Physiological Changes
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Development from egg to adult is a continuous process, just like the development
of a human embryo from a single cell to a completely formed baby. However, it is
useful to think about development during the egg, larval, pupal and adult stages
separately. Most of the work described below has actually been done on other insects,
but the conclusions apply equally well to monarchs.
Egg
The study of egg development through hatching is often 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.
Review the Female Monarch Reproductive System
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.
Larva
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 to
the right shows these five stages, illustrating the incredible change in size that
occurs in only nine 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), beginning with the separation of the old cuticle from the epidermal (skin) cells
that underlie it, a process called apolysis, and ending
with the shedding of the old cuticle, a process called ecdysis.
The old cuticle is partially broken down by enzymes, and some of its constituents
recycled. When it is first secreted, the new cuticle is protected from these enzymes
by a layer of wax. The new cuticle is soft and flexible, thus permitting expansion
before it undergoes sclerotization, or hardening.
Table 1. Sequence of Events in Molting
- apolysis (separation of old cuticle)
- new cuticle production
- wax secretion (protects new cuticle)
- activation of molting enzymes
- ecdysis (shedding of old cuticle)
- expansion of the new cuticle
- 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 still contains.
There is a progressive increase in mass through the larval stages (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. Elaine Dunham, working in our MN laboratory
in 1998, studied individual larvae to see just how much mass they gained throughout
development, and how this was affected by the age of the milkweed that they ate
(Dunham, Prysby and Oberhauser 1999).
While the primary function of the larval stage 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 (called 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 larva pupates, major changes to the adult form have already been
made.
Pupa
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 above 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.
Adults
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 (Does mating cause eggs to mature?).
The process of egg development is triggered by hormonal changes.
Left: This dissected ovary is from a newly-emerged female and contains no
eggs. The white material on the ovary consists of fat bodies. Right: This
ovary is from a 7-9 day old female and contains eggs in several stages of development.
Note the row of regularly shaped objects within the tubular ovarioles.
Even though sperm are produced in late instar larvae and pupal stages, males are
not ready to mate until they are three or four days old. The male reproductive tract
grows in size after emergence as it builds up accessory gland materials that will
be transferred to the female during mating (What
factors affect the size and composition of monarch spermatophores?).
Both male and female reproductive development are influenced by environmental conditions.
For example, monarchs exposed to decreasing daylength at the end of the summer do
not become reproductively mature for several months (see reproductive diapause).
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