Objectives. RECEPTION & INTEGRATION: The Nervous System. Human Olfaction

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1 RECEPTION & INTEGRATION: The Nervous System Objectives 1. Describe the origin of the insect nervous system. 2. Identify the major structures of the insect nervous system and describe their function. 3. Compare and contrast the physical structure and functions of compound eyes and simple eyes. 4. Differentiate between the two types of simple eyes. 5. Describe the four types of mechanical receptors insects possess. 1 2 Introduction Have you ever thought about how insects receive information from their environment? We use our five senses, but what about insects? Sight Sound Touch Smell Taste Human Olfaction 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 are prepared to eat. Actually insect senses are different from those of vertebrates. nerve 3 (All this talk about food is making me hungry.) 4 Insect Olfaction Insects smell in a similar way. Their olfactory neurons are not enclosed in a nasal cavity, but within their antennae, mouthparts, or 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 signals and stimulates neurons that cause the male to fly--migrating toward the female's scent. Nervous System Development The insect nervous system arises during embryonic development from cells called neuroblasts located in the ectoderm. These neuroblasts first develop into a mass of nerve cells called a ganglion (plural ganglia). Two ganglia form in each segment and they begin to connect with each other as neuron fibers grow out from each ganglion. Hemiptera: Pentatomidae Antennae of a male moth of Trictena atripalpis: (a) anterior view of the head; (b) cross-section through the antenna showing the antenna is branched three times; (c) enlargement of the tip of one of the branches showing olfactory hairs. (modified from Gullan & Cranston, 2005, pg. 100) 5 Representation of nervous system development. (a) neuroblast formation; (b) neurblasts form ganglia and 6 ganglia interconnect; (c) ganglia fusion. (modified from Elzinga, 2000, pg. 91) 1

2 Ganglia fusion Nervous System Functions 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 (supra-, above). It is made up of three main lobes, the protocerebrum, the deutocerebrum and tritocerebrum. We will 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 labium. Insect brain and associated structures (modified from Elzinga, 2000, pg. 93). Ganglia fusion from primitive to specialized insects. (modified from Elzinga,2000, pg. 92) As development continues, the first three ganglia pair eventually fuse to form a structure called the 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. 7 Note: The human nervous system is just opposite-it is dorsally located along our back. 8 Supraesophageal Ganglia Functions The protocerebrum receives and processes nerves 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 labrum nerves, subesophageal ganglion and assists in controlling the digestive, circulatory and endocrine systems. It helps control the corpora allata, an endocrine gland which secretes Juvenile Hormone. Neurons To get some background knowledge of how the nervous system is organized, please fill in the definition section of your study guide. Soma (perikarya) - the cell body of the neuron that contains the nucleus and typical cellular organelles. Axon the long, thin cytoplasmic extension that conducts the nerve impulse. Dendrite the region of information input. (modified from Daly, et. al, 1998, pg. 125) 9 10 Eyes There are three kinds of insect eyes you will learn about including the compound eye and two simple eyes, the ocelli and stemmata. Simple Eyes - Ocelli Ocelli (singular, ocellus) are usually located on the frons or on the top of the head. Each ocellus is made up of a corneal lens, several rhabdoms and neurons. Pigment cells surround the outer rim of the rhabdom cluster. Simple eyes: (a) Stemmata of caterpillar; (b) dorsal ocelli of a stinkbug; (c) three dorsal ocelli, cicada (modified from Romoser & Stoffolano, 1998, pg. 32). Longitudinal sections through the rhabdoms in simple eys. Stemma of a caterpillar. Ocellus of an adult bug. (modified from Gullan & Cranston, 2005, pg. 107) carpenter ant head showing position of ocelli

3 Simple Eyes - Stemmata Stemmata (singular, stemma) are structurally intermediate between ocelli and compound eyes. Compound Eyes Each facet is called an ommatidium. The ommatidium has a corneal lens, crystalline cone, rhabdom, and pigment cells. Light is focused by the lens and the cone onto the color pigments located in the rhabdom. Simple eyes: (a) Stemmata of caterpillar; (b) dorsal ocelli of a stinkbug; (c) three dorsal ocelli, cicada (Romoser & Stoffolano, 1998, pg. 32). Longitudinal section through one ommatidium, with enlargement showing transverse section (Gullan & Cranston, 2005, pg. 108). Longitudinal sections through the rhabdoms in simple eys. Stemma of a caterpillar. Ocellus of an adult bug. (Gullan & Cranston, 2005, pg. 107) Predator Eyes Video Bee Cam large compound eyes - damselfly large compound eyes tiger beetle large compound eyes deer fly 15 IMPORTANT NOTE: Throughout the course units, you will be asked to view short video clips. Please understand that many of these video clips are copyrighted and are NOT to be used outside of this class and only may be used for this semester. Please do not copy or distribute these clips. 16 Eye Summary Table 1 Summary of eye types Receptors chemoreceptors Eye: Ocellus Stemma Ommatidium Type: Simple Simple Compound Insects: Nymphs and adults of many kinds of insects holometabolous larva Nearly all adult insects and some hemimetabolous nymphs Many rhabdoms in one ocellus. One rhabdom per stemma. One rhabdom per ommatidium. Many ommatidia in one eye. Lens and crystalline lens Structures: Lens, no crystalline structures. similar to lens and crystalline One lens and crystalline cone. cone in compound eye. Pigment cells surround and Pigment cells surround and Pigment cells surround and shield adjacent shield adjacent structures from shield adjacent structures from ommatidia from light rays. light rays. light rays. Insects smell by capturing small molecules of scent floating in the air. These molecules enter a sensillum, (sensilla, plural) like the one depicted d on the left, through tiny pores. There are two classes of sensilla, uniporus and miltiporus. Image: Poor. Blurry. Poor. Blurry. But as the number of stemmata increase on the insect, the better the vision. Good vision. As the number of ommatidia increase per eye, so does the image resolution. Forms a mult-faceted image. 17 Longitudinal section through a multiporous sensillum. (modified from Gullan & Cranston, 2005, pg. 99) 18 3

4 Chemoreceptors Chemoreception is the ability to sense molecules through taste and smell (olfaction). Taste and olfaction are very similar processes and the sensory structures involved are likewise similar. Chemoreceptors are what give insects the ability to taste. The sense of taste is extremely well developed in some insects. Olfaction For many, if not most insects, olfaction is the single most important type of sensory perception. A common use of olfaction is for the location of food. The other important use of olfaction is the location of a suitable mate. This is done by releasing odors we call pheromones. (Just think about that the next time one lands on you or your hamburger.) house fly on soda can UF 19 Regal moth, Citheronia regalis (Fabricius) 20 Mechanoreceptors Trichoid Sensilla Trichoid sensilla are hair-like setae that have a single dendrite attached to their base. When these sensilla are brushed, the movement of the hair is immediately sensed by the firing of a neuron. That firing transmits an impulse to the rest of the nervous system. These trichoid sensilla are not only used to sense what s passing by the insect s body, they are also used to sense the relative position of the insect's appendages. Longitudinal section of a trichoid sensillum (modified from Gullan & Cranston 2005, pg. 87). 21 Sensilla located at a joint, called hair-plate sensilla, showing how the hairs are stimulated by contacting adjacent cuticle. (modified from Gullan & Cranston 2005, pg. 88) 22 Conclusion Mechanoreceptors Chemoreceptors Photoreceptors All these receptors send impulses down their neurons to the ganglia of the nervous system. This system of neurons develops from masses of tissue during egg growth and then develops intricate structures that perform a variety of functions. Now when you see a housefly land on a piece of food, you will be able to think about how it detected the food, what it looked like to the fly, which taste receptors he may be using, and which parts of the nervous system were used by the fly to determine whether the food is palatable 24 4

5 References Daly, H.V., Doyen, J.T. and Purcell III, A.H., Introduction to Insect Biology and Diversity, Oxford University Press, New York, 2nd ed. Elzinga, R.J., Fundamentals of Entomology. Prentice Hall, Upper Saddle River, New Jersey, 5th ed. Gullan, P.J. and P.S. Cranston, The Insects: An outline of entomology. Blackwell Publishing. Romoser, W.S. & J.G. Stoffolano The Science of Entomology. McGraw-Hill Companies, Inc. Boston. 4th ed. 25 5