvae then feed on. For this reason, besides “Monarch”, another common name is Milkweed butterfly.

Genus, Danaus:
The twelve Danaus species are found worldwide, including Australia, Indonesia, Asia, Africa, South America, and North America. North American species include the two from subfamily Danainae, D. plexippus (Monarch) and D. gilippus (Queen), as well as D. eresimus (Soldier). This genus appears to be named for its display of regal colours. According to Greek mythology, Danaus, who ruled in Libya, was the son of Belus, king of Egypt. Myth has it that his twin brother, Aegyptus, drove him out of Egypt, into Argos of Greece.

Following the regal theme, due to the bright orange colour of D. plexippus, it is believed that the common name, “Monarch,” is in honour of King William III of England, whose secondary title was, “Prince of Orange.”

Description/Features:
As adults, D. plexippus are recognized by wings with an upper tawny orange surface, with veins traced in black and black borders dotted white. These markings serve as a warning to predators of poison. The underside is similar, but the tips of all wings are yellow brown and the white spots are larger. Their wingspan ranges from 8.9 to 10.2 cm (3.5 – 4.0 in). Males are distinguished from females by being slightly larger, with narrower vein tracing and a small, oblong “sex” spot on each hind wing. Pheromones are dispersed from a similar spot on some other species, but this is not known to be the case with Monarchs.

Like all creatures of the class, Insecta, Monarchs have 3 pairs of 3 legs; however, they only use their middle and hind leg pairs. The forelegs are vestigial and held against the body. From head to abdomen tip they are black with white spots.

A species often confused with the Monarch is Limenitis archippus (Viceroy butterfly). Together, these two species are considered Müllerian mimics, that is, well-defended, foul-tasting species that have evolved to mimic one another against their shared predators.

Range:
Native to North and South America, this migratory species has spread to other warm places where their host plants, various milkweed species, grow. While they are now populating Hawaii, Portugal, Spain and Oceania islands, unfortunately they are no longer to be found in South America.

There are two major populations of Monarchs, subspecies D. plexippus plexippus, in North America. From southern Canada and the United States, the western population migrates south to California, while the eastern population, of up to 500,000 individuals, travels as much as 3000 km to mountainous forests in central Mexico.

Curiously, migration affects the appearance of the Monarch. Wings size and shape differ between those who don’t migrate and those who do, and of the ones who do, early migrants appear redder and more elongated than those who migrate later. In terms of North American migrants, Monarchs in the east have larger and more angular forewings than those in the west.

Host Plant(s):
The Monarchs’ host plants, of the genus Asclepias or milkweeds, instigate their migration. The leaves of milkweed plants offer the required surface for egg laying as well as necessary food nutrients for larvae. Milkweeds contain toxins that Monarch larvae are adapted to handle. These toxins, incorporated into their bodies, provide protection, as make the larvae and metamorphized butterflies distasteful to many predators.

Habitat:
The reproductive habitat of this migratory butterfly is that of the milkweed. Common milkweed (Asclepias syriaca), the most common species in eastern NA, grows in old fields and roadsides. Swamp milkweed (A. incarnata) grows in wet areas, including marshes, streambanks and moist meadows. Within the southern part of Monarch’s range, Green or Spider milkweed (A. viridis) grows in rocky wooded openings, prairies, glades, and roadsides, while Whorled milkweed (A. verticillata) grows in fields, pastures, dry prairies, dry open woods, glades and roadsides.

Behaviour:
Native to North America, D. plexippus plexippus (Monarch) was introduced to Australia in the 1870s, and there this species gained the name, “Wanderer Butterfly”, for its migratory habit.

D. plexippus plexippus is the Monarch subspecies that migrates from the northern part of their range each autumn (eastern/northeastern NA or western/northwestern NA) and from the southern part of their range each spring (the mountainous forest of central Mexico or the west coast of California, respective). Other subspecies make minor migrations or not at all.

Most North American individuals do not migrate, but rather live for 4-5 weeks, during which time they mate and lay eggs. For the generation that emerges from late August and into September, this reproductive urge is replaced by the impetus to migrate. Before the great flight, their energy goes into storing the fat that will serve to fuel their late-August to mid-October journey and throughout their subsequent southern stay. Slow and sailing, Monarch flight speed has been estimated at around 9 km/h (5.5 mph).

Monarchs tend to move on cold fronts, and when rain, wind or unfavourable temperatures prevent them from crossing Lake Erie, they will use the southernmost tip of Canada, the site of Point Pelee National Park, Ontario, as their resting point. Thousands of these beautiful butterflies can be seen during their brief (a few days) Autumn stay. For best viewing, use binoculars in the early morning or before sunset. Look into sheltered areas near the treetops. You may see them resting with open wings or, with wings together, camouflaged as dead leaves.

The migrating butterflies live for more than 6 months. They enter into a state of ‘active’ diapause that suppresses reproduction and initiates the storing of fats (lipids, proteins and carbohydrates). This store carries them through migration and their roost for the duration of the North American winter (total, approx 5 months). The cool, mountainous forest slows their metabolism so that they do not need to eat very much while roosting; however, they do require replenishment of water, so on warm days they will fly out in the millions to drink. When the weather becomes amiable for milkweed growth (mid-March), these same butterflies come out of diapause and lay eggs. The migratory trip northward begins with these children. Along the journey, females continue to lay eggs for subsequent generations to take over the flight, so that an annual cycle involves four generations in all!

Life Cycle:
Mating behaviour may be observed as adult butterflies move about their milkweed flower nectary. Look for males perching on exposed sunny spots. When a butterfly passes by he will fly out in hopes of a female Monarch. If he’s in luck, he will fly behind her and bump into the tip of her abdomen. Together, they will engage in a speedy, erratic chase. The male will then grab her from above and hold his wings straight out, while the female flutters and glides to the ground. Hidden among weeds, the male attaches his abdomen to hers and then carries her off to dense vegetation. There they remain attached from 2-14 hours, the time it takes for a packet of sperm to be transferred.

D. plexippus undergoes complete metamorphosis – egg, larval, pupal, and adult stages. The fertile adult female butterfly will lay hundreds of eggs in one day, but only one at a time. An egg is about the size of a pinhead, off-white to yellow, with longitudinal ridges running from tip to base. They can often be found on the bottom of a milkweed leaf near the top of the plant. About 4 days later the eggs hatch into larvae. Over the following 10 days the larvae, black-white-yellow caterpillars, feed on milkweed leaves as they proceed through 4 molts (5 instars). Pupae attached under a milkweed leaf or other solid object and remain in this state for about 12 days. The emergent butterflies will continue to mate and lay eggs, with as many as 3-4 broods. Born in (the North American) late-summer, the last brood enters an ‘active diapause’ that ceases reproduction in favour of the accumulation of fat storage, and migrates south to overwinter there.

Conservation:
For Monarchs, it’s all about milkweed, milkweed, milkweed … warm temperatures, cool forests, and quality water. For the NA eastern population, protection of milkweed habitats throughout their migratory range and summer breeding grounds, along with the preservation of their overwintering site, mountainous oyamel forests of Mexico and its watershed, are paramount to the survival of our beloved relation, Danaus plexippus plexippus, Monarch butterfly.

What is true of the Nymphalidae family as a whole is certainly true of Monarch. This highly visible species lends well to study and is an excellent model system not only for understanding complexities of Earth life, but also for conserving life well beyond that of one species. Conservation of Monarch habitats in North America and Mexico is conservation of all species who live within these habitats. Much love and gratitude extending to this beautifully fascinating creature!

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