Subphylum
Chelicerata
Subphylum Chelicerata

70,000 species

Spiders, scorpions, mites,
and horseshoe crabs

Most are terrestrial

Very diverse, with many
interesting adaptations to
terrestrial existence
General Body Form

Possess two tagmata



Cephalothorax





Cephalothorax
Abdomen
No antennae, but typically two
compound eyes and four ocelli
Chelicerae – most anterior
appendages; used for feeding
Pedipalps – second appendages
with various functions
Walking legs – four pairs
Abdomen



Typically lacks appendages
Houses many visceral organs
Respiratory structures
Class Xiphosura

Horseshoe crabs

Largest chelicerates


Up to 75cm long
Only three genera,
comprised of four species


Limulus is the most famous;
lives off N. American Atlantic
coast
Remaining three species
inhabit the Indo-Pacific Ocean
Body Form

Cephalothorax is shaped
like a horse’s hoof

Dorsal exoskeleton is called a
carapace

Two large lateral compound
eyes and two medial ocelli

Appendages are located
ventrally; most have chelae




Chelicerae
Pedipalps
Three pairs of walking legs
Pusher legs for pushing soft
sediment

Abdomen

Ventral concavity that forms
branchial chamber

One pair of large flat genital
opercula, with two gonopores
(either male or female)

Five pairs of book gills are
housed here

Long tail spine (telson) that
extends posteriorly from last
abdominal segment



Used for pushing
Righting body
Anus located near base of tail
spine
Nutrition

Omnivorous

Eat molluscs, worms, algae,
etc.

Walking leg coxae have
gnathobases

Medial space between
gnathobases forms a food
groove

Gnathobases crush food and
particles are moved towards
mouth
Digestive System

J-shaped gut

Mouth opens posteriorly at
anterior end of food groove

Stomach

Has two pairs of large digestive
ceca



For extracellular digestion and
absorption
May be white as they take up
extra calcium from blood, which
is later voided with feces
Not edible; some have high
concentrations of saxotoxin
Hemal System

Hemocoel is main body
cavity

Heart has 9 pairs of ostia

Blood contains:


Hemocyanin
Two types of hemocytes (for
clotting and producing more
hemocyanin)
Respiratory System

Book gill surface area is
approximately 2m2

Gill movements ventilate
branchial chamber

Inhalant aperture on each
lateral aspect of carapace (at
junction of cephalothorax
and abdomen)

Exhalant aperture at both
sides of tail spine
Excretory System

Ammonotelic, so wastes removed across gill surface

Four pairs of saccate nephridia assist with
osmoregulation in brackish water

Two nephridiopores located near the last pair of
walking legs
Nervous System

CNS




Large brain
Two longitudinal nerve cords
Five pairs of segmental ganglia that
serve book gills
Sensory structures

Lateral compound eyes



1000 ommatidia each
Do not form images, but detect
movement
Flabellum


Located on pusher leg, in the path of the
inhalant current
Chemosensor that monitors water
quality
Reproduction

Gonochoric

External fertilization via
flagellated sperm

Have specific mating seasons in
spring and summer





Influenced by lunar cycles and tides
Mating occurs near shore
Smaller male climbs onto female’s
dorsal abdomen and claps her with
pedipalps
Female digs a depression in the sand
and oviposits 2000 – 30,000 eggs
Male releases sperm into depression

Larvae are called trilobite
larvae





One cm long
Small tail spine and two pairs
of book gills
Burrow in sand
9 – 12 years to reach sexual
maturity
May live up to 19 years
Class Arachnida

70,000 described, with an estimated
one million more undescribed

Spiders, scorpions, solifuges, and
mites

Almost entirely terrestrial

Very well adapted for terrestrial
existence




Waxy epicuticle
Book gills transformed into book lungs
for breathing air
Malpighian tubules
Possess other interesting adaptations


Silk
Poison glands
Body Form

Two tagma are sometimes
connected by a pedicel

Similar appendages in all



Chelicerae – one pair that
may have fangs and poison
glands
Pedipalps – one pair that
have many functions, and
may have gnathobases
Walking legs – four pairs
that may terminate in claws
Nutrition

Mostly carnivorous, but some are
ectoparasites

Digestion begins extraorally





Prey is captured and killed
Chelicerae and gnathobases of
pedipalps macerate
Digestive enzymes digest
Liquids are ingested
Gut is adapted for a liquid diet



Either pharynx or stomach
functions as a muscular pump
Gut has a very small diameter
No gizzard
Gas Exchange

Small arachnids use body surface

Large arachnids have two types of organs

Book lungs





Invaginations of ventral abdominal wall with a slit-shaped spiracle
Atrium contains lamellae supported by pedestals
Muscle contractions expand atrium to facilitate ventilation
Atrium surrounded by pulmonary sinus
Trachea



Do not make direct contact with cells as in insects
Blood delivers gasses (possesses hemocyanin)
Tracheae conserve more water than book lungs (huge SA)
Excretion

Uricotelic

Possess either saccate nephridia, Malpighian tubules,
or both

Also possess nephrocytes



Large phagocytic cells that police the hemocoel
Sequester metabolic wastes indefinitely
May detoxify wastes and return to blood
Nervous System

Highly cephalized

Sensory organs

Trichobothria



Slit sense organs





Setae found in high
concentrations on legs
Can sense air currents from fly
wings, and aid in prey capture
Slits in exoskeleton with a thin
membrane covering
Detect strain on exoskeleton
May be found in clusters called
lyriform organs
Some have as many as 3000 slit
sense organs
Flagellum

Lack antennae, so some posses a
posterior sensory flagellum
Reproduction

Always gonochoric

Always internal
fertilization

Sperm transfer

Direct


Penis
Indirect

Spermatophore placed
on ground, or
transferred by pedipalp
Order Scorpiones

1200 species

Found on all continents except
Antarctica


Secretive and nocturnal




Baja California has greatest
diversity
Hide under rocks, logs, or in
burrows during the day
Emerge at night to hunt
Fluoresce green under UV light
Most are 3 – 9 cm, but Hadogenes
from Africa can reach 21 cm
Body Form

Long segmented abdomen

Posses a telson (sting)

Pedipalps are enlarged to form
pincers

No pedicel

Two comb-like sensory structures
(pectines) on ventral surface

Several segments possess
spiracles that open into book
lungs
Nutrition

Feed on other inverts,
especially insects and
arachnids

Ambush predators


Wait at burrow entrance
Claws open and ready to
strike

Some dig up prey

Some can catch prey in
midair

Prey is held with pedipalps and
stabbed with sting (if needed)

Metabolic rates are among the
lowest recorded for animals


Conversion of prey to biomass
is very efficient


Can survive over one year
between meals
One meal can increase body
weight by 1/3
Poison glands

Only 25 species are potentially
lethal to humans


Centruoides in SW US
Androctonus in N. Africa
Organ Systems

Respiratory


Hemal


Four pairs of book lungs,
with spiracles located
ventrally
Standard
Excretory


Two pairs of Malpighian
tubules
One pair of saccate
nephridia

Nervous system

Ventral nerve cord

Seven segmental ganglia

Giant axons for rapid closing of
claws and stinging motion

Pectines




Sensory role is unclear
Sensitive to ground vibrations
Suspected chemosensory role in
spermatophore deposition
Ablation of pectines results in
failure of male to deposit
spermatophore
Adaptations to Arid Existence

Lowest evaporative water loss of any
invertebrate

0.01% body weight / hour

Tolerate up to 40% body water loss

Obtain water from food and rarely drink

Uricotelic

Impermeable exoskeleton

Burrow up to one meter in some

Nocturnal

Raise bodies off of ground (stilting)
Reproduction

Indirect sperm transfer via
spermatophore

Usually a courtship ritual where
claws are grasped and the two
walk around as if dancing

Male deposits spermatophore




Stands approximately 6 mm high
Consists of a lever, stalk, ejection
apparatus and sperm reservoir
Male positions female over it
Female pectines cause sperm reservoir to
eject into open gonopore

Viviparous

Direct development

1 to 95 young depending on
species

Juveniles crawl onto mother’s
back and remain through first
molt (4 weeks)

High degree of parental care

Six months to six years to reach
sexual maturity

May live 25 years
Order Uropygi

100 species of whip scorpions

Some such as the North American
vinegaroon (Mastigoproctus giganteus) can
reach 8 cm in length

Interesting structures



Large pedipalps with pincers
Flagellum
Repugnatorial glands near anus, that spray acetic
acid at predators

In some, males use pedipalps to transfer
spermatophore to female’s gonopore

Females lay eggs in a sac that she attaches to
her body
Order Amblypygi

Whip spiders

Secretive and nocturnal

4 to 45 mm

Flattened bodies that share
characteristics of spiders and crabs

Often move sideways like crabs

Lack poison glands

Large spiny pedipalps

First legs are extremely long and thin



Sensory
One pointed forward and one sideways
while animal moves
Have a pedicel
Order Araneae

40,000 species of spiders

Huge population densities


Five million / hectare in one field
in Great Britain
Silk is probably most important
adaptation to spider existence

Only arachnids with silk spinning
appendages (spinnerets)

Possess a pedicel, which allows
abdomen to move independently
of cephalothorax, in order to
distribute silk

Carnivorous
Body Form

Large variation in body size (0.5 mm to 9
cm)

Typically have eight eyes

Chelicerae possess fangs and poison
glands

Distal tips of male pedipalps are knoblike
and modified for sperm transfer

Four pairs of walking legs that terminate in
either two (cursorial) or three (web
building) claws

Gonopore and book lung spiracles located
ventrally

Tracheal spiracle located posteriorly, near
spinnerets
Silk

Produced by abdominal silk glands

Extruded from spigots on distal tips of
spinnerets

Different types of glands (six known)
that produce different silks

It is a protein that is emitted as a liquid ,
but hardens as it is drawn out

Strong, but resistance to breakage is
mainly due to its elasticity

Many functions:




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Prey capture (web building)
Shelters
Egg cases
Draglines
Balooning in spiderlings
Prey Capture

Web building spiders



Most famous type of web is the orb web




Webs are strategically placed and oriented
according to prey habits
Web type differs between species, which
partitions resources
One plane
Radial threads aren’t sticky (spider walks on
these)
Catching spiral is sticky
Spiders are sensitive to web vibrations

Can sense type, size and location of prey

Cursorial (hunting) spiders

Do not build webs

Ambush or actively forage

Heavier legs

Possess scopulae




Tufts of setae near claws
Each seta may have over
1000 branches (huge SA)
Use for adhering to slick
surfaces and prey
Can walk up walls, glass, or
upside-down across ceilings
Nutrition

Fangs on chelicerae penetrate prey

Muscles surrounding poison
glands contract

Low metabolic rates

Can double body weight with one
meal

Mainly carnivores of insects, but
some wasps (tarantula hawks and
mud daubers) use spiders as food
for larvae

Most spider venom is not toxic to
humans

A few genera are toxic, but rarely lethal

Latrodectus



Loxosceles




Black widows have neurotoxic venom
Symptoms of bites include nausea, pain, and
muscles spasms
Recluses have hemotoxic venom
Six eyes, and a violin shape on cephalothorax
Causes tissue necrosis
A few genera are lethal

Atrax



Australia’s funnel-web spider
World’s deadliest spider
Trechona

Lives in South America
Gas Exchange

Respiratory structures located
ventrally

Primitive spiders (such as
tarantulas) possess two pairs of
book lungs

Most possess one anterior pair of
book lungs and one posterior set
of tracheae (with one spiracle)

Some only possess tracheae
Hemal and Nervous Systems

Hemal


Heart only has 2 or 3 pairs of ostia
Spiders lack extensor muscles, so
legs are extended by elevating
hemocoel pressure



Legs are always flexed in dead
spiders
Jumping spiders can jump 50X
body length by quickly elevating
blood pressure
Nervous


Eyes lack ommatidia
Vision is important to cursorial
spiders


Jumping spiders have best vision of
all arachnids
Main eyes form images while
secondary eyes detect movement
Reproduction

Sexually dimorphic (males usually
much smaller)

Highly modified pedipalps are
used for sperm transfer

Have a sperm reservoir

Male builds sperm web and
extrudes sperm (not
spermatophore) onto web

Dips pedipalp into sperm, fills
reservoir, and searches for
receptive female

Courtship behavior,
chemical and tactile cues
are important so that the
female doesn’t eat the
male

Orb-weaver males pluck
radial threads of female’s
web during courtship


Number, frequency and
intensity of plucks is
species specific
Female lays up to 300
eggs in a silk case

Egg case is attached to web,
inside of home, or spider
itself

Spiderlings remain in egg
case until completing first
molt

Disperse via ballooning

Most live one to two years,
but some have lived for 25
years in captivity
Order Pseudoscorpiones

Small, less than 7mm

2500 species

Live in leaf litter

Large chelate pedipalps with poison
glands

Feed on small arthropods such as mites

Exhibit a phenomenon known as
phoresy

Grab leg of a flying insect and hitch a ride
Order Solifugae

Many names: solifuges (with
many variations on spelling),
sun spiders or wind scorpions

Body size: a few mm to 7 cm

Large chelicerae with the
largest mandibles (relative to
size) in the entire animal
kingdom

Two median eyes

Long, leg-like pedipalps

First legs are used as
antennae

Carnivorous or omnivorous

Many eat termites

Tracheae are similar to
those of insects

Oddly shaped sensory
organs


Called racquet organs
Two to five pairs hang from
fourth legs
Order Opiliones

5000 species of daddy longlegs
or harvestmen

Lack silk glands and poison
glands

Many are herbivores or feed on
dead organic matter

Gut is adapted for ingesting and
processing particulate matter

Body length sometime
reaches 22 mm, but leg
length ca reach 160 mm

May be brightly colored

Long legs are excellent
for maneuvering over
rough terrain

Males possess a penis
and females possess an
ovipositor
Order Acari

40,000 species of mites

Most mites are free-living, but some are
parasites of people, animals, and crops

Mites are the smallest arachnids



Some can inhabit the tracheae of honeybees
Some live in human hair follicles
The largest mites (called ticks) can reach 3
cm when engorged with blood

Lack segmentation;
cephalothorax is fused with
abdomen

Can be internal or external
parasites



Larvae of Trombicula are referred
to as chiggers
Mange mite burrows under skin,
lays eggs, and causes a skin
condition known as scabies
Can also be vectors of diseases

Example – Lyme disease
Class Pycnogonida

1000 species of sea spiders

Marine, but not really
spiders

Crawl around on eight long
legs

One mm to 10 mm long
normally, but Colossendeis
can achieve leg spans of 70
cm

Mostly feed on sessile
animals like hydroids, soft
corals, and anemones

Some are ectoparasites

Ovary has branches that
extend into femurs


Gravid females have swollen
legs
Males have ovigerous legs
(ovigers) that are used to
carry egg masses

Gathers released eggs and
cements them to ovigers