When a tiny creature with a lot of legs walks in front of you, you might not take the time to wonder if it was a bug or a spider. This might especially be true if you are one of the many people who suffers from arachnophobia or entomophobia, fear of spiders and insects respectively.
Both arachnids and insects are multi-legged creatures, and many people are afraid of both. While bugs and spiders have a lot in common, there are some major differences between the two — mostly in their body structure and legs.
Insects have three body segments — a head, a thorax and an abdomen — while arachnids have just two body parts — a cephalothorax and an abdomen. And while insects have six legs arranged in three pairs, arachnids have eight legs arranged in four pairs. In addition, insects have two antennae, while spiders do not have antennae.
The life cycles of insects and arachnids are also different. Insects have a four-part life cycle — egg, pupa, larva and adult. Most insects undergo metamorphosis, the process of transforming from an immature to an adult form. On the other hand, arachnids typically have a three-part life cycle: egg, immature and mature.
Words to know
Thorax: The middle section of the body between the head and the abdomen.
Cephalothorax: The head and the body combined or fused together as one part.
Abdomen: The part of the body following the head and the thorax. The abdomen contains important organs and the digestive tract.
Antennae: A pair of long, thin sensory extensions on the heads of insects, crustaceans and some other arthropods.
Insects and arachnids do have some similarities. Both are arthropods, a group in the animal kingdom that also includes centipedes, millipedes and crustaceans like crabs, crayfish, lobsters and shrimp.
Like all arthropods, arachnids and insects are both invertebrates, which means they do not have backbones. Both also have exoskeletons, which is a rigid body covering that provides protection and support.
The world of insects is bigger than you may think. There are more than 1 million insect species identified worldwide! In fact, between 80% and 90% of all animal species on Earth are insects. Insects are divided into 32 different orders. Of these, beetles are the largest, with more than 350,000 species worldwide. Other orders of insects include flies; ants, bees and wasps; and butterflies and moths.
When we think of arachnids, spiders are probably the first — and maybe only — thing to come to mind. However, there are other animals that are classified as arachnids, including mites, scorpions, ticks and harvestmen. In all, there are about 60,000 known arachnid species. Spiders are the most numerous of these, with more than 50,000 known species.
Another thing insects and arachnids have in common is that they are essential to a healthy ecosystem. So before you squish or stomp on the next arachnid or insect you see, keep in mind how important they are, although mainly for opposite reasons. Many arachnids eat insects, including many that humans consider to be nuisances. That means that one of the most important ecosystem roles of insects is to serve as a food source for animals higher up on the food chain.
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