Chapter 18
I. Phylum Arthropoda
 ¾ of all known animal species; abundance and wide
ecological distribution makes them most diverse
 Dates from late Precambrian
 Distinct metameres: linear somites with jointed
appendages; somites may be fused or combined
into specialized groups called tagmata; appendages
specialized for division of labor
 Size ranges from 0.1 mm to 4 m
 All modes of feeding are used but most herbivorous
II. Body Plan
A. Exoskeleton
 Cuticle protective but jointed for mobility; inner thick
procuticle and outer thin epicuticle
 Consists of chitin bound with protein making a
lightweight, flexible barrier that protects against
 Laminated and tanned making it harder
 Thins at joints to allow movement
 Infolds to line foregut, hindgut, and trachea
 Ecdydis, molting, sheds outer covering and growing
new, larger one; exocuticle secreted by procuticle
prior to molt and endocuticle secreted afterwards.
 Arthropods molt 4-7 times, with weight of
exoskeleton the ultimate limit of size of animal
B. Segmentation & Appendages
 Each somite bears pair of segmented legs
 Segments and appendages modified for various
adaptive functions such as food handling,
sensing, walking, or swimming
 Limb segments are hollow levers with internal
striated muscles
C. Respiration
 Tracheal system
efficiently delivers
oxygen to cells in
terrestrial arthropods
 Gills are used by
aquatic arthropods
D. Sense Organs
 Eyes vary from simple ocelli to a compound
mosaic eye
 Other senses are used for touch, smell, hearing,
balancing, and chemical reception
E. Behavior
 Surpass most other
invertebrates in
complex and
organized activities
 Most behavior innate
but some is learned
F. Metamorphoses
 Most undergo
metamorphoses that
result in different larval
and adult forms
 Larval and adult forms
occupy different
habitats and eat
different foods to avoid
competing with each
III. Subphylum Trilobita
 Arose before Cambrian, flourished, died out 200 mya
 Trilobed body shape; head, thorax, pygidium
 Head fused segments; bore compound eyes, antennae, mouth,
and jointed appendages
 Gills
 Bottom dwellers; probably scavengers
 2- 67 cm long; could roll up like pill bugs
IV. Subphylum Chelicerata
A. General Features
 Six pairs of
chelicerae (most
anterior on head),
pedipalps (2nd pair),
and 4 pairs of legs
 No mandibles or
 Most suck liquid from
B. Class Merostomata
 Horseshoe crabs; unchanged since Triassic
 5 species
 Unsegmented carapace covers body; 2 compound eyes and 2
simple eyes
 Cephalothorax bears 1 pair chelicerae and 5 pairs of walking
 Abdomen bears book gills on appendages in median line
 Larvae resemble trilobites
C. Class Pycnogonida
 Sea spiders
 Many have chelicerae
and palps; 5-6 pairs of
 Proboscis sucks fluids
from cnidarians and
other soft-bodied
marine organisms
 Live in all oceans but
more common in polar
D. Class Arachnida
1. General Features
 Spiders, scorpions, daddy longlegs, ticks, and
mites; over 70,000 species
 Live mainly in warm dry regions
 Body consists of cephalothorax (chelicerae,
pedipalps, 4 pairs of walking legs) and abdomen
 Predaceous with claws, fangs, poison glands, or
 Sucking mouth parts ingest fluids and soft
 Most harmless and control undesirable insects
2. Order Araneae: Spiders
a. Feeding
 Pair chelicerae with
terminal fangs
 Pedipalps help handle
 All spiders predaceous,
feeding mainly on
insects which are killed
with poison and fangs
 Injected venom liquefies
and digests the tissues;
sucked into stomach
b. Respiration
 Book lungs or tracheae used
 Book lungs unique to spiders; parallel air pockets
extend into blood-filled chamber; air enters
chamber through lit in body wall
 Tracheae are tubes that carry air from the inside
directly into the tissues
c. Excretion
 Malpighian tubules are used for excretion
 Potassium, other solutes, and wastes are
secreted into tubules
 Rectal glands reabsorb the potassium and water,
leaving wastes and uric acid for excretion; this
conserves water allowing spiders to live in dry
d. Sensory Systems
 Most have 8 simple
eyes, each with a
lens, optic rods, and a
retina; used to detect
movement and may
form images
 Sensory setae detect
air currents, web
vibrations, and other
e. Web-spinning
 Spinning silk critical
 2-3 pairs of spinnerets
contain tubes that go to
silk glands
 A liquid protein hardens
as it is extruded; the
silk is very strong and
will stretch
considerably before
 Silk is used for webs,
lining burrows, forming
egg sacs, and wrapping
f. Reproduction
 Male stores sperm in
pedipalps before mating
 A courtship ritual is
required by female
 Male inserts pedipalps
into female genital
 Eggs develop in a
cocoon in the web or
may be carried by the
 Young hatch in 2 weeks
g. Harmfulness
 Spiders consume undesirable
 American tarantulas rarely bite
and bite is not dangerous
 Some black widow spiders are
venomous to humans, being
 Brown recluse spider has
hemolytic venom that destroys
tissue around bite
 Some Australian and South
American spiders are the most
dangerous and aggressive
3. Order Scorpionida: Scorpions
a. Features
 Most common in tropical and subtropical areas
 Nocturnal and feed largely on insects or spiders
 Sand scorpions detect prey by feeling surface
waves with leg sensillae
 Cephalothorax has appendages, medial eyes,
and 2-5 lateral eyes
 Preabdomen has 7 segments
 Postabdomen has tail that ends in a stinger
 Stinger has venom that varies from mildly painful
to dangerous
b. Reproduction
 Comblike pectines
under abdomen are
used in sex
 During mating dance,
male guides female
over spermatophore
 Ovoviviparous or
viviporous producing
6-90 young
4. Order Opiliones:Harvestmen
 Daddy long legs
 Common in tropical regions
 Cephalothorax and abdomen join broadly without
any narrowing as in spiders
 Chelicerae are pincer-like and used for scavenging
5. Order Acari: Ticks and Mites
a. Features
 30,000 species
 All environments
 Fusion of cephalothorax
and abdomen with no
external segmentation
 Mouth on end of tube
 Chelicerae pierce, tear,
or grip food
 pedipalps, and 4 pairs
of legs
 Hatchlings are 6-legged
larvae, followed by 8legged nymphal stage
b. Relationship with Humans
 House dust mites are free-living and often cause
 Spider mites suck out plant nutrients and is common
agricultural pest
 Chiggers are larval mites that feed on skin and cause
 Hair follicle mite is harmless but others can cause
mange in domestic animals
 Human itch mite causes intense itching
 Ticks may transmit Lyme disease, Rocky Mountain
spotted fever, or Texas cattle fever