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Discovering the diverse group of animals that do not have backbones, from insects to jellyfish.
Did you know that of all animal species on Earth do not have a backbone? Imagine wearing your skeleton on the outside of your body like a suit of armor or living your whole life as soft as a marshmallow!
Most animals you know, like dogs, birds, and fish, have a spine or 'backbone' running down their backs. These are called vertebrates. However, the vast majority of animals on our planet are invertebrates. An invertebrate is an animal that has no backbone. Because they don't have a bony internal skeleton, they have developed other amazing ways to stay shaped and protected. Some are soft and squishy like a jellyfish, while others are hard and crunchy like a crab. Without invertebrates, our food webs would collapse because they do important jobs like pollinating flowers and cleaning up waste in nature.
Think about the difference between a human and an earthworm: 1. A human has a hard spine you can feel if you touch your back. 2. An earthworm is an invertebrate; it uses muscles and fluid to move because it has bones. 3. Both are animals, but they belong to different groups based on that one backbone!
Quick Check
If you found a new animal in the woods and it had a hard spine inside its body, would it be an invertebrate?
Answer
No, because invertebrates are defined by having no backbone.
Since they don't have bones inside, many invertebrates like insects and lobsters wear their 'bones' on the outside! This is called an exoskeleton. Think of it as a permanent suit of armor. It protects their soft insides from predators and keeps them from drying out. However, there is one problem: a hard shell cannot grow! To get bigger, the animal must perform molting. This is when they crack open their old shell, crawl out, and wait for a new, larger one to harden. During this time, they are very soft and must hide to stay safe.
Imagine a Cicada (a loud buzzing insect) growing up: 1. The Cicada grows too large for its current exoskeleton. 2. It stops eating and finds a safe tree trunk. 3. It splits the back of its shell and wiggles out. 4. For a few hours, it is white and soft until the air hardens its new, larger 'armor'.
Quick Check
What is the main purpose of an exoskeleton for an insect?
Answer
It provides protection for soft body parts and support since they have no internal bones.
Scientists group invertebrates by their body shapes. Insects are the largest group; they always have legs and body parts (head, thorax, and abdomen). Arachnids, like spiders, are different because they have legs and only body parts. Then there are Mollusks. These animals have soft, fleshy bodies. Some, like snails, grow a hard shell for protection, while others, like the octopus, are completely soft and use camouflage to hide. Even though they look different, they all share the common trait of having no backbone!
Let's classify two garden creatures: 1. Creature A has legs and wings. It is an Insect. 2. Creature B has legs and spins a web. It is an Arachnid. 3. Creature C has a slimy foot and a spiral shell. It is a Mollusk.
Which of these is the best definition of an invertebrate?
How many legs does an insect have?
An octopus is considered a mollusk even though it does not have a hard outer shell.
Review Tomorrow
In 24 hours, try to list the three main body parts of an insect and explain why a spider is not an insect.
Practice Activity
Go on a 'Backyard Safari.' Find three different small creatures and try to decide if they are insects (6 legs), arachnids (8 legs), or mollusks (soft/slimy).