Danaus plexippus (“Monarch butterfly”)
Aliona Pitchkar Biology 7; LAVC Spring „07
Background & Phylogeny
The Monarch butterfly, as it is commonly called, is a beautiful creature belonging to class Insecta, order Lepidoptera, family Danaidae – or family Nymphidae, depending on which sources you go by, genus Danaus, and lastly species Plexippus. The diverse order Lepidoptera is divided into several families – all of which are called either moths or butterflies. Based on molecular testing and DNA analysis, it has been concluded that the two closest relatives of Danaus plexippus are Danaus cleophile, also known as the Jamaican Monarch, and Danaus erippus, usually called the Southern Monarch. These three species are so similar as adults that for many years they were mistakenly classified as one species. As evidence has proved, however, they vary drastically in terms of their development and genotype – although their phenotypes are exactly the same! Although there is currently some debate on the issue, the most widely accepted theory of the evolution of Danaus suggests that the species evolved in Central America after the recession of the ice sheets some 12,000 years ago.
other insects, the monarchs are less dependent on chemical pheromones for their sexual attraction mechanisms and more so on technique. D. plexippus’ courtship ritual consist of two basic steps: in the aerial phase, the male pursues, nudges, and eventually takes down the female; next comes the ground phase, during which actual copulation occurs and involves the transfer of a spermatophore from the male into the female, which bears the sperm as well as supplemental energy resources which aid the female with future reproduction. Once fertilization has taken place, the female travels alone to a safe breeding ground – typically a milkweed plant – and lays her eggs on the host plant, making sure to attach them well by secreting a glue-like fluid over them. Now it is time for a brand new life cycle to begin!
Life Cycle
The life cycle of a Monarch butterfly is characterized by a method of complete metamorphosis, meaning that the larva matures into an adult who looks completely different and usually functions completely differently compared to the larval form. The first stage of the life cycle is the fertilized egg. Usually the egg has been left on the bottom of a milkweed leaf and will probably never see its biological parents again.
Courtship & Reproduction
All the Monarch butterflies reproduce sexually during their mating season, which always occurs in the spring. Compared to
Depending on the temperature, the eggs will hatch in about three to five days into the two inch long larval form, commonly called a caterpillar. The caterpillar is very unlikely to venture too far from its host milkweed plant, especially because milkweed leaves are the only thing it can eat. After about two weeks worth of eating the caterpillar finally has enough stored energy to progress into the next stage of the life cycle. It will find a safe place – usually a twig or small branch – and attach itself head down in the shape of a „J‟ by spinning silk from its posterior spinneret. It then sheds its outer layer of skin by molting and grows a thicker exoskeleton. Once this hardened green exoskeleton is in place, the caterpillar enters a dormant stage with relatively low metabolic rates and is now officially in the third, pupal, stage. In terms of Monarch terminology this third stage of the life cycle which is normally called a pupa, is actually called a chrysalis. For about ten to twelve days during pupation, the entire body of the caterpillar is reorganized and altered into one which resembles the form of a typical butterfly. Once the chrysalis is fully transformed, the fourth and final stage of the life cycle is in place, the fragile adult (now called a butterfly) emerges. It pumps stored liquids into the wings to inflate them but must wait
about thirty minutes for them to dry before it can fly. This half an hour of waiting is technically the most vulnerable state of the adult butterfly‟s life. Although they are usually able to escape predators by swiftly flying away, they cannot do so until their wings have dried. The Monarch is characterized by beautiful wings, always in a pattern of black and orange or yellow. In Figure A.1 below, the entire life cycle is visually summarized.
Figure A.1 The Monarch goes through 4 radically different stages 1) egg 2) larva pupa adult 4) 3)
There is no further growth once the animal has reached its adult form. As previously stated, the mating season occurs in the spring, and since the entire purpose of the Monarch‟s adult life is to procreate and then die, the individuals which are born later in the year typically have a longer lifespan (about 8-9 months) compared to the short lives of those born closer to the spring (about 2-5 weeks).
Morphology
The morphological characteristics of Danaus plexippus vary drastically depending on which phase of the life cycle the organism is currently exhibiting. In the first stage, the premature butterfly is surrounded by a hard outer shell, called the chorion. This shell and the developing larva inside of it collectively make up what is referred to as the egg. The outer layer of the shell is made up of a waxy substance which prevents desiccation. Micropyles are small funnel shaped openings which run from one end of the egg to the other that enable sperm to pass into the egg during copulation – keep in mind that unlike most other animals, the Monarch‟s egg is already hardened even before fertilization has occurred. It is not uncommon to associate egg shells with being smooth and hard; the Monarch‟s egg however, is not smooth at all. Instead it is completely covered by small longitudinally raised areas called ridges, which are formed inside the female before she lays the egg. These “ribbed” eggs are one of the primary developmental differences between butterflies and their close relative the moth. The larval form of the animal is usually about 2 inches long and may vary greatly in width. It is so morphologically different from the adult butterfly that the two phases
are often mistaken for two completely different species. The caterpillar, as the larval form is called, has three main anatomical parts: the head, which consists of two short antennae, a mouth, and six pairs of eyes called ocelli; the thoracic body, which is segmented and possesses one pair of jointed “true” legs per segment; and the abdominal body, which is also segmented and contains prolegs, also referred to as “false” legs because they function to hold the caterpillar onto a leaf, and not in locomotion. Like the larva of many other members of order Insecta, D. plexippus performs gas exchange with the environment through holes in the sides of its thorax and abdomen called spiracles. These spiracles are connected to a highly complex network of long airtubes called tracheae, which carry O2 gas throughout the body. The next stage, pupation, is also very different physically compared to the egg and larval forms. When the caterpillar is ready to pupate it spins a silk pad on a milkweed leaf and then undergoes a final molt by wiggling out of its exoskeleton. Once this outer layer is shed from the body, the cremaster, a spiny appendage located posterior to the abdomen, becomes exposed. The caterpillar then hooks its cremaster to the silk pad it has spun and hangs upside down this way while
it develops into a butterfly. This fresh pupa is usually a green color, as shown in Figure B.2 below, but will become increasingly transparent as hormonal changes propel the development of the butterfly within it.
Figure B.2 This Monarch chrysalis (pupa) is a waxy substance with a green hue which becomes increasingly transparent as the process progresses.
is a straw-like organ used to suck nectar and water from plants and is rolled up when not in use – basically the proboscis is the butterfly‟s tongue. The thorax is composed of three segments, each with its own pair of legs. There is also a pair of forewings attached to the second segment and a pair of hindwings attached to the third segment. The abdomen consists of eleven much finer segments, the last two of which are joined. The male Monarch also has a pair of claspers on his posterior abdomen which are used to grasp the female during copulation. As you can see, the morphology of D. plexippus is just as diverse as its life cycle!
In about two weeks all the color will have faded from the chrysalis and the mature form emerges. The body of the adult, now called a butterfly, is divided into the same three sections as the caterpillar – head, thorax, and abdomen – except they have become more complex. The head now contains compound eyes which are enormous compared to the rest of its physiognomy and consist of thousands of light-sensitive structures called ommatidia. The head also still has its two antennae but they have gotten bigger and better in comparison with those of the caterpillar. The newly formed parts of the head include the two palpi, which are olfactory organs, and the proboscis, which
Diet & Defense
In most organisms, the daily intake of nutrients is a topic discussed separately from the methods they have adapted to escape predation. In the case of the Monarch butterfly, however, the two issues are deeply intertwined. The first meal a newly hatched caterpillar consumes is its own eggshell, after that it will never eat anything besides the leaves of milkweed plants and will continue to do so once it has become a butterfly as well. This milkweed-rich diet is indeed the essence of the Monarch‟s defense against its three main predators: the oriole, the black-headed grosbeak, and the forest mouse. Although they pose no direct threat
to humans, the Monarch is technically a poisonous butterfly. The leaves of the milkweed plant, genus Asclepias, produce the poisonous cardenolide called cardiac glycoside which although virtually harmless to the Monarch, is highly toxic to other animals because it inhibits their internal mechanisms of sodium-potassium exchange and makes them very sick. When D. plexippus eats the milkweed leaves this substance becomes infiltrated throughout the entire body of the Monarch; therefore, sometimes a bird or other predator will take a bite out of the wing of the butterfly and be completely repelled by its foul taste and usually will not continue eating the animal. If a predator does happen to consume the entire Monarch butterfly, it will surely be very sick to its stomach for the rest of the day and will definitely learn to avoid the black and orange insect in the future. In fact, this unique chemical defense characteristic has lead one of the Monarch‟s relatives, the completely non-toxic and palatable Viceroy butterfly, to adapt its black and orange wing coloration – an example of Batesian mimicry.
such as meadows, fields, or marshes. Although the species is technically a North American butterfly, it also inhabits other regions of the world such as New Zealand, Australia, the Canary Islands, Portugal, Spain, and Mexico. The distribution of the Monarchs is also highly dependent on the distribution of the core of their life cycle: milkweed plants, of course! One of the most distinct yet peculiar character traits of D. plexippus is its lengthy annual migrations. They typically make a massive southbound migration from August until the first frost and then a northbound migration during spring, which is also the season when they mate. During migration huge flocks of butterflies can be seen traveling together. The peculiar aspect of these mass migrations is the Monarch‟s exceptional navigational skills. The insects are guided by means of an internal magnetoclinic compass which directs them towards the blooming of the milkweed. In a study performed by the Monarch Watch Organization of America in 1999, even new generation Monarch‟s which were separated from their flock were somehow able to navigate to the migratory fields – a remarkable discovery which is still being studied. More than anything however, what makes this species so unique is the fact that
Habitat & Migration
Monarchs can typically be found all around the globe in sub-tropical or tropical regions usually with a preference for open areas
it is the only insect that migrates – a characteristic usually reserved for birds – and also the only insect which has managed transatlantic flights! Overall, the Monarch butterfly is a beautiful, complex, and highly valued member of the ecosystem of numerous countries and diverse places all over the world.