Control of Insect Metamorphosis
Hormones and Environment Stimulate Molting and Life Stage Changes
Mar 15, 2009
Almost all insects change shape as they develop from a newly hatched larva or nymph to the adult capable of reproduction. For some insects, the changes are minimal, but for others, the young and adult stages have little or no resemblance to each other. The changes that occur are termed metamorphosis.
How Metamorphosis Occurs
When an insect embryo develops, several clusters of stem cells, called “imaginal discs,” remain undifferentiated. These discs do not form organs or become functional as long as the insect remains in larval form.
As the larvae approach the adult stage, their imaginal discs develop into new organs and alter the shapes of larval organs. The development of imaginal discs differ in insects that undergo different modes of metamorphosis.
Differences Between Molting and Metamorphosis
Although metamorphosis occurs at the same time as molting, molting and metamorphosis are different processes under the control of different hormones.
Molting the exoskeleton is stimulated by the hormone ecdysone produced by glands in the thoracic segments that the legs and wings are attached to. This hormone causes the surface cells to digest much of the old, rigid exoskeleton and product a new, flexible one below the old one. In this process, the old exoskeleton becomes brittle and easily splits when the insect is ready to molt. When a larval insect molts, it fills up with fluids and food until it looks like it is about to burst. The old, exterior skin splits and the insect crawls out. It then sucks air into the body to help stretch the new skin. After the new skin hardens, the air and excess fluids and fecal material are expelled causing the insect to appear very thin. It rapidly resumes eating and may be ready to molt again within two or three days under highly favorable conditions.
Metamorphosis occurs when the corpora allata, glands attached to the brain in the head, secrete neotenin, the juvenile hormone (JH), throughout most larval stages. When molting occurs in the presence of high levels of JH, the imaginal discs are inhibited, and the product of molting is another juvenile stage. As JH levels drop during preparation for the last one or two molts, the imaginal discs differentiate into adult structures while larval structures may atrophy. The result is that shortly before metamorphosis, a completely different insect resides inside the old insect exoskeleton and a dramatic change in structure occurs at the next molt.
If JH is produced or injected into a larval stage that would normally undergo metamorphosis, metamorphosis is delayed at the next molt and a larger than normal adult will result. If JH is removed or inhibited, the insect metamorphoses prematurely to a smaller than normal adult.
Control of Metamorphosis
The ultimate control of molting and metamorphosis comes from several sources: growth rate of the insect, food availability, temperature, the ratio of day and night, and whether the proportion of daylight is increasing or decreasing. Molting and metamorphosis are sometimes delayed or accelerated depending on these factors, and the adults produced will be smaller or larger than normal.
Kinds of Metamorphosis
There are four types of metamorphosis, and each has several variations:
- Ametamorphic: The juvenile and adult look identical in body proportions and structures. The only difference is in size. Only a few insect species are ametamorphic.
- Gradual: A juvenile insect such as a grasshopper, roach, aphid, true bug, or praying mantis looks much like an adult of its species except it has a larger head, smaller abdomen, and its wings are either absent or reduced.
- Incomplete or Abrupt: The juvenile may look quite different from the adult, but the change occurs in one step. Dragonflies, homopterans (cicadas and psylids) display this mode. The aquatic or subterranean juvenile crawls out of the pond, stream, or ground into a twig or stem and a winged adult emerges from its skin.
- Complete: Insects that undergo complete metamorphosis pass through four life stages: egg, larva, pupa, and adult. The more specialized groups of insects do this: beetles, flies, bees and ants, butterflies, and a few others.
This article is composed of four pages:
Page 1: The control of metamorphosis.
Page 2: Types of metamorphosis.
Page 3: Possible functions of metamorphosis.
Page 4: The most probable functions of metamorphosis.
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