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Zoology Chapter 31

Page history last edited by Shelly Turner 14 years, 7 months ago

Chapter 31:  Arthropods


Using your vocabulary words, make notecards/flashcards that you can use outside of class to study. 


Work on your Directed Reading in class every chance you get.  Remember that I will assist you on the harder questions.  This assignment is due the day we take the test on this chapter.  Use the information in this assignment as a study guide for your test.

This study guide is to help you study for your test.  It should not be the only item you use to study w hen preparing for the test.  Remember to complete your Directed Reading for every chapter along with writing your vocabulary words on notecards to help you remember them.  And always remember to listen carefully in lecture because all of the information in this study guide will be covered.


Jointed Appendages

         An appendage is a structure that extends from the arthropod’s body wall. 

         Unlike the parapodia and setae of annelids, arthropod appendages have joints that bend.

         A variety of jointed appendages are found in arthropods, including legs for walking, antennae for sensing the environment, and mouthparts for sucking, ripping, and chewing food.


Arthropod Diversity


         About 900,000 species of arthropods have been recorded, and probably at least as many remain to be classified.

         There are more species of beetles alone than there are of vertebrates. 

         Living arthropods are traditionally divided into two groups, arthropods with jaws and arthropods with fangs or pincers.


Phylogenetic Diagram of Arthropods





Characteristics of Arthropods





Arthropod Body Plan





         In arthropods, individual body segments often exist only during the larval stage. 

         In most arthropods the many body segments fuse during development to form three distinct regions—the head, the thorax (midbody region), and the abdomen. 

         In some arthropods the head is fused with the thorax to form a body region called the cephalothorax.




Compound Eyes


         A compound eye is an eye composed of thousands of individual visual units, each with its own lens and retina. 

         The brain receives input from each of the units, and then composes an image of an object. 

         While the image formed is not as clear as what you see, arthropods see motion much more quickly.




Function of the Compound Eye





         The outer layer of the arthropod body is a rigid exoskeleton (often called a shell) composed primarily of chitin.

         The exoskeleton is thin and flexible where the joints of the appendages are located. 

         Muscles attached to the interior surfaces of the exoskeleton can pull against it, causing the animal’s joints to bend.





         In a process called molting or ecdysis, they shed and discard their exoskeletons periodically. 

         Just before molting, a new exoskeleton forms beneath the old one. 

         When the new exoskeleton is fully formed, the old one breaks open.




         The majority of terrestrial arthropods respire through a network of fine tubes called trachea. 

         Air enters the arthropod’s body through structures called spiracles and passes into the tracheae, delivering oxygen throughout the body. 

         Valves that control the flow of air through the spiracles and prevent water loss were a key adaptation for the first arthropods that invaded land more than 400 million years ago.


Tracheal System of a Beetle





         Terrestrial arthropods have an excretory system that is composed of units called Malpighian tubules. 

         Malpighian tubules are slender, fingerlike extensions from the arthropod’s gut that are bathed by blood. 

         Water and small particles in the blood move through the tubules and into the gut. Metabolic wastes remain in the gut until they exit through the anus.


Digestive Tract of a Bee



Spiders and Other Arachnids


Characteristics of Arachnids



Arachnid Modifications


         Arachnids form the largest class in subphylum Chelicerata. The members of subphylum Chelicerata have mouthparts called chelicerae that are modified into pincers or fangs. 


         The arachnid body is made up of a cephalothorax and an abdomen. 

         There are no antennae, and the first pair of appendages are chelicerae. The second pair of appendages are pedipalps, which are modified to catch and handle prey.





         The chelicerae of spiders are modified into fangs. Poison glands located in the spider’s anterior end secrete a toxin through these fangs. 

         Most spiders can secrete sticky strands of silk from appendages called spinnerets located at the end of the abdomen.



         Tubes located on some spinnerets do not produce silk. Instead, they excrete a sticky substance that the spider can use to make some silk strands adhesive.


Anatomy of a Brown Recluse Spider




Scorpions and Mites




         Scorpions have long, slender, segmented abdomens that end in a venomous stinger used to stun their prey. 

         The stinger-tipped abdomen is usually folded forward over the rest of the scorpion’s body, a trait that makes scorpions instantly recognizable. 

         The pedipalps of scorpions are large, grasping pincers, which are used not for defense but for seizing food and during sexual reproduction.




         Mites are by far the largest group of arachnids. They are easily recognizable because their head, thorax, and abdomen are fused into a single, unsegmented body.  

         Most mites are not harmful, but some are plant and animal pests. 

         While feeding, plant mites may pass viral and fungal infections to the plant. Blood-sucking ticks attach themselves to a host, often a human.


Insects and Their Relatives 


Insect Diversity


         Ants, mosquitoes, gnats, flies, bees, crickets all belong to the arthropod subphylum Uniramia, an enormous group of mostly terrestrial arthropods that have chewing mouthparts called mandibles (jaws). 

         Uniramians consist of three classes: Insecta (insects), Diplopoda (millipedes), and Chilopoda (centipedes). 

         The insects are by far the largest group of organisms on Earth, with more than 700,000 named species.


Insect Diversity






More than 90 percent of insect species belong to one of the four orders shown in this table.



Insect Body Plan


         All insects share the same general body plan, made up of three body sections.


            1. Head. Located on an insect’s head are mandibles, specialized mouthparts, and one pair of antennae. 

            2. Thorax. The thorax is composed of three fused segments. 

            3. Abdomen. The abdomen is composed of 9 to 11 segments.


Insect Mouthparts





Insect Life Cycle


         The life cycles of most insects are complex, and often several molts are required before the adult stage is reached. 

         During the last molt, the young insect undergoes a dramatic physical change called metamorphosis.


Insect Life Cycle: Complete Metamorphosis


         Almost all insect species undergo “complete” metamorphosis.   

         In complete metamorphosis, the wingless, wormlike larva encloses itself within a protective capsule called a chrysalis. Here, it passes through a pupa stage, in which it changes into an adult. 

         A complete metamorphosis is a complex life cycle. The larvae can, however, exploit different habitats and food sources than adults.


Complete Metamorphosis




Types of Insects That Go Through Complete Metamorphosis



Insect Life Cycle: Incomplete Metamorphosis


         A smaller number of species develop into adults in a much less dramatic incomplete metamorphosis.  

         In these species, the egg hatches into a juvenile, or nymph, that looks like a small, wingless adult. 

         After several molts, the nymph develops into an adult.


Incomplete Metamorphosis




Types of Insects That Go Through Incomplete Metamorphosis



Anatomy of a Grasshopper










Feeding Habits of Grasshoppers



Grasshopper Circulatory and Respiratory Systems





         An insect’s wings develop from saclike outgrowths of the body wall of the thorax. 

         The wings of adult insects are composed entirely of chitin, strengthened by a network of tubes called veins (which carry air, not blood). 

         In most insects, the power stroke of the wing during flight is downward, and it is produced by strong flight muscles.


Social Insects


         Two orders of insects, Hymenoptera (ants, bees, and wasps) and Isoptera (termites), have elaborate social systems. 

         Within these insect societies, there is a marked division of labor, with different kinds of individuals performing specific functions. 

         The role played by an individual in a colony is called its caste. Caste is determined by a combination of heredity; diet, especially as a larva; hormones; and pheromones, chemical substances used for communication. 



Insect Relatives


         Centipedes and millipedes have similar bodies. Each has a head region followed by numerous similar segments.  

         Centipedes have one pair of legs per segment and can have up to 173 segments. Modern millipedes have from 11 to 100 or more body segments, and most millipede segments have two pairs of legs. 

         While centipedes are carnivores, most millipedes are herbivores.




Characteristics of Crustaceans



Crustacean Habitats


         While primarily marine, members of subphylum Crustacea are also found in fresh water and in a few terrestrial habitats. 


         Crustaceans include crabs, lobsters, crayfish, shrimps, barnacles, water fleas (Daphnia), and pill bugs. 

         Almost all crustaceans have a distinctive larval form called a nauplius.



         A microscopic, freeswimming nauplius larva is a developmental stage of almost all crustaceans.


         The nauplius has three pairs of branched appendages.




Comparison of Crustaceans and Insects



Terrestrial Crustaceans

         The most widespread group of terrestrial crustaceans is composed of the pill bugs and sow bugs. 

         Another group, the sand fleas, includes several thousand species typically found along beaches. 

         In addition, a few species of land crabs live in damp areas. Land crabs are only partly adapted to terrestrial living. 


Aquatic Crustaceans


         The members of some orders of crustaceans are quite small. Common are fairy shrimps, water fleas, ostracods, and tiny copepods.  

         Copepods are among the most abundant multicellular organisms on Earth and are a key food source in the marine food chain. 

         Another small marine crustacean, Euphausia superba, swarms in huge groups and is known by its common name, krill. 





         Large marine crustaceans such as shrimps, lobsters, and crabs, have five pairs of legs and are often referred to as decapods. 

         The head and thorax of decapods are fused into a single cephalothorax, which is covered on top by a protective shield called a carapace. 

         In crayfish and lobsters, the anterior pair of legs are modified into large pincers called chelipeds.


External Anatomy of a Crayfish


Internal Anatomy of a Crayfish





Sessile Crustaceans


         Barnacles are a group of crustaceans that are sessile as adults. 

         Free-swimming larvae attach themselves to a rock, post, or some other submerged object, where they remain. 

         Unlike most crustaceans, barnacles are hermaphrodites. However, they do not usually fertilize their own eggs. 

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