subphylum chelicerata 2 ppt
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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:
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