Phylum Arthopoda - Arthropods
includes spiders, scorpions, horseshoe crabs,
centipedes, crustaceans, insects
The largest and most diverse animal phylum.
Almost 2/3 of all species that have been described are arthropods
There may be as many as 30,000,000 species of insects alone
Abundant in all habitats, but dominate terrestrial regions
Most arthropods are small, but a few may be as large as 3.6 m
Economically important,
especially insects
Pollinate crops, control
insects and weeds
Many species are eaten especially crustaceans
Compete with humans
for food
Cause extensive damage
to food crops
Spread diseases
Key Innovations of Arthropods: Jointed appendages, complete
exoskeleton, segmental body with specialization of body regions
Arthropod means "jointed feet"
Jointed appendages are
specialized for different
functions: legs, mouthparts,
antennae
Joints in appendages make
them highly functional for
walking, and grasping
Antennae are sensory sounds and chemicals
Mouth parts specialized for
different food sources
Rigid exoskeleton, made of chitin and protein helps to protect
against predators and reduces water loss - allows life in dry
environments
Skeleton functions as attachment for muscles
muscles attach to interior of skeleton
Exoskeleton limits arthropods maximum size
although chitin is tough, it is brittle and cannot support great
weight without increasing its thickness greatly
Exoskeleton must be shed in order for increase in size - “ecdysis”
Segmental body plan clear in all forms, but segments often
fused to form specialized body regions - “tagmats”
Insect: head, thorax, abdomen
Crustacean: cephalothorax, abdomen
Living arthropods divided into three subphyla - based on appendages
Chelicerata:
first appendages are for
feeding - chelicerae fangs of spiders
feeding appendages of
horseshoe crabs
Crustacea and Uniramia (Tracheata) together called the
Mandibulates: first appendages are antennae, first feeding
appendages are called mandibles
Mandibulates are divided into two subphyla - Crustaceans and
Uniramians (Tracheata) - based on appendages
Crustaceans have biramous (branched) appendages
Uniramians (insects, millipedes, centipedes) have uniramous
(unbranched) appendages
General Biology of Arthropods
Exoskeleton
All arthropods covered by hardened exoskeleton
made of chitin and protein - secreted by and fused with
epidermis
varies in toughness and thickness
many crustaceans add calcium carbonate - increases rigidity
Functions of exoskeleton
prevents excessive water loss
protects from predators and injury
Molting - a liability of the exoskeleton
Growth requires periodic ecdysis, shedding of outer cuticle
New exoskeleton grows beneath old one
Separated by a fluid that dissolves components of old skeleton
Old skeleton cracks open and is shed
New skeleton is soft and must be expanded to full size
Hardens with exposure to air or water
A new exoskeleton
forms inside old old one must be
shed - “ecdysis”
The new
exoskeleton and all
of the body parts it
encloses must be
pulled out of old
exoskeleton includes legs,
claws, gills, etc.
Each new life
stage is called an
“instar”
Eyes
Ocelli are simple eyes with single lenses
Sometimes occur together with compound eyes
Function in distinguishing light and dark
Compound eyes
Composed of many ommatidia - independent units
Each covered with a lens - contains eight
retinular cells and central light-sensitive rhabdom
Each ommatidium receives its own
image and is connected to a nerve
cell - entire image is integrated in
the brain
Gut is tubular and extends from mouth to anus - with specialized
regions - crop, stomach (midgut), hindgut, intestine, rectum
Circulatory System is open
heart extends through
thorax and abdomen
contraction sends blood
forward
relaxation draws blood
from tissues
one-way valves in heart
allows blood to flow
forward only
blood from anterior end
flows through tissues to
posterior end
Respiratory System
Crustaceans have feathery gills under carapace
Chelicerates have book gills and book lungs - series of plates with
thin respiratory epithelium
Terrestrial Uniramians and some Chelicerates have trachaea
Trachea are branched tubules that allow air to diffuse into the body
smaller tracheoles bring air to individual cells
air entry controlled through external spiracles
closing spiracles conserves water
flow is mostly passive - muscular movements can increase flow
limits body size because all cells must be able receive oxygen
Excretory System
Several forms of excretory systems
Terrestrial Uniramians have Malpighian tubules
Slender tubular projections off the digestive tract at the
junction of the midgut and hindgut
Water and solutes pass through walls of tubules
nitrogenous wastes are precipitated as uric acid or guanine
Fluid with waste is emptied into hindgut and eliminated
Most water and valuable solutes reabsorbed by hindgut
An efficient system
that conserves water
- a good adaptation
for terrestrial life
Nervous System
Double chain of ganglia runs along ventral surface
Three fused pairs of dorsal ganglia form the brain
ventral ganglia control local activity of body regions
many activities continue with brain removed
Brain appears to be inhibitor, not stimulator (as in vertebrates)
some activities begin spontaneously when head is removed
Three Classes of Chelicerates
Class Arachnida - spiders, scorpions, ticks
Class Merostomata - horseshoe crabs
Class Pycnogonida - sea spiders
Class Arachnida - Arachnids
Largest class of chelicerates includes spiders, daddy longlegs,
scorpions, mites & ticks
All have a pair of chelicerae, pair of pedipalps, four pairs of legs
Chelicerae are first appendages, fangs with poison glands
Pedipalps are next, similar to legs - rarely used for locomotion
often used for catching and handling prey
may also chew with basal portion
may function as copulatory organs or
sensory organs
scorpion pincers are pedipalps
Most are carnivorous, some mites are herbivorous
Most ingest only liquified foods, digestion begins externally
Most are terrestrial, direct transfer of sperm for reproduction
Respire with trachea, book lungs or both
Class Merostomata - Horseshoe Crabs
Example: Limulus, common on North Atlantic coasts
Ancient group, Limulus fossils date to 220 million years old
Live in deep water, migrate to shallow coastal waters to mate
Feed at night on mollusks and annelids
Shell-like carapace over cephalothorax protects most body parts
Swim with carapace down moving abdominal plates
Possess four pairs of walking legs, chelicerae, and pedipalps
Respire via five pairs of book gills
Class Pycnogonida - Sea Spiders
Common in marine habitats, especially in cool waters
rarely observed because of small size
Not closely related to spiders
Adults are parasites or predators on other animals
Have sucking proboscis with terminal mouth
Body consists mostly of cephalothorax, no well-defined head
Possess four to six pairs of legs
Males exhibit parental care of young,
carry eggs on legs
Subphylum Crustacea - Crustaceans
includes shrimp, crabs, crawfish, lobsters, pillbugs, copepods,
brine shrimp, barnacles
have biramous (two branch) appendages, two pairs of antennae,
and 3 pairs of feeding appendages,
larger forms have feathery gills near base of legs
Most have a nauplius larva
Have legs on thorax and abdomen like millipedes and centipedes
but unlike insects
Having two pairs of antennae is unique to crustaceans
Many have compound eyes
Have tactile (touch sensitive) hairs over whole body
Excretion of nitrogen wastes occurs mostly across surface of cuticle
Variety of sexual styles and care of young
Crustacean Diversity
Decapod ("ten-footed") Crustaceans includes lobsters, shrimp, crabs, crawfish
Exoskeleton reinforced with calcium carbonate
Anterior segments fused into cephalothorax, covered by carapace
Crushing pincers common, used to obtain food and in defense
Swimmerets
used in
reproduction and
locomotion
Snapping of
telson and
uropods causes
forceful, rapid
movement to the
rear
Crustaceans Diversity
Terrestrial forms - pillbugs, sowbugs, isopods
Amphipods are both terrestrial and aquatic
Planktonic crustaceans
Copepods (Copepoda)
Water fleas (Cladocera)
Ostracods (Ostracoda)
Fairy shrimp and brine shrimp (Anostracoda)
Sessile Crustaceans
Include barnacles (Cirripedia)
Sessile adults, free-swimming larvae
Head attaches to substrate, food swept
into mouth by feathery legs
have shell-like plates covering body
Subphylum Uniramia (or Tracheata) - has three classes
Class Chilopoda - centipedes
Class Diplopoda - millipedes
Class Insecta - insects
Well adapted to terrestrial life
Respire with trachea
Malpighian tubules for excretion
Waxy cuticle on exoskeleton
Centipedes and Millipedes - both have head tagmat followed by
numerous repeating segments - each with paired appendages
Centipedes (hundred legs) have one pair of legs per segment
Millipedes (thousand legs) have two pairs of legs per segment
Centipedes - carnivorous, most eat insects
Appendages of first body segment are poisonous fangs
Millipedes - most are herbivorous
Can roll bodies into a flat coil
May secrete defensive fluids and cyanide gas
Reproduction similar in both groups
Sexes separate, fertilization is internal, copulate to
transfer sperm, all species lay eggs
Juveniles are similar to adults in appearance
Class Insecta - Insects
Largest group of organisms on earth - with great diversity
Especially numerous in the tropics
Most are terrestrial but many are aquatic in freshwater
Have three body segments: Head, thorax, abdomen
Have three pairs of legs, all attached to thorax
Have one pair of antennae
May have one or two pairs of wings
Sexes separate with internal fertilization
Wings arise as sack-like
outgrowths
Wings are solid except
for veins
Wings are not
homologous to other
appendages
Digestive tract is tubular and slightly coiled
digestion occurs within stomach or midgut
Excretion by Malpighian tubules
Respiration via trachea that extend throughout body
may have air sacs with muscles that create a bellows system
to allow deeper air circulation in body
Spiracles can be closed by muscles to retard water loss
Possess wide variety of sensory systems - in addition to eyes
Sensory hairs located all over bodies especially on legs and
antennae
Sounds detected by tympanum
Sensory hairs may also detect sound waves
Produce sounds which may inaudible to humans
Chemicals (pheromones) are also used to
communicate
Development
Most insects hatch from laid eggs - rarely develop within mother
After hatching young insects undergo regular ecdysis through a
series of instars
Often the larva is very different from the adult and undergoes
metamorphosis to become adult
In simple metamorphosis, wings develop during juvenile stage
there is no "resting stage" before last molt
Immature stages are called “nymphs”
Seen in beetles, grasshoppers, dragonflies, cockroaches, silverfish
Simple Metamorphosis
In complete metamorphosis, wings develop during a resting
stage just prior to final molt
resting stage called a pupa or chrysalis - at this time cellular
reorganization occurs to form adult body
Pupa does not normally move, except in mosquitoes
Complete
Metamorphosis
Juveniles and adults live in distinct habitats larvae often worm-like, lacking large eyes, and legs
Larvae usually have chewing mouth-parts, even if adults have
sucking mouth-parts
Pupae are usually inactive and do not feed
seen in moths, butterflies, beetles, bees, wasps, ants, flies, fleas