Invertebrate Zoology

Lecture 17: Phylum Arthropoda, Part 1

Lecture outline Phylum Arthropoda

Diversity/ClassificationPhylogenyBauplan BasicsFocus: Arthropod exoskeletonFeeding/digestionCirculation/gas exchangeOsmoregulation/excretionNervous system/sensoryReproduction

Diversity/Classification Subphylum

Trilobitomorpha Two longitudinal

furrows divide body into three sections

Marine; Paleozoic Extinct (end of

Permian)

Diversity/Classification Subphylum Crustacea: Lobsters,

crabs, barnacles, copepods, etc…

Antarctic krillEuphausia superba

Subphylum Hexapoda: Insects

Dennis Kunkel

Brusca & Brusca

Subphylum Myriapoda: Centipedes and Millipedes

Subphylum CheliceriformesSpiders, mites, horseshoe crabs,

scorpions, sea spiders et al…

PhylogenyBased on body plan & development

Hypothesis 1:

Bauplan basics Features shared with Phylum Annelida

Classic protostome development Segmentation; segments added posteriorly Nervous system architecture

Derived characters (differ from Annelida) Rigid cuticle: contains chitin; forms

exoskeleton Open circulatory system; hemocoel Reduced coelom

Key hypothesis: loss of functionality (i.e. peristalsis)

Where are coelom “remnants” located?

Intermediate phylaWhy considered intermediate?

Phylum Onychophora

Phylum Tardigrada

Bauplan basics Other features

Tagmatization

Paired jointed appendages “Primitive condition”: appendages associated

with each segment, Lack motile cilia, except for some sperm

Why? Which other group has non-motile cilia?

Focus: cuticle Epidermis

Secretes cuticleDermal glands

Variety of secretionsSensory structures

ProcuticleChitinCalcium carbonate

CrustaceaSclerotizationJoints

EpicuticleCement layerWax layer

Molting

Feeding/digestion

Different modes of feeding Variety of mouthparts Complete digestive system

Note digestive cecum

Circulation/gas exchange Open circulatory system; ostiate heart

Hemocoel: space bounded by basement membrane of epidermis

Re-entry of hemolymph into heart?

Circulation/gas exchange

Respiration: Subphylum Crustacea Gills; body surface (small forms)

Circulation/gas exchange

Respiration: Hexapoda & Myriapoda Tracheal system (openings = spiracles)

Circulation/gas exchange

Respiration: Cheliceriformes book lungs; book gills

Crustacea: Gills (ammonia release only Antennal (green) gland

Entry via hemocoel channels & active transport Selective secretion and uptake along tubule

Osmoregulation & ammonia excretion

Hexapoda, Myriapoda, Chelicerata: Malpighian tubules

Closed distally: hemocoelActive ion transport

fluid uptake Open into hindgut

Adaptive value?

Osmoregulation & ammonia excretion

Organization Brain (=cerebral ganglia) Subesophogeal (subenteric)

ganglion Ventral nerve cord (paired,

fused partly or completely) Ganglia: one per segment

or fewer (consolidated)

Nervous system

Crab: Note consolidated thoracic ganglia

Nervous system

Sensory systems: Compound eye

Compound eyes comprised of multiple ommatidia Interommatidial hairs in some…

Sensory systems: Compound eye

Key structuresCorneaCrystalline

cone/stalkIris (=corneal

pigment cells)Retinular cell

w/rhabdome

Sensory systems: Compound eye Light-adapted vs. dark-adapted eye

Shift in pigment position (within iris)Alteration in light pathwayConsequences for sensitivity and acuity

Sensory systems: Compound eye Mosaic theory of insect vision

Each ommatidium views piece of visual field Adaptive advantages of compound eye Acuity varies with

Ommatidia concentrationSizeFlatness

Key: angular distance

Sensory systems: Sensillae Mechanosensory & chemosensory

Reproduction: Overview

Sexual reproduction Usually gonochoristicSome hermaphrodites

Example: Barnacles

Parthenogenesis in some groupsPart of aphid life cycleSome crustaceans living in temporary ponds

Egg production often food limited. Why?Example: Antarctic krill

Reproduction: Male system Testes Vas deferens Seminal vesicle Accessory glands

Types of secretions?

Ejaculatory duct Gonopore

Spermatophores (in some)

Penis or modified appendages for copulation

Damselfly penis Barnacle penis

Spider pedipalps

Reproduction: female system Ovaries

Ovariole structureEgg batches

Oviduct Accessory glands

Types of secretions?

Copulatory bursa Spermathecae

Function? Bees and lobsters…

Reproduction & molting Mate guarding

Adaptive value for male?Adaptive value for female?

Reproduction & cannibalism Hexapoda & Cheliceriformes (i.e. spiders)

Adaptive value for male?Adaptive value for female?