Growth and Development
Monarchs, like other insects with complete metamorphosis, undergo a development process that appears very different from humans. They go through stages in which their body form changes radically, whereas human infants have approximately the same form that they will have as human adults. However, we humans do an incredible amount of developing before birth. Insects hatch from the egg at a relatively earlier stage of development, so many of the changes that take place are simply more visible to us. On this page, read more about physiological changes in developing monarchs and environmental effects that may impact growth and development.
Development from egg to adult is a continuous process, just like the development of a human embryo from a single cell to a baby. However, it is useful to think about development during the egg, larval, pupal and adult stages separately.
The study of egg development through hatching is called embryology. The developmental process for most animals begins with fertilization, or the joining of an egg and sperm cell. In monarchs, this occurs right before the egg is laid. As each egg passes down the female's oviduct, a few sperm are released from the sperm storage organ (spermatheca). The nucleus of the sperm and egg join to form a zygote (fertilized egg). Soon after the egg is laid, the nucleus of the zygote starts to divide and the resultant cells form a new larva. The yolk nourishes the developing egg until the larva is ready to hatch four to six days later.
The cells that will become different parts of the monarch differentiate early in the development process. Some of the cells form the blastoderm, a layer of cells surrounding the yolk. These cells eventually separate into three categories. The ectoderm cells form part of the digestive system (the mouthparts and crop), the central nervous system, and the exoskeleton. Outgrowths from these cells also form the appendages such as antennae, mandibles, and legs. The mesoderm cells form the muscles, fat bodies and reproductive organs. The endoderm cells form the rest of the digestive system. Cells which will become eggs or sperm (called germ cells) separate from the blastoderm cells relatively early in development, and are eventually folded within a separate membrane. Butterfly and moth embryos make extensive movements within the egg as they develop, and it is possible to see the larva moving within the egg chorion just before it hatches.
Amazingly, the formation of a monarch larva from a single fertilized egg cell takes only four to five days. It is even faster in some other insects, taking only about 30 hours in one kind of mosquito! In still other insects it takes over a month.
Once hatched from the egg, the larva begins to feed and grow. There is little change in body form throughout this period, but many changes are occurring within the larva.
Since the larval cuticle, or exoskeleton, will only stretch to a limited extent it must be shed periodically. Monarchs have five larval instars: stages between molting or shedding the cuticle. The picture above shows these five stages, illustrating the incredible change in size that occurs in only 9 to 14 days. Molting is controlled by a hormone produced in glands in the thorax. It actually involves a whole sequence of events, summarized in Table 1.
|Table 1. Sequence of Events in Molting|
|1. Apolysis (separation of old cuticle from the epidermal (skin) cells that underlie it)|
|2. New cuticle production|
|3. Wax secretion (protects new cuticle)|
|4. Activation of molting enzymes|
|5. Ecdysis (shedding of old cuticle)|
|6. Expansion of the new cuticle|
|7. Sclerotization (hardening of new cuticle)|
After molting, monarch larvae (and the larvae of many other insects) usually eat the shed skin, thus recycling useful nutrients that it contains.
There is a progressive increase in mass through the larval instars, although the larva loses mass as it molts due to a period of fasting, and the loss of its cuticle and some water. As soon as the molting process is over, the larva starts eating again and rapidly gains mass.
While the primary function of the larva is to eat and gain weight, many developmental changes begin during this stage. There are tiny clusters of cells present inside the larva that will become the wings of adult monarchs (imaginal disks). The proboscis, palpi, antennae, eyes and reproductive organs also begin to develop. The larval legs will turn into the adult legs. Growth and development of many of these organs speeds up during the last one or two days before pupation, so by the time the pupa forms, major changes to the adult form have already been made.
Some books describe the process of metamorphosis as one in which the larva "turns to liquid" and is then completely reorganized into an adult. Nothing could be farther from the truth. As described in the larval development section, many of the adult features begin forming in the larva. However, an immobile pupa stage is required when the larval and adult forms are as different as they are in monarchs. The most dramatic changes that occur in the pupa are the growth of the wings and the development of flight muscles. These things could not occur in an active larva.
When the larva molts into the pupa, the wings and other features that have been developing inside the larva move to the outside, and are visible through the pupa casing. The sperm begin to mature in male pupae, although most egg development takes place after the adult female emerges. The digestive tract changes radically to accommodate the change in diet from milkweed leaves to nectar.
Once the adult monarch has emerged, there are few changes in outward appearances except for the gradual deterioration of the wings and often, a loss of mass over time.
Internal changes, most of them connected with reproduction, do occur in both sexes. It takes about four days for the eggs to develop after females emerge from the pupa, and females appear to be unwilling to mate until egg development is complete. The process of egg development is triggered by hormonal changes.
Even though sperm are produced in late instar larvae and pupal stages, males are not ready to mate until they are 3-4 days old. The male reproductive tract grows after emergence as it builds up accessory gland materials that will be transferred to the female during mating.
Both male and female reproductive development are influenced by environmental conditions. For example, monarchs exposed to decreasing daylength at the end of summer do not reproductively mature for several months, this is referred to as reproductive diapause.
Short-term Environmental Changes: Effects of Temperature on Larval Development
Several researchers have studied how temperature affects the rate and success of larval development. Rawlins and Lederhouse (1981) and Zalucki (1982) showed that monarch larvae do not develop at temperatures below 10°C or above 34°C. Monarchs generally develop faster at higher temperatures, with those experiencing 27°C taking about 12 days to go through all five instars, and those raised at lower and higher temperatures taking more and less time, respectively. There seems to be a positive relationship between temperature and larval survival at temperatures between 15 and 29°C, while above 29°C survival decreases (Zalucki 1982). Work in our lab shows that monarchs can survive very hot temperatures during the day (up to 40C, or 104F), as long as the temperature cools down at night (York and Oberhauser 2002, Nail et al. 2015), but that their development slows down at hot temperatures.
Since larvae tend to develop faster and have higher survival rates at higher temperatures, at least within the range of 15 to 29°C, it would be beneficial for them to be able to increase the temperatures they experience. Females prefer to lay eggs on milkweed plants exposed to the sun (Urquhart 1960, Zalucki 1982), and monarch larvae often select portions of plants that receive direct sunlight (Rawlins and Lederhouse 1981). Larvae also orient their bodies in ways that increase their exposure to the sun's rays, sometimes increasing their temperature to as much as 8°C above air temperature (Rawlins and Lederhouse 1981). If it is too hot for successful development (above 34°C), larvae will crawl off the milkweed plant and rest in leaf litter, or crawl to the undersides of leaves (Urquhart 1960, Rawlins and Lederhouse 1981).
Temperature may also affect the appearance of larvae. A high school student working in our laboratory in 1998 studied larvae reared at three different temperatures, and found that those reared in cooler temperatures had wider black stripes. She hypothesized that this may help them to absorb heat from the sun more effectively (Larkin 1999).
Other Environmental Influences on Monarch Development
Many researchers have studied the effects of crowding, humidity, host plant species and quality, and light on development in a wide variety of insect species. Middle school, high school and university researchers in Minnesota and other places have studied how monarchs are affected by a wide variety of environmental conditions.
The condition and species of milkweed eaten by larvae can affect the rate of development and adult size. For example, in their study of diapause induction, Liz Goehring and Karen Oberhauser found that monarch larvae fed older Asclepias syriaca plants developed into larger adults than those fed young plants of the same species. Beth Lavoie fed larvae milkweed fertilized with varying amounts of nitrogen, and found no difference in mass, but slower growth of larvae fed plants that received less nitrogen.
Although monarch larvae typically occur at low densities in the wild, crowding has been shown to influence monarch larvae development (Lindsey et al. 2009). High density conditions can cause physiological stress, increased competition for food, and an increased susceptibility to pathogens in monarch larvae. Results from Lindsey et al. showed that monarchs reared at moderate-low larval densities had greater body size, shorter development time, and lower mortality compared to larvae reared at high densities. This study also researched the effects of density on the protozoan parasite Ophryocystis elektroscirrha (Oe). Results showed an increase in infection probability with greater larval density. Thus, wild monarchs that occur in high density may be more likely to become infected with the parasite. These results are ecologically-relevant, as monarch densities in the wild can reach high levels during the late part of the breeding season and in non-migratory or year-round populations.
Exposure to light has interesting effects on monarch development and physiology. Light can even affect the direction that pupae face; students from Rochester MN found that the larva in its prepupal "J-shape" and the resulting pupa both face away from the light. Exposure to light is also a critical environmental clue that triggers diapause in migrating butterflies. Goehring and Oberhauser (2002) reported a significant effect of shorter day length on initiating diapause in female monarchs. Futhermore, shorter exposure to light resulted in smaller ejaculatory duct mass in males. The varying exposure to light is important in triggering diapause in migrating monarchs.