Transformations

Arthropods grow by breaking out of their rigid exoskeletons. Most species molt, or shed their exoskeletons, only as larvae (LAR-vee). Larvae are immature animals that are not able to reproduce. However, crustaceans, arachnids, some insects, and other arthropods will continue to grow and molt throughout their adult lives. Each stage between molts is called an instar (IHN-star). The number of instars varies among species, ranging from three to more than twenty times in insects. The number of larval molts remains the same for each species. With each molt a soft pale body escapes from its old exoskeleton through a special escape hatch. After a few hours, days, or weeks the new ex-oskeleton darkens and hardens. This process of change and growth is called metamorphosis (MEH-teh-MORE-feh-sihs).

There are four basic types of metamorphosis. Some millipedes and centipedes, as well relatives of insects known as pro-turans, develop by anamorphosis (ANN-eh-MORE-feh-sihs). Their larvae hatch from eggs with fewer body segments than they will have as adults. Additional segments and legs are added as they molt. When wingless diplurans, springtails, silverfish, and bristletails molt, the only noticeable change is that they are larger. They molt many times as larvae and will continue to molt after they reach adulthood. Grasshoppers, true bugs, drag-onflies, and some other winged insects develop by gradual metamorphosis. The larvae strongly resemble the adults when they hatch, but they lack developed wings and reproductive organs. These insects stop molting once they reach the adult stage. Beetles, butterflies, moths, flies, fleas, ants, bees, wasps, and others develop by complete metamorphosis. They have four very distinct stages: egg, larva, pupa, and adult. They do not continue to grow or molt once they reach adulthood.

Insect metamorphosis:

A. Ametabolous development;

B. Incomplete metamorphosis;

C. Complete metamorphosis. (Illustration by Patricia Ferrer. Reproduced by permission.)

Insect metamorphosis:

A. Ametabolous development;

B. Incomplete metamorphosis;

C. Complete metamorphosis. (Illustration by Patricia Ferrer. Reproduced by permission.)

Spiders and other arachnids lay their eggs in a protective sac. The eggs hatch into helpless prelarvae that are unable to move. Their legs are not fully developed, and their bodies show traces of segmentation not visible in the adults. The prelarvae molt into larvae, which still show traces of segmentation on the abdomen but have legs that are more fully developed. The larvae molt into nymphs (nihmfs), or spiderlings. The very active spiderlings leave the egg sac and resemble small versions of the adults. Many arachnids continue to molt after they reach adulthood.

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