Coleop-Terra Physiology - Metamorphosis of Beetles

Metamorphosis Eggs Larvae Pupa Hormones

Metamorphosis

Metamorphosis is a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal's form or structure through cell growth and differentiation. In complete or complex metamorphosis, or holometabolism, the larvae differ markedly from the adults. Insects with complete metamorphosis pass through a larval stage, then enter an inactive state called pupa (or chrysalis), and finally emerge as adults.

Eggs

Most beetles reproduce sexually — that is, male sperm cells unite with female egg cells. When mating takes place, the male usually deposits enough sperm inside the body of the female to fertilize all of the many hundreds of eggs she may produce during her lifetime. The sperm remain stored in the female body in a structure called the spermatheca while the female’s eggs mature in the ovaries. Fertilization, in which sperm unite with eggs, occurs just before the eggs are laid. The female beetle lays her eggs in a protected place near a food supply, such as in wood or under the bark of trees, on leaves, or in carcasses of decaying animals. Depending on the species, a female may lay a single egg or a mass of eggs. Most beetle eggs range in size from microscopic to about 2.5 mm (0.1 in), and they are usually white. After laying her eggs, female beetles leave the eggs to hatch on their own. Hatching typically occurs after several days, and the wormlike larva stage emerges. In some beetle species the mother watches over the developing larvae to protect them from predators.

Larvae

Beetle larvae, also known as grubs, vary in appearance, but most have well-developed heads with chewing mouthparts and three pairs of legs. Many species have a pair of leglike structures at the end of the body that help the larvae move. As the larva grows, it molts (sheds its skin) several times. Each molt yields a progressively larger and more developed larval stage. The moulting process occurs in a series of steps.

1. Apolysis--the outer cuticle separates from the epidermal cells.

2. Epidermal cells secrete a molting fluid containing enzymes to break down the old cuticle. Material from the broken down exoskeleton will be recycled to form part of the new cuticle.

3. Epidermal cells secrete the foundation for a new cuticle and recycle parts of the digested old cuticle. The new cuticle is continually added to which shoves the old cuticle up and away. Eventually the old cuticle will break from the pressure.

4. Ecdysis--the insect will wriggle and move its way out of the old cuticle. The old exoskeleton is left by the wayside. The insect, vulnerable to the environment, will need to remain still until the new cuticle hardens.

At the end of the larval stage, the larva secretes silky strands to form a cocoon. Inside the cocoon, the larva transforms into the pale, soft pupa.

Fig.2: Transformation of a golofa-larvae (Dynastidae) (pictures from Alex Weigelt)

Pupa

The pupal stage is often referred to as the resting stage of the beetle’s life cycle because externally the pupa appears inactive. In fact, a great deal of activity occurs internally during this stage, as larval tissues break down and reform into adult structures. The changes that occur during the pupal stage include the development of the adult legs, wings, antennae, and reproductive system.

In this stage is easy to see if a pupa belongs to the Tigerbeetles, Stagbeetles or other groups.

Fig.3 : Different types of pupa, Tigerbeetle (Cicindelae), Stagbeetle (Lucanidae), Longhorn-Beetle (Cerambycidae)

Hormones

Insect growth and metamorphosis are controlled by hormones synthesized by endocrine glands near the front of the body (Fig.4). Some cells of an insect's brain secrete a hormone that activates thoracic glands, which secrete a second hormone, usually Ecdysone (a steroid), that induces metamorphosis. Moreover, the corpora allata produce the juvenile hormone, whose effect is to prevent the development of adult characteristics while allowing ecdysis. Therefore, the insect is subject to a series of moult, controlled by Ecdysone, until the production of juvenile hormone ceases and metamorphosis occurs.