In the Classroom

Activity

A discussion with Students, Incorporating the Following Points:

Ask students if they know how land invertebrates, such as spiders and insects, grow?

• All insects start life as an egg, but being an invertebrate, they have no internal skeleton.

• Their muscles are attached to an outer skeleton, or hardened exterior surface, called an exoskeleton.

• The exoskeleton can be soft (as in spiders) or hard (as in cicadas).

• A hard exoskeleton provides a problem for growth, it doesn’t stretch! Students may imagine how they might increase in size if they were encased inside a suit of armour.

• Some invertebrates emerge from the egg as a tiny adult, called a nymph. Nymphs grow larger by successively discarding their outer covering, called moulting.

Moulting

1. In order for a nymph to grow, it must crack open the exterior. How do they do it? Their heart rate increases, and blood is pumped from the hind end (abdomen) to the front end (cephalo-thorax), until the pressure inside the cephalo-thorax increases. The pressure pushes on the old exoskeleton until it cracks open.

2. Most insects distribute oxygen to their muscles through a system of channels, called trachea. These have hardened walls to protect the insect body from drying out, and must be pulled out of the body each time the animal increases its size. Is this a reason why these arthropods are small?

3. For an insect that moults, each time that it loses its protective coat, it is exposed to predation, increasing the chance of being eaten by a carnivore.

4. You might ask students how invertebrates could escape from being eaten:

• live underground, like nematode worms. (Four out of five animals on Earth are nematode worms!)

• stay underground, like cicadas (as a nymph, often for many years).

• in a tunnel (like a trap-door spider).

• in hardened above-ground mounds (like termites).

Metamorphosis (‘meta’ - change, ‘morph’ – shape, or form)

Other insects emerge from the egg as a tiny worm like larva. This form concentrates on eating, and successively changing its body shape. Each stage may be outlined, as follows:

1. Adult (or imago) has wings for mobility and dispersal, and for finding a mate.

2. Eggs (noorak) like seeds, contain energy and are a source of food for many carnivorous animals. The success of the insect species depends on the survivorship of the eggs; thus, they are usually laid in a secure place (on, or under a leaf, in a hole in a tree trunk, underground) and, in large numbers. Each egg, containing the genes of the adult, hatches either as a nymph, or as a larva. If a nymph, it grows by moulting. If a larva, it grows by eating.

3. Larva (miya). The larva is the morph that eats. They can be worm-like, with legs for moving in search of food, such as caterpillars (ngarna); or they can be a sit-and-wait predator, with a hardened exoskeleton, and build a trap, like the antlion. When sufficient food energy is acquired, the body form changes into a pupa.

4. Pupa - The adult features are developed during this stage, such as wings, legs, reproductive organs, weapons of defence, etc. The pupa is sheltered during this time, usually inside a cocoon.

Many insects have an aquatic stage in their lifecycle, such as dragon flies, mosquitos and midges

The lifecycle of the non-biting midge has 4 stages:

1. The female lays eggs on the surface of the water in a gelatinous egg mass; it may contain over 1000 eggs The eggs sink to the bottom and hatch, as a larva, within a week.

2. The larva burrows into the mud and builds a small tube around itself, feeding on the suspended organic matter in the water and in the mud.

• The larva lives for 2 to 7 weeks, depending on the temperature of the water.

• While it grows, it turns a ‘blood’ red colour due to a compound called haemoglobin, explaining why they are called “blood worms”.

3. The larva transforms into a pupa while still in its tube, and swims to the surface.

4. After 3 days, the pupa emerges as an adult midge.

5. The adult midge lives for only three to five days feeding on nectar, during which time, mating occurs.