31 Protostome Animals Lecture Presentation by Cindy S

Chapter 31 Opening Roadmap. © 2017 Pearson Education, Inc.

Introduction • Protostome animals are spectacularly diverse and abundant • Of the 30 animal

Table 31. 1 © 2017 Pearson Education, Inc.

Introduction • Some protostome phyla have little diversity, such as the phylum Priapula, which contains only 16 named species • On the other end of the spectrum, over 85, 000 mollusks have been named thus far, and about 1. 2 million arthropod species, of which about a million are insects • Scientists estimate that the actual number of arthropod species may be over 10 million © 2017 Pearson Education, Inc.

Figure 31. 1 Major protostome phyla Major non-protostome phyla Insects s an ce ta us Cr Ch el ice r ate s Arthropoda M ia yr p s od Nematoda Mollusca Platyhelminthes Annelida Chordata Echinodermata Cnidaria Porifera Other invertebrates © 2017 Pearson Education, Inc.

Introduction • Protostomes fill very important ecological roles • As a group, they live in virtually every aquatic and terrestrial habitat • They can be detritivores, herbivores, or carnivores • The ecological importance of protostomes extends to human health © 2017 Pearson Education, Inc.

Introduction • Importance of protostomes: • Major direct source of food for humans • Provide ecosystem services • Some damage crops • Some produce materials such as silk and pearls • Cause or transmit human diseases and are parasites • Include two of the most important model organisms • Fruit fly Drosophila melanogaster and roundworm Caenorhabditis elegans © 2017 Pearson Education, Inc.

What Is a Protostome? • The exact order of branches is still under debate, but researchers agree that the sponges (phylum Porifera), comb jellies (phylum Ctenophora), and jellyfish and corals (phylum Cnidaria) were the first lineages to emerge during the early radiation of animals • Bilaterian ancestor was likely bilaterally symmetric and triploblastic, which gave rise to an array of body plans among bilaterians during the Cambrian explosion © 2017 Pearson Education, Inc.

What Is a Protostome? • Biologists traditionally distinguished the protostomes from deuterostomes using developmental characteristics: 1. Embryonic development of the mouth before the anus during gastrulation 2. Inability of isolated early embryonic cells to develop into a complete embryo 3. The formation of a coelom by the splitting of blocks of mesodermal cells © 2017 Pearson Education, Inc.

What Is a Protostome? • Newer view is that there are many exceptions to the “protostome versus deuterostome” developmental patterns • For example, the anus develops before the mouth in some protostomes • Some similar developmental characteristics evolved independently in different lineages • Convergent evolution • Other characteristics thought to be synapomorphies were lost in some groups © 2017 Pearson Education, Inc.

What Is a Protostome? • Recent DNA sequence data support two major subgroups within the protostomes: 1. Lophotrochozoa include mollusks and annelid worms 2. Ecdysozoa include the nematodes and arthropods © 2017 Pearson Education, Inc.

Figure 31. 2 Porifera Ctenophora ANIMALS Cnidaria LOPHOTROCHOZOA Rotifera Platyhelminthes Annelida ECDYSOZOA Nematoda Tardigrada BILATERIANS PROTOSTOMES Mollusca Onychophora Arthropoda Echinodermata Chordata © 2017 Pearson Education, Inc. DEUTEROSTOMES

The Water-to-Land Transition • Fossils indicate that protostome lineages originated in the ocean • Like land plants and fungi, protostomes made the transition from aquatic to terrestrial environments • However, protostomes made this transition multiple times as they diversified © 2017 Pearson Education, Inc.

Figure 31. 3 Cnidaria LOPHOTROCHOZOA Rotifera Platyhelminthes Annelida Mollusca All aquatic ECDYSOZOA Nematoda Tardigrada Onychophora Arthropoda Echinodermata © 2017 Pearson Education, Inc. Chordata Some terrestrial All terrestrial

The Water-to-Land Transition • Water-to-land transitions are important because they open up entirely new habitats and new resources to exploit • To make the transition to land, new adaptations must allow protostomes to 1. Exchange gases 2. Avoid drying out 3. Hold up their bodies under their own weight • Terrestrial protostomes have evolved many solutions to these challenges © 2017 Pearson Education, Inc.

The Water-to-Land Transition • Roundworms and earthworms have a high surfacearea-to-volume ratio • This increases the efficiency of gas exchange across their body surface in their moist environments • Some terrestrial arthropods and mollusks have gills or other respiratory structures located inside the body • This minimizes water loss when moving onto land © 2017 Pearson Education, Inc.

The Water-to-Land Transition • Insects evolved a waxy layer to minimize water loss from the body surface • If the environment dries, openings to respiratory passages can be closed • Desiccation-resistant eggs evolved repeatedly in populations that made the transition to life on land • Insect eggs have a thick membrane that keeps in moisture • Snail and slug eggs have a thick shell that retains water © 2017 Pearson Education, Inc.

Modular Body Plans • Morphological and physiological diversification has a genetic basis • Until recently, biologists wrongly assumed that very different organisms required very different genetic instructions • Multicellular animals have a common tool kit of genes that establish the animal body plan during development • Hox genes are an important part of this tool kit • Expressing the genes in the tool kit at different times and places during development can lead to dramatic differences © 2017 Pearson Education, Inc.

Modular Body Plans • Diversification of animal body plans can occur by the generation of new genes over time • However, changing the expression pattern of existing genes likely had an even larger impact on animal body plan diversification • When comparing diverse organisms within a lineage, genetically based modularity is evident • A small set of elements can be reused and rearranged to produce a large variety of outcomes © 2017 Pearson Education, Inc.

What Is a Lophotrochozoan? • The lophotrochozoans are a monophyletic group—all descendants of a common ancestor • The 13 phyla within the Lophotrochozoa include the rotifers, flatworms, annelids, and mollusks • Two morphological traits occur in some, but not all, members: 1. Feeding structure called a lophophore, which is found in three phyla 2. A type of larva called a trochophore, which is common to many of the phyla © 2017 Pearson Education, Inc.

What Is a Lophotrochozoan? • A lophophore is a specialized structure that rings the mouth of these animals and functions in suspension feeding • Trochophores are a type of larva common to several phyla of lophotrochozoa • Trochophore larvae have a ring of cilia around their middle that functions in sweeping and, sometimes, in feeding © 2017 Pearson Education, Inc.

Figure 31. 4 (a) Lophophores function in suspension feeding in adults. (b) Trochophore larvae swim and may feed. Food particles Water current Mouth surrounded by the lophophore (ring of ciliated tentacles) © 2017 Pearson Education, Inc. Anus Mouth Anus Gut 0. 1 mm Cilia used in locomotion and feeding 0. 1 mm

What Is a Lophotrochozoan? • Trochophore larvae occur in animals that undergo indirect development, live in different habitats, and eat different foods • Recent analyses suggest that trochophore larva originated early in the evolution of lophotrochozoans and later evolved into different larval types in some groups • However, larvae are not unique to lophotrochozoans —even sponges have larvae © 2017 Pearson Education, Inc.

What Is a Lophotrochozoan? • Spiral pattern of cleavage in embryos is a synapomorphy for this monophyletic group • When cells divide at oblique angles to each other during early embryogenesis, a spiraling pattern of cells in the blastula results • In contrast, in radial cleavage, cells divide at right angles to each other • Spiral cleavage has been highly conserved in some lophotrochozoan phyla, but has been modified or lost in others © 2017 Pearson Education, Inc.

Figure 31. 5 (a) Spiral cleavage is unique to lophotrochozoans. (b) Radial cleavage, for

What Is a Lophotrochozoan? • In sum, you could be confident that an animal belongs to the Lophotrochozoa if you observe any of the following: • A lophophore • A trochophore • Spiral cleavage • However, not all lophotrochozoans possess all three of these characteristics © 2017 Pearson Education, Inc.

What Is a Lophotrochozoan? • Many lophotrochozoan phyla include species that have long, thin, tubelike bodies that lack limbs; they are worms with a basic tube-within-a-tube design • The outside tube is the skin, which is derived from ectoderm • The inside tube is the gut, which is derived from endoderm • Muscles and organs derived from mesoderm are located between the two tubes • Worms may or may not have a coelom © 2017 Pearson Education, Inc.

What Is a Flatworm? • Flatworms are named for the broad, flattened shape of their bodies • They lack a coelom and structures specialized for gas exchange and circulation of oxygen and nutrients • Molecular data confirms the placement of flatworms within the Lophotrochozoa © 2017 Pearson Education, Inc.

What Is a Flatworm? • Biologists hypothesize that the flattened body is an adaptation that provides a large surface area for gas exchange • This flat body plan allows nutrients and gases to diffuse efficiently to all of the cells inside the animal with minimal expenditure in complex internal structures • This requires flatworms to live in an aquatic or moist environment © 2017 Pearson Education, Inc.

Figure 31. 6 (a) Free-living turbellarian (b) Parasitic fluke Male Eye spots Gut Pharynx

What Is a Flatworm? • The flatworms are a large and diverse phylum consisting

What Is a Flatworm? • Turbellaria is a paraphyletic group of free-living flatworms • Most are freshwater or marine • Prey on protists or small animals, or scavenge dead animals • Have a blind digestive tract • Mouth is near middle of body’s ventral surface, at the end of a tubelike structure called the pharynx • Can extend and pierce live prey or other food © 2017 Pearson Education, Inc.

What Is a Flatworm? • Cestoda are strictly endoparasitic tapeworms that parasitize diverse vertebrates, absorbing nutrients by diffusion across their body wall • Humans most often acquire tapeworms by eating undercooked pork, beef, or fish © 2017 Pearson Education, Inc.

What Is a Flatworm? • Trematoda are endoparasitic or ectoparasitic flukes that parasitize vertebrates, arthropods, annelids, and mollusks • A fluke is responsible for schistosomiasis—a serious public health issue in many tropical and subtropical nations, infecting more than 200 million people worldwide • They feed by gulping host tissues and fluids, often causing blood in the host’s urine or stool, kidney failure, and other problems © 2017 Pearson Education, Inc.

What Is a Flatworm? • Monogenea are tiny ectoparasites that parasitize specific tissues of

What Is a Segmented Worm? • Most annelids have a coelom, a fully developed digestive tract with a mouth and an anus, and a segmented body • Traditional view grouped annelids and arthropods together in the same clade due to their segmentation © 2017 Pearson Education, Inc.

What Is a Segmented Worm? • Molecular data suggests that segmentation arose independently in these groups • But, some of the same developmental tool-kit genes are involved in segmentation in both phyla • Convergent evolution at the morphological level • Homology at the genetic level © 2017 Pearson Education, Inc.

What Is a Segmented Worm? • Common ancestor of annelids had a key synapomorphy in addition to segmentation: • Numerous, bristle-like extensions called chaetae that extend from lobe-like appendages called parapodia • These appendages appear very different from appendages of other bilaterian phyla • But homologous expression patterns of genes such as Dll, in appendages of diverse phyla, suggest a common origin of the genetic tool kit for appendages © 2017 Pearson Education, Inc.

Figure 31. 6 (c) Marine polychaete (d) Terrestrial earthworm Bristle-like chaetae on parapodia 1

What Is a Segmented Worm? • Three main groups: 1. Polychaeta 2. Oligochaeta 3.

What Is a Segmented Worm? • Polychaeta • Diverse worms that live in a wide variety of marine habitats • Polychaetes that are highly mobile often have large parapodia and chaetae • More sedentary polychaetes have reduced parapodia and smaller, but still numerous, chaetae © 2017 Pearson Education, Inc.

What Is a Segmented Worm? • Oligochaeta • Earthworms and other oligochaetes • Deposit feeding in soils • Ecosystem services include: • Tunnels that are important in aerating soil • Feces contribute large amounts of organic matter © 2017 Pearson Education, Inc.

What Is a Segmented Worm? • Hirudinea • Leeches • Ectoparasites that attach themselves to fish, humans, or other hosts and suck blood and other body fluids • Nonparasitic leech species are predators or scavengers © 2017 Pearson Education, Inc.

What Is a Segmented Worm? • Molecular data have added complexity to this threegroup categorization • The Polychaeta are paraphyletic • Unsegmented worms called Sipunculida and Echiura (traditionally thought to be independent phyla) are now grouped within Annelida—their ancestors were segmented, but this character was lost over time © 2017 Pearson Education, Inc.

What Is a Mollusk? • Mollusks are a highly diverse monophyletic group of lophotrochozoans • Characteristic body plan: • Foot—a large muscle located at base of the animal, used in movement • Visceral mass—the region containing most of the main internal organs and external gill • Mantle—outgrowth of the body wall that covers the visceral mass, forming an enclosure called the mantle cavity © 2017 Pearson Education, Inc.

Figure 31. 7 Shell (when present) Mantle cavity Gill Radula Muscular foot © 2017

What Is a Mollusk? • Once the molluscan body plan evolved, subsequent diversification was largely driven by adaptations that allowed the mollusks to move, feed, or reproduce in novel ways • Dramatic radiation of mollusks into several lineages: • Chitons (mollusks with dorsal shells made of plates) • Bivalves (clams and mussels) • Gastropods (slugs and snails) • Cephalopods (squid and octopuses) © 2017 Pearson Education, Inc.

Table 31. 2 © 2017 Pearson Education, Inc.

The Foot Is a Muscular Hydrostat • Snails and chitons have a large, muscular foot at the base of the body that works as a type of hydrostatic skeleton called a muscular hydrostat • Waves of muscle contractions sweep backward or forward along the length of the foot, allowing individuals to crawl along a surface • In bivalves, the foot is modified as a digging appendage • In cephalopods, the foot is modified to form tentacles for crawling and grasping © 2017 Pearson Education, Inc.

Figure 31. 8 Stripes are waves of muscle contraction © 2017 Pearson Education, Inc.

The Visceral Mass Separates Internal Organs from the Hydrostatic Skeleton • The visceral mass is the region in all mollusks where organs and surrounding fluids are located, separate from the muscular foot • Separation of the foot from the visceral mass may have enabled greater diversification of both features across the phylum © 2017 Pearson Education, Inc.

The Visceral Mass Separates Internal Organs from the Hydrostatic Skeleton • The coelom is highly reduced in most mollusks, functioning mostly in reproduction and excretion of wastes • Organs occupy a different type of body cavity called a hemocoel, where body fluids bathe organs directly in an open circulatory system • Different from a coelom because it is not lined in mesoderm and has a distinct developmental origin © 2017 Pearson Education, Inc.

The Visceral Mass Separates Internal Organs from the Hydrostatic Skeleton • At the anterior end of the visceral mass, the mouth has a feeding structure called a radula, which functions like a rasp or file • Moves the radula back and forth over the food source, causing the many sharp plates to scrape material so that it can be ingested • Radula probably evolved early in molluscan evolution • Lost in the bivalves, which acquire food by suspension feeding © 2017 Pearson Education, Inc.

Figure 31. 9 50 µm Rasp-like radula scrapes off pieces of food © 2017

The Mantle Has Diverse Functions • In many species, the mantle secretes a shell made of calcium carbonate • Some mollusk species have a shell with one, two, or eight parts, called valves; others have no shell at all • In bivalves, the protective shell is hinged and closes • Many marine and terrestrial snails can retract into their shells when they are attacked or when their tissues begin to dry out © 2017 Pearson Education, Inc.

The Mantle Has Diverse Functions • A protective shell may have been an important adaptation • But, shells made of calcium carbonate are heavy • Thus, there is a trade-off between protection and mobility • Mollusks with thick shells are constrained to aquatic habitats, where buoyant forces help support the load • The largest mollusks have highly reduced shells or none at all • Terrestrial mollusks have thin shells or none at all © 2017 Pearson Education, Inc.

The Mantle Has Diverse Functions • Numerous adaptations of the mantle allow diverse functions other than secreting shells • In some terrestrial snails, the mantle forms an internal lung • In bivalves and cephalopods, the mantle is lined with muscle and forms tubes called siphons © 2017 Pearson Education, Inc.

The Mantle Has Diverse Functions • Numerous adaptations of the mantle allow diverse functions other than secreting shells • In clams, two siphons extend into the water column above, controlling incurrent and excurrent water flow over the gills • In cephalopods, the mantle cavity fills with water and then mantle muscles contract, forcing a stream of water out of a single siphon • The force of the expelled water provides a form of locomotion called jet propulsion © 2017 Pearson Education, Inc.

Figure 31. 10 Cavity enclosed by mantle fills with water © 2017 Pearson Education,

What Is an Ecdysozoan? • Unlike lophotrochozoans that grow continuously and incrementally, ecdysozoans grow intermittently by molting • Shedding of the soft cuticle or hard exoskeleton • Once the animal molts, fluid causes the body to expand—then a new, larger cuticle or exoskeleton forms • Ecdysozoans undergo a succession of molts as they grow • Sometimes dramatic morphological transformations occur during these molts © 2017 Pearson Education, Inc.

What Is an Ecdysozoan? • The cuticle and exoskeleton • Protect these animals from predators • Provide an effective structure for muscle attachment • During molting, the animal’s soft body is exposed and vulnerable © 2017 Pearson Education, Inc.

Figure 31. 11 (a) Lophotrochozoans grow incrementally. Growth bands © 2017 Pearson Education, Inc.

Table 31. 3 © 2017 Pearson Education, Inc.

What Is a Roundworm? • Species in the phylum Nematoda are commonly called roundworms or nematodes • Unsegmented worms with a pseudocoelom, a tubewithin-a-tube body plan, no appendages, and an elastic cuticle that is molted during growth • Lack specialized systems for exchanging gases and circulating nutrients • Gas exchange occurs across the body wall, and nutrients move by diffusion from the gut to other parts of the body © 2017 Pearson Education, Inc.

What Is a Roundworm? • Roundworms play an important role in their ecosystems • Feed on a wide variety of materials including bacteria, archaea, fungi, plant roots, small protists, and detritus • In most species, the mouthparts are structured in a way that increases the efficiency of feeding on a particular type of organism or material • Most roundworms are free-living • Some species parasitize animals, including humans © 2017 Pearson Education, Inc.

What Are Tardigrades and Velvet Worms? • Tardigrada and Onychophora are of special interest due to their close relationship to arthropods • Similar to arthropods in having a segmented body and segmented limbs • Unlike arthropods, their cuticle is not hardened as an exoskeleton, and their limbs are not jointed © 2017 Pearson Education, Inc.

What Are Tardigrades and Velvet Worms? • The tardigrades, or “water bears, ” are microscopic animals that live in diverse marine, freshwater, and terrestrial environments, such as in the film of water that covers plants in moist habitats • The onychophorans, or velvet worms, are small, caterpillar-like organisms that live in moist leaf litter and prey on small invertebrates © 2017 Pearson Education, Inc.

What Is an Arthropod? • In terms of duration in the fossil record, species diversity, and abundance of individuals, arthropods are the most important phylum within the Ecdysozoa • Appear in the fossil record over 520 million years ago • Most abundant animals observed in both aquatic and terrestrial environments • Over a million living species have been described © 2017 Pearson Education, Inc.

Arthropod Body Plan • Arthropods are defined by three key features: 1. Segmented body organized into prominent regions, or tagmata (singular: tagma) • Body of grasshopper and other insects is divided into a head, thorax, and abdomen • Body of a spider or a crayfish is divided into the cephalothorax (“head-chest”) and abdomen © 2017 Pearson Education, Inc.

Arthropod Body Plan • Arthropods are defined by three key features: 2. Exoskeleton made primarily of the polysaccharide chitin, strengthened by calcium carbonate (Ca. CO 3) in crustaceans 3. Jointed appendages, which enable the rigid body to move © 2017 Pearson Education, Inc.

Figure 31. 12 (a) Insects have three tagmata. Head Thorax Tagmata Abdomen Jointed limbs Exoskeleton (covers body) Segmented body Tagmata (b) Spiders have two tagmata. Abdomen Cephalothorax Chelicerae Pedipalp © 2017 Pearson Education, Inc.

Why Has the Arthropod Body Plan Been So Successful? • Like mollusks, arthropod bodies are modular in evolutionary terms • Studies of Hox genes and other tool-kit genes show that • Small changes in the timing and location of gene expression can result in novel shapes and sizes • Variation in gene expression, combined with ecological opportunity through natural selection: • Can result in the diversification of arthropod body segments and appendages © 2017 Pearson Education, Inc.

Figure 31. 13 Primary functions: Sensory Feeding Defense Walking Swimming © 2017 Pearson Education,

Origin of the Wing • Insects were the first animals on Earth that had wings and could fly • Hypotheses about the origin of wings vary, and include • Independent origin hypothesis • Wings arose as outgrowths from the thorax independent from the legs • Gill co-option hypothesis • Wings arose from gill-like projections on the branched legs of a wingless ancestor © 2017 Pearson Education, Inc.

Figure 31. 14 (a) Independent origin hypothesis Body crosssection Wing Leg (b) Gill co-option

Origin of the Wing • The hypotheses were tested using developmental genetics • Results

Arthropod Diversity • Arthropod taxonomy traditionally recognizes four major lineages: 1. Myriapods 2. Insects

Table 31. 4 -1 © 2017 Pearson Education, Inc.

Myriapods Have Long, Segmented Trunks • Myriapods have relatively simple bodies: • A head region • A long trunk with segments that each have one pair of legs (centipedes) or two pairs of legs (millipedes) • Some species have no eyes, but others have from a few to many simple eyes clustered on the sides of the head • Myriapods inhabit terrestrial environments all over the world © 2017 Pearson Education, Inc.

Insects Have Three Tagmata, Unbranched Appendages, and One Pair of Antennae • Insects have three tagmata—a head, thorax, and abdomen • Three pairs of walking legs extend from the sides of the thorax • Most species have one or two pairs of wings on the back of the thorax © 2017 Pearson Education, Inc.

Insects Have Three Tagmata, Unbranched Appendages, and One Pair of Antennae • Head has four pairs of mouthpart structures, one pair of slender antennae that are used to touch and smell, and a pair of compound eyes • A compound eye contains many lenses, each associated with a light-sensing, columnar structure • ~1 million species of insects have been named thus far, but it is certain that many more exist • Insects dominate terrestrial environments • Larvae of some species are common in freshwater aquatic environments © 2017 Pearson Education, Inc.

Table 31. 4 -2 © 2017 Pearson Education, Inc.

Crustaceans Have Two or Three Tagmata, Branched Appendages, and Two Pairs of Antennae • Crustaceans live primarily in aquatic environments, where they play important ecological roles: • Planktonic copepods—consume phytoplankton and, in turn, are an important food source for fish and marine mammals • Just a few species of crabs and some isopods are terrestrial © 2017 Pearson Education, Inc.

Crustaceans Have Two or Three Tagmata, Branched Appendages, and Two Pairs of Antennae • The segmented body of most crustaceans is divided into three tagmata: the head, thorax, and abdomen • Some species have a carapace (platelike section of exoskeleton) that covers and protects head and thorax • Crustacean appendages are usually branched, but some have lost their branch, becoming secondarily unbranched © 2017 Pearson Education, Inc.

Crustaceans Have Two or Three Tagmata, Branched Appendages, and Two Pairs of Antennae • Two pairs of antennae; sophisticated, compound eyes (usually on stalks) • Most have 4– 6 pairs of mouthparts derived from jointed appendages • Many have mouthparts on head (maxillae) • Some have a pair of mouthparts called mandibles that can bite or chew • Many have feeding appendages on thorax segments, called maxillipeds or claws © 2017 Pearson Education, Inc.

Chelicerates Have Two Tagmata and Chelicerae • The most prominent lineages of chelicerates are terrestrial (spiders, scorpions, ticks, mites, and daddy longlegs) • Some are marine (horseshoe crabs and sea spiders) © 2017 Pearson Education, Inc.

Chelicerates Have Two Tagmata and Chelicerae • Chelicerate body consists of two tagmata: the cephalothorax (prosoma) and abdomen opisthosoma) • Chelicerate tagmata are not homologous to those in other arthropods, even though they are functionally similar • Cephalothorax lacks antennae, but usually has eyes © 2017 Pearson Education, Inc.

Chelicerates Have Two Tagmata and Chelicerae • Named for a pair of clawlike appendages called chelicerae, located near the mouth • Depending on the species, the chelicerae are used in feeding, defense, copulation, movement, or sensory reception • Also have a pair of pedipalps located just behind the chelicerae, which may be used to manipulate food, transfer sperm, or carry out other functions © 2017 Pearson Education, Inc.

New Phylogeny of Arthropods • Relationships of lineages within the arthropods are currently undergoing a major revision • Insects and myriapods have long been considered sister groups within the arthropods due to shared morphological characteristics • However, recent phylogenetic studies provide strong support for the placement of the insect clade within the crustacean lineage, making the traditional concept of crustaceans paraphyletic © 2017 Pearson Education, Inc.

Figure 31. 15 ONYCHOPHORA CRUSTACEA Crabs Lobsters Shrimp Isopods Copepods Two pairs of antennae Barnacles Brine shrimp Remipedes Loss of one pair of antennae ARTHROPODA Single pair of antennae Chelicerae and pedipalps Insects MYRIAPODA Centipedes Millipedes CHELICERATA Sea spiders Mites Ticks Horseshoe crabs Scorpions Spiders © 2017 Pearson Education, Inc.

Two Types of Insect Metamorphosis • In insects, the presence or absence of a larval stage defines two distinct types of metamorphosis 1. Hemimetabolous metamorphosis (incomplete metamorphosis) • Form of direct development • Juveniles called nymphs look like smaller versions of the adult © 2017 Pearson Education, Inc.

Figure 31. 16 a (a) Aphid: Incomplete metamorphosis (hemimetabolous metamorphosis) Nymphs look like smaller

Two Types of Insect Metamorphosis 2. Holometabolous metamorphosis (complete metamorphosis) • There is a distinct larval stage • Larvae and adults often live in different habitats and feed on different foods • In holometabolous metamorphosis, larvae stop feeding and moving and secrete a protective case, becoming a pupa • During pupation, the pupa’s body is completely remodeled into a new, adult form © 2017 Pearson Education, Inc.

Figure 31. 16 b (b) Mosquito: Complete metamorphosis (holometabolous metamorphosis) Adult Pupae Larvae look

What Is the Adaptive Significance of Metamorphosis? • Complete metamorphosis is 10 times more common than incomplete metamorphosis • Biologists hypothesize that this is because of feeding efficiency • If adults and juveniles feed on different materials in different ways, they do not compete with each other for resources © 2017 Pearson Education, Inc.

What Is the Adaptive Significance of Metamorphosis? • Another hypothesis is based on the advantages of functional specialization • Specialization can lead to higher efficiency in feeding and reproduction and thus higher fitness © 2017 Pearson Education, Inc.