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Phylum Arthropoda Nervous System

Instructor: Taormina Lepore

Taormina has taught advanced high school biology, is a science museum educator, and has a Master's degree in museum paleontology.

How does an arthropod sense the world around it? How do these animals react to external stimuli? In this lesson, we'll discuss the structure and function of the arthropod nervous system, and compare a few special examples of arthropod nervous system adaptations.

Arthropods: Segmentad Animals With A Simple and Wonderful Nervous System

A tiny black speck skitters across a white tile floor. You know what it is immediately: a spider, and it reacts lightning fast as you try to squash it! Beady little eyes detect your movement and eight legs power the little creature's body out of harm's way. All of this happens because of the spider's system of nerves, coordinated by its brain.

Arthropods have a small but intricate nervous system that allows them to detect and react to the world around them. The nervous system is how a fly evades a flyswatter, how a lobster scavenges for food in the oceans, and how a scorpion swipes its tail to sting its prey. The segmented and armored body of an arthropod houses bundles of nerves that help it survive and thrive.

The Phylum Arthropoda is a huge group, encompassing over a million species - and making up more than 80% of all living animals. From spiders and scorpions, to millipedes and centipedes, to insects like flies and beetles and bees, and crustaceans such as shrimps and lobsters, arthropods are broadly adapted to lots of different environments.

So with all that diversity, it makes sense that there is a lot of variation in how the arthropod nervous system has helped each arthropod adapt to its specific environment.

Here we'll take a look at the basic structure and function of the arthropod nervous system, with a couple of special examples from different arthropod groups.

Arthropod Nervous System Structure

The arthropod nervous system might be fairly simple, but it's got a few different parts to it that we might recognize. At the head end of an arthropod sits a brain, the bundle of nerve cells that helps coordinate movement and sensory reaction. The brain sits above and sometimes encircles the feeding tube, or esophagus, of the insect's digestive system.

Arthropod body plan, with the nervous system in blue. Each segment has a blue ganglion, with the brain at the head end.
arthropod body plan

From the brain to the rear end of an arthropod, something similar to our spinal cord exists - but these nerve cords are quite a bit different than ours. For a start, there are two of them instead of one, and they run along the stomach, or ventral, side and not the back, or dorsal, side of the animal. In structure they look a bit like a ladder.

This paired set of nerve cords, known as ventral nerve cords transmits messages along the body. The two parallel cords are connected by repeating segments of nerves that look just like the rungs of a ladder. These repeating, rung-like segments of nerves are known as commissures, and they branch out from each of an arthropod's body segments to connect the parallel nerve cords across the ventral surface.

The spots where the commissures branch out form big bundles of sensory and motor nerves in each body segment; these bundles are called segmental ganglia (singular ganglion), and they serve as the central warehouse where other nerves branch to the arthropod's extremities in that segment. Remember from basic nervous system biology that sensory nerves carry sense-based messages like taste and touch toward the brain; motor nerves carry messages from the brain in order to move an arthropod's muscles and react to a sensory stimulus.

Most arthropods have a secondary ganglion bundle beneath the esophagus, called the subesophageal ganglion. An easy way to remember this organ is the Greek prefix sub-, which means below or beneath - so, subesophageal means 'beneath the esophagus'.

Arthropod Nervous System Function

Unlike us, the arthropod brain doesn't serve as central a role in nerve coordination. Some insects, for example, have been known to survive for days without a head! They can survive like this because of the branching nerves that run along the length of the arthropod body, many of which play just as powerful a role as the brain in nerve signaling.

The ventral nerve cords and commissures transmit and coordinate messages from around the arthropod's body. Should I run from the shadow of that shoe? Is that animal predator or prey? Is there something caught in my web? These sensory messages are read on a basic level by the arthropod segmental ganglia, the brain, and the subesophageal ganglion.

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