Arthropods
Insects were a winning combination
• Insects cause hugh economic losses (but a lot
of benefits, as well) each year
• They are the dominant group of animals on
the earth today
• The diversity of insects is far greater than all
other taxa combined
• There are an estimated 200 million insects
alive for every human today
Characteristics of Arthropoda
• Arthropods includes spiders, scorpions, ticks, mites,
crustaceans, millipedes, centipedes, insects, and
some smaller groups as well
• There is a rich fossil record extending back to the
mid-Cambrian period
Characteristics
• Arthropods have a well-developed organ
system and a chintinized cuticular exoskeleton
• Segments have coalesced into tagmata
• They range in size from .1 mm to 13 feet!
• Arthropods compete with us for food and
spread disease; they also produce silk, honey,
and beeswax
Ecological Relationships
• They are found in all environments and
virtually all altitudes and latitudes
• Species are adapted to land and to fresh,
brackish, and marine water
• Most species fly to their favored habitats
• All modes of feeding occur among arthropods
though the majority are herbivorous
• Nothing else matches their divesity
Characteristics of the arthropoda
• An exoskeleton with a cuticle that is highly
protective but is jointed, providing mobility
• Both layers of the cuticle contain chitin bound
with protein
• Chitin is a tough resistant polysaccharide
insoluble in water
Characteristics of the arthropoda
• Terrestrial arthropods use an efficient tracheal
system that delivers oxygen directly to cells
• Aquatic arthropods respire by various forms of
efficient gills
• Arthropods have highly developed sensory
organs
– Sensory organs vary from a compound mosaic eye
to other senses of touch, smell, hearing, balancing
and chemical reception
Segmentation and Appendages
• Each segment usually has a pair of jointed
appendages
• Segments and appendages are modified for
various adaptive functions
• Appendages may function in sensing, food
handling, walking or swimming
Characteristics of the arthropoda
• As the cuticle is thin between segments, it allows
movement at the joints
• Muscles attach to the cuticle
• The cuticle also folds inward to line the foregut,
hindgut, and the trachea
• Ecdysis, or molting, is the process of shedding its
outer exoskeleton
• Arthropods usually molt 4 to 7 times; the weight
of the exoskeleton limits their body size
Taxonomy of the arthropods
• There are four subphyla:
– Trilobita
– Chelicerata
– Crustacea
– Uniramia
Subphylum Trilobita
• Trilobita arose before the Cambrian, flourished, and then
became extinct 250 million years ago
• They have a trilobed body shape due to a pair of longitudinal
grooves
• They were bottom dwellers and probably were scavengers
• They ranged from an inch to 20 inches or more and could roll
up like pill bugs
Subphylum Chelicerata
• Include the horseshoe crabs, spiders, ticks,
mites, scorpions, and sea spiders
• Chelicerates have six pairs of appendages
including chelicerae, pedipalps and four pair
of legs
• They lack mandibles and antennae
• Most suck liquid food from prey
Class Merostomata
• Includes the modern horeshoe crab; Limulus
polyphemus
• This species is nearly unchanged from its
Cambrian ancestors
• Five species in three genera survive
The Horseshoe Crab
• Most live in shallow water
• They have an unsegmented carapace that covers
the body in front of a broad abdomen
• A telson or spinelike tail
• Book gills are exposed on some of the abdominal
appendages
• They walk with their walking legs and swim with
abdominal plates
• They feed at night on worms and small molluscs
Class Pycnogonida
the sea spiders
• They vary from a few millimeters to centimeters; all have
small, thin bodies
• Some males use legs to carry developing eggs
• The mouth at the tip of a proboscis, drinks juices from
cnidarians (hydroids) and soft-bodied animals
• They have a greatly reduced abdomen attached to an
elongated cephalothorax
Class Arachnida
• Consists of a great diversity among scorpions,
mites, ticks, daddy longlegs and others
• Of 80,000 species, most are free living and
more common in warm, dry regions
• Arachnids are dived into a cephalothorax and
abdomen
Order Araneae
the spiders
• About 40,000 species of spiders are known
• The body consists of an unsegmented
cephalothorax and abdomen joined by a
slender pedicel
Order Araneae
the spiders
• The anterior appendages are a pair of
chelicerae with terminal fangs
• All spiders are predaceous, mostly on insects,
which are killed b poison and fangs
• The injected venom liquefies and digests the
tissues which are then sucked into the spider’s
stomach
• Spiders breath by book lungs and/or trachae
Characteristics of the spiders
• Book lungs are found only in the spiders; parallel
air pockets extend into a blood-filled chamber
• Air enters the chamber through a slit in the body
wall
• The tracheae system is less extensive than in
insects; it carries air directly to tissues
• Spiracles are openings to the trachea
• Most spiders have eight simple eyes, each with a
lens, optic rods and a retina
• They detect movement and may for images
• Sensory setae detect air currents, web vibrations,
and other stimuli
Web-spinning Habits
• Spinning silk is a critical ability for spiders and
some other arachnids
• Two or three pairs of spinnerets contain
microscopic tubes that run to silk glands
• A liquid protein secretion hardens as it is
extruded from the spinnerets
• Silk threads are very strong and will stretch
considerably before breaking
• Spiders are often camouflaged or cryptic
Web-spinning Habits
• Jumping spiders (Salticidae) have excellent vision and stalk their prey
• Silk is used for orb webs, lining burrows, forming egg sacs, and wrapping
prey
• Wolf spiders, jumping spiders, and fisher spiders chase and catch their
prey
Spider Diversity
Reproduction in spiders
• Males court females before mating
• The male spins a small web, deposits a drop of sperm
on it, and then stores the package in his pedipalp
• Mating involves inserting the pedipalps into the female
genital openings and depositing the spermatophore
• Sperm are stored in a seminal receptacle for weeks or
months until the eggs are ready
• Eggs may develop in a cocoon in the web or may be
carried by the female
• The young hatch in a few weeks and may molt before
leaving the cocoon
Venomous spiders
• Most are feared for no reason at all
• Spiders help to control the populations of insects
• American tarantulas rarely bite and the bite is not
dangerous
• The black widow (Lactrodectus mactans)
however, can be fatal
– The venom is neurotoxic
• The brown recluse spider, Loxosceles recluse, has
hemolytic venom that destroys tissue around the
bite
• Some Australian and South American Spiders are
the most dangerous and aggressive
Order Scorpionida: Scorpions
• More common in tropical and subtropical (such
as Florida!) zones but do occur in temperate
areas
• They are nocturnal and feed largely on insects
and spiders
• The short cephalothorax has the appendages and
1 to 6 pair of eyes
• The postabdomen has the long slender tail of five
segments that end in a stinging apparatus
• Scorpions bear live young carried on the mothers
back
Order Opiliones: Harvestmen
• Harvestmen or Daddy longlegs are common,
especially in tropical areas
• Unlike spiders, their abdomen and
cephalothorax join broadly without a narrow
pedicel
• They can lose one or mor legs of their eight
legs without ill effect
• Their chelicerae are pincer-like and they feed
more as scavengers than do spiders
Order Acari: Ticks and Mites
• They are medically and economically the most
important arachnids
• Their mouthparts are on the tip of the
anterior capitulum
• They are both aquatic and terrestrial; some
parasitize vertebrates and invertebrates
• About 40,000 species have been described;
many more are estimated to exist
Diversity of mites and ticks
• House mites are free-living and cause allergies
• Spider mites are one of the many important agricultural
pest mites that suck out plant nutrients
• Chiggers are larval Trombicula mites; they feed on dermal
tissues and cause skin irritation
• The hair follicle mite Demodex is harmless but related
species cause mange in domestic animals
• Ticks are usually larger than mites
• Tick species of Ixodes carry Lyme disease
• Tick species of Dermacentor transmit Rocky Mountain
Spotted Fever
• The cattle tick transmits Texas cattle fever
Diversity of Mites and Ticks
Subphylum Myriapoda
• Includes the centipedes, millipedes,
pauropods and symphylans
• Several classes have two tagmata – a head and
trunk with paired appendages on the trunk
• Myriapods only have one pair of antennae,
mandibles, and maxillae
• Legs are always uniramous
• Respiration occurs through the body surface,
trachea, or gills in juveniles
Class Chilopoda: Centipedes
• Centipedes are terrestrial carnivorous predators found under
logs, bark and stones eating earthworms, cockroaches and
other insects
• Their flattened bodies have up to 177 segments
Class Chilopoda: Centipedes
• Each segment, except the one behind the head and the last
two, bears a pair of appendages
• Appendages of the first body segment form venom claws
• The head has a pair of eyes on either side of the head that
consist of groups of ocelli
• A pair of spiracles in each segment allows air to diffuse
through branched air tubes of the trachae
• The sexes ar separate; all are oviparous and the young
resemble the adults
• One genus of house centipede has 15 pairs of legs and
another has 21 pairs
• Most are harmless to humans but a few large, tropical
centipedes are dangerous
Class Diplopoda
• Millipedes have many legs, but not a thousand as they
are sometimes called
• Their cylindrical bodies have from 25 to 100 segments
• Two pairs of legs are present per segment, probably
from the fusion of two segments
• Each abdominal segment has two pair of spiracles
opening into air chambers and tracheal air tubes
• Most eat decayed plants but a few eat living plant
tissue
• After copulation, the female lays eggs in a nest and
guards them
• Larvae have only one pair of legs to each segment
Millipedes
Subpylum Crustacea
• 67,000 species include lobsters, crayfish,
shrimp, crabs, and copepods
• Most are aquatic, and free living, many are
sessile, commensal, or parasitic
• The main distinguishing characteristic of
crustaceans is that they have two pairs of
antennae
• The head also has a pair of mandibles and two
pair of maxillae
Typical Body Plan
Subphylum Crustacea
• There is one pair of appendages on each of the
additional segments; some segments lack
appendages
• All appendages, except perhaps the first
antennae, are biramous with two main branches
• Primitive crustaceans may have up to 60
segments; modern crustaceans have 16-20
• The tagmata are usually head, thorax and
abdomen
• The dorsal covering is the carapace; it may cover
most of the body or just the cephalothorax
Form and Function of Crustacea
• Crayfish and lobsters show modifications to their
appendages
• Swimmerets retain the primitive biramous condition
and consist of an endopod and exopod which are
attached to one or more basal segments collectively
called a protopod
Form and Function of Crustacea
• Crayfish appendages have evolved into walking
legs, mouthparts, swimmerets, etc from
modification of the basic biramous appendage
– The evolutionary trend is reduction and modification
of appendages
• Three pairs of thoracic appendages are called
maxillipeds; the first pair of walking legs are
called chelipeds, and the last pair of appendages
are called uropods
Form and Function of Crustacea
• Abdominal swimmerets are used in
locomotion, the first pair are named gonopods
• Gonopods in males are modified for
copulation; females attach eggs and young to
them
• Uropods serve as paddles for swift backward
movement
• The telson also protects eggs and young on
the swimmerets
External Structure of Crayfish
Important Internal Features
• Major body space is a blood-filled hemocoel
• Muscular and nervous sysems show the
metamerism of annelid-like ancestors
• Most muscles are antagonistic; flexors draw a
limb toward the body and extensors straighten a
limb out
• Abdominal flexors of a crayfish allow it to swim
backward
• Strong muscles located on each side of the
stomach control the mandables
The molting process
• Molting is necessary for a crustacean to increase in size; the
exoskeleton does not grow
• The physiology of molting affects reproduction, behavior and many
metabolic processes
• Inorganic salts are withdrawn from the old cuticle during premolt
• The underlying epidermis secretes the cuticle
• Enzymes released into the area above the new epicuticle dissolve
the old endocuticle
• The animal then swallows water or air to expand and burst the old
cuticle
• The new soft cuticle stretches and then hardens with the
deposition of inorganic salts during postmolt
• Molting occurs often in young animals and may cease in adults
• Temperature or day length may trigger molting
Ecdysis or Molting
Molting sequence of a lobster
Feeding Habits
• Many crustaceans shift from one type of feeding
to another, depending on food availability
• The same fundamental mouthparts are adapted
to a wide array of food availability
• Mandibles and maxillae ingest food; maxillipeds
hold and crush food
• Suspension feeders generate water currents in
order to eat plankton, detritus and bacteria
• Predators consume larvae, worms, crustaceans,
snails and fishes
Feeding Habits
• The shrimp-like Lygiosquilla pierces prey with
a specialized digit on a walking leg
• The pistol shrimp Alpheus catches prey with a
large chela that snaps shut
• Scavengers eat dead animal and plant matter
• Crayfishes have a two-part stomach; a gastric
mill grinds up food in the first compartment
Internal anatomy of crustaceans
• Gills that vary in shape
• Excretory and osmoregulatory organs are located
in the head
• Decapods have antennal glands called green
glands
• Waste products consist of ammonia with some
urea and uric acid
• Crustaceans and other arthropods have an open
circulatory system; there is no system of veins to
separate blood from interstitial fluid
Internal anatomy of crustaceans
• Movement of organs and limbs circulate blood in the
open sinuses
• Hemocyanin and hemoglobin are respiratory pigments;
clotting also occurs
• There is a brain with a double ventral nerve cord
• A median eye and compound eyes are present
• The median eye consists of three pigment cups, retinal
cells, and possible a lens
• Crustacean compound eyes are similar to insect eyes
• Attached to moveable stalks, compound eyes detect
motion and analyze polarized lights
• Statocysts, tactile setae, and chemosensitive setae are
also present
Reproduction and Life Cycles
• Crustaceans have separate sexes with specializations
for copulation
• Almost all barnacles are monecious but generally crossfertilize
• In some ostracods, males are scarce and reproduction
is by parthenogenesis
• Most crustaceans brood eggs in brood chambers, in
brood sacs attached to the abdomen, or attached to
abdominal appendages
• Crayfish develop directly without a larval form
• Most crustaceans have a larva unlike the adult in form,
and under metamorphis
• The Nauplius is a common larval form
Life Cycle of Gulf Shrimp
Brief Survey of Crustaceans
• Ostracods are enclosed in a bivalve carapace and
resemble tiny clams and are less than 1/16 inch long
• Most live in marine or freshwater sediments but
some scavenge or feed on detritus
• Copepods have numerous species
• They lack a carapace and retain the simple,
median eye in the adult
• They have four pairs of flattened, biramous,
thoracic swimming appendages
• Free-living copepods may be the dominant
primary consumer in aquatic communities
• Parasitic forms are highly modified and
reduced, often unrecognizable as arthropods
• Brachiurans lack gills and most are parasites of
fish
• Found on both marine and freshlwater fish
Cirripedia – the barnacles
• Barnacles as adults are sessile and attach
directly or by a stalk to the substrate
• The carapace surrounds the body and secretes
a set of calcareous plates
• The head is reduced, the abdomen is absent
and the thoracic legs are long with hair-like
setae
• The many-jointed cirri that bear the setae are
extended from the plates to feed on small
particles
The barnacle
• Isopods are dorsoventrally flattened, lack a carapace and have
sessile compound eyes
• The abdominal appendages bear gills
• Common land forms include the sow bugs and pill bugs
• Some isopods are highly modified as parasites of fish or
crustaceans
• Amphipods resemble isopods except they are
somewhat compressed laterally
• They lack a carapace and have sessile compound
eyes
• Many are marine, others are beach-dwelling,
freshwater or parasitic
• Euphausiacids or “Krill” has only 90 species but
includes the important ocean plankton called krill
• They form a major component of the diet of baleen
whales and of many fishes.
• Some are over 2 inches long
• Decapods have five pairs of walking legs, the first
forming pincers or chelae
• They range from a few millimeters to the larges
arthropod, a Japanese crab with a 12 foot legspan
• They are true crabs with a broader cephalothorax
and reduce abdomen, compared to crayfish or
lobsters
• Fiddler crabs have a reduced abdomen and
burrow in the sand
• Hermit crabs are adapted to live in snail shells
Variety of Decapods