Introduction
'Minibeasts' is a useful lay term to use when generally describing a broad group of small animals. However, it is more scientifically accurate to classify the animals we are looking at as arthropods.
The diagram below shows the scientific criteria used for classifying various 'minibeasts' into the large group called arthropods and its smaller sub-groups.
Examples of the arthropod sub-groups are:
- Insects include bees, butterflies, beetles, ants, moths, praying mantids, cicadas, cockroaches, fleas, wasps, and flies.
- Arachnids include spiders, scorpions, pseudoscorpions, ticks, and mites.
- Crustaceans include crabs, shrimps, prawns, lobsters, crayfish, and slaters.
- Myriapods include centipedes and millipedes.
Most arthropods have a hard exterior casing known as an exoskeleton, which they must shed in order to grow. This process is called moulting.
Insects have six jointed legs and a body that is divided into three parts: head, thorax and abdomen. Insects have a head that has a pair of antennae and have mouthparts that are adapted for particular diets. 75% of all animal species described by scientists are insects.
More information
- Why are most animals insects?
Points covered:
- Why are most animals insects?
- How did insects evolve to live on land?
- What is the evolutionary importance of being able to fly?
- What is the importance of the co-evolution of insects and plants?
Words to know:
- EPICUTICLE - the outermost layer of insects' exoskeleton.
- ARTHROPOD - the name for animals with jointed legs and an exoskeleton.
- TERRESTRIAL - pertaining to or living on land.
- SPECIATION - the process of evolving new species.
- IMPERMEABLE - water can't pass through.
- DEVONIAN PERIOD - approximately 410 - 362 million years ago.
Insects comprise 75% of all animal species that scientists have named and described, and most of these insects have wings. The key to insect success is their ability to survive on land and take to the air.
Insects have adapted well to the terrestrial environment, which demands that an organism must prevent excessive water loss from its body. Insects solved this problem by modifying many aspects of their structure, physiology and habits. For example, the exoskeleton of insects has a special outer layer, the epicuticle, which is impermeable. Also, insects evolved unique solutions to the pressures of breathing, excreting waste without losing too much water and moving about on land.
The evolution of wings is an obvious key to insect success and diversity because it means that most insects can disperse widely and escape unfavourable environmental changes. Flight also provides a means of escaping predators and allows insects to colonise new environments where they may exploit new food sources.
Fossils show that insects were among the first animals to invade land during the Devonian Period, about 400 million years ago. This permitted them to utilise food resources that had not previously been consumed, such as terrestrial plants. The co-evolution of insects and plants has been very important in the histories of both groups. For example, insect mouthparts have evolved specialisations for different styles of biting and sucking plant tissues. Also, the pollination of flowering plants by insects has led to fresh avenues for the speciation of plants.
Arachnids (spiders, scorpions, mites and ticks)
Arachnids have a body divided into two parts: the cephalothorax (head + thorax) and the abdomen. They have four pairs of walking legs and lack both antennae and wings. They usually have 8 simple eyes.
Spiders are arachnids that have chelicerae (jaws with fangs) that inject poison, and they possess silk glands and spinnerets on the abdomen.
Scorpions, mites and ticks are also part of the Arachnid family. Scorpions have a long tail that ends in a stinger. They also have a pair of pedipalps (front limbs) that end in grasping pincers at the front of their bodies.
Most ticks and mites are parasites on other animals. A parasite is an animal that lives off the blood, body fluid or tissues of another animal. However, some mites feed on plant material rather than on animals.
More information
- Spider Fact Sheets
http://www.amonline.net.au/factsheets/index.htm#spiders - Scorpions Fact Sheet
http://www.amonline.net.au/factsheets/scorpions.htm
Most centipedes have 20 - 30 pairs of legs. A very few have as many as 100 legs. Centipedes are carnivores that catch other invertebrates for food. Large centipedes may sometimes even eat vertebrates such as frogs, geckos and mice. Centipedes have fangs which inject venom into their prey. Don't touch a centipede as it can give a painful bite.
Millipedes are herbivores. They feed on vegetable matter and are found in moist places such as under logs. They have hundreds of tiny legs that move together in waves to carry the millipede along. They don't have venom but some species produce an irritating fluid.
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Crustaceans (crabs, shrimps and slaters)
All crustaceans have a body divided into a head, thorax and abdomen, and have more than eight legs. Most of them have a harder section protecting the thorax called a carapace. Many crustaceans live in water (both fresh and saltwater). Slaters are the most obvious crustacean that lives on land. You can find these in your backyard - check under pot plants and logs. Australia has at least 7 species of freshwater crab and many more species of saltwater crab.
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