Unit 6 in Entomology [1] Unit six. Reception and Integration: The Insect Nervous System. [2] In this unit, you'll need to describe the origin of the insect nervous system, identify the major structures of the insect nervous system and describe their function, compare and contrast the physical structure and functions of compound eyes and simple eyes, differentiate between the two types of simple eyes and describe the four types of mechanical receptors insects possess. [3] Have you ever thought about how insects receive information from their environment? We use all of our five senses, but what about insects? Think about this. Do they have eyes? Yeah, mostly. Do they have a nose? The answer may seem obvious to you: insects don't have noses, but have you ever thought about how they smell or do they even smell? Well, yes, they do. They have receptors on their antenna and other parts of their body to pick up scents. In order to understand how an insect picks up a scent, let's first look at how humans do it. [4] Someone is baking luscious bread in the kitchen. As you walk by the kitchen, chemical molecules mixed with the steam waft up from the cooking food and enter your nose. The molecules then bind to tiny hairs in the nasal cavity. These hairs are extensions of olfactory nerve cells. Nerve cells are also called neurons. The binding of the chemical causes your olfactory nerves to fire and send a message to your brain. There, the brain interprets the message and fires another nerve cell in response that stimulates your salivary glands. You begin to salivate and you're ready to eat. [5] Insects smell in a similar way. Their olfactory neurons are not enclosed in a nasal cavity, but within their antennae, mouthparts or even their legs. When a female moth sends out a chemical to attract a mate, the male moth picks up the chemical molecules with his antennae, where his olfactory neurons are located. These neurons fire a message to his brain which interprets the signal and stimulates neurons that cause the male to fly, migrating towards the female's scent. If you take a look at the diagram below, you can see the moth. Male moths usually have very large antennae and these antennae have small projections on the sides you can see where they can pick up of lots and lots of scent. If you look at diagram C, you can see an enlargement of the tip of one of the branches showing all of the olfactory hairs on the end of the antenna. [6] The insect nervous system arises during embryonic development from cells called neuroblasts, located in the ectoderm. These neuroblasts first developed into a mass of nerve called a ganglion. Two ganglia form in each segment and they begin to connect with each other as neuron fibers grow out from each ganglion. If you take a look at the diagram on the right, you see a representation of nervous system development. At the top you see the neuroblast formation. In the middle the neuroblasts form ganglia and then the ganglia interconnect. And then by C, you have ganglia fusion. So, as insects become more and more advanced, they have more and more ganglia fusion. Notice that insects are segmented and there's a mass of nerve cells within each segment. [7] Speaking of ganglia fusion, as development continues the first three ganglia pair eventually fuse to form a structure called the supraesophageal ganglion. Notice the word esophagus there, so supraesophageal ganglion. And the fourth through sixth pairs fuse to form the subesophageal ganglion. The remaining ganglion pairs fuse and form the ventral nerve cord. In some highly evolved or specialized insects, all the ganglia will fuse to form one large mass in the head and prothorax. This ganglia fusion can be seen in water striders. The primitive thysanurans, the silverfish, show very little fusion. [8] The insect nervous system is ventrally located, running from the head down below the digestive system. The supraesophageal ganglion, as its name implies, lies above the esophagus. It's made up of three main lobes: the protocerebrum, the deutocerebrum and the tritocerebrum. We'll discuss each of their functions on the next slide. The subesophageal ganglion is located below the esophagus. It coordinates and controls the maxilla, mandibles and the labium—all parts of the mouth. This makes sense since the ganglion is located so close to these structures. If you take a look at the diagram below, you can see some of the different ganglia and which of the insect structures they innervate. You can see the eyes, the optic nerve, you can see some of the mouth parts. The pharynx, some of the digestive system, you can see the parts of the heart at the top of the diagram. [9] Let's take a look at the supraesophageal ganglion functions. The protocerebrum receives and processes nerve signals from the insect's eyes then interprets the message and sends a response. The deutocerebrum receives impulses from the antenna, interprets the message and controls the movement of the antenna. As a male moth detects pheromones with his antenna, the antennal nerves send a message to the deutocerebrum. This lobe will then fire the appropriate neurons to get the male moth moving towards the female. The tritocerebrum receives input from the labium nerves, subesophageal ganglion and assists in controlling the digestive, circulatory, and endocrine systems. It helps control the corpora allata, which is the endocrine gland which secretes juvenile hormone, so part of the function is in molting. [10] So let's take a look at exactly how these nerves are put together. For some background information, please refer to your textbook and fill out the study guide section about the nervous system. Take a look at the diagram to your right to see how a nerve is put together. The cell body of the neuron that contains the nucleus and the typical cellular organelles is called the soma. The long, thin cytoplasmic extension that conducts the nerve impulse is called the axon and the region of information input is called the dendrite. Neurons serve as information highways within the insect’s body and they may be unipolar with one end, bipolar with two or multipolar with many ends. These cells generate electrical impulses called action potentials that travel as waves of depolarization along the cell’s membrane. Every one of the neurons has the nerve body, the soma, and have the dendrites and axons that propagate the action potential. That just means it continues it on from nerve to nerve to nerve until an action is met. This happens because individual nerve cells connect with one another through special junctions called synapses. When a nerve impulse reaches the synapse, it releases a chemical messenger. This is called a neurotransmitter and this chemical messenger defuses across the gap, the synapse, and triggers a new impulse in the next dendrite of one or more connecting neurons. Nerve cells are generally found grouped in bundles, and a nerve is simply a bundle of these dendrites or axons that serve the same part of the body. A ganglion is a dense cluster of interconnected neurons that process sensory information or control motor output. So let's take a look at some of the functions of the nervous system. [11] The first system we’ll tackle as insect vision. As with most other animals, insect eyes are located at the anterior end of the body. There are three kinds of insect eyes: they’re the compound and two simple eyes. The ocelli and the stemmata. Let's begin with the simple eyes. If you take a look at the picture of the ant head, you can see the three ocelli right in the center of the head. This is an example of the simple eyes. [12] Ocelli are usually located on the frons or on the top of the head. They can be seen on nymphs or on adult insects. Each ocellus is made up of a corneal lens, several rhabdoms and neurons. Light is focused by the lens onto color pigments located in the rhabdoms. The light hitting the color pigments causes a chemical reaction which fires neurons surrounding the rhabdoms. Impulses pass down the axon and are sent to the protocerebrum. There they are processed and the insect accomplishes vision. Pigment cells surround the outer rim of the rhabdom cluster. These pigment cells shield surrounding structures from the piercing light rays. Ocelli do not form clear images, but are most likely used to detect changes in light, such as when a shadow is cast by an approaching predator. If you take a look at the diagrams below, you can see the simple eyes located in a caterpillar, in a stinkbug, and in a cicada. Notice how they are located on the top of the head, to protect it from predators. On the right-hand side, you see a longitudinal section through the rhabdoms in a simple eye. Notice the corneal lens, the groupings of rhabdoms, the pigment cells, and then at the bottom you’ll see the nerve attachment. [13] If you noticed in the diagram we just looked at, there were two types of simple eyes: the stemmata and the ocelli. Now let's take a look at the stemmata. They’re structurally an intermediate between the ocelli and the more complex compound eyes. They have a corneal lens, but only have one rhabdom. You only see stemmata on caterpillars and the larva of other holometabolous insects.