Onychophora
Tardigrada
Arthropoda
Ecdysozoa
Nematoda
Nematomorpha
Priapulida
Loricifera
Kinorhyncha
Phylum Onychophora

Phylum Onychophora: Velvet worms, Peripatus.

All modern Onychophorans are terrestrial but fossils,
which date back to the Cambrian, are mainly from
marine sediments.

Onychophorans look a bit like slugs with legs and when
first described in 1825 were thought to be mollusks.

However, now placed at the base of the Arthropod family
tree.
http://www.jennifermarohasy.com/blog/archives/Peripatus.jpg
Phylum Onychophora

About 10 genera and approximately 110 species. 1.415cm in length.

Geographically widely distributed in suitable moist, humid
habitats such as tropical (e.g. Congo, northern South
America, Himalayas) and southern temperate forest (e.g.
New Zealand, Australia, South Africa, Andes).

Limited habitat distribution (and exclusively nocturnal
behavior) related to problem of water loss.

Like other arthropods, Onychophorans have a chitinous
cuticle, which is molted as they grow, but it is thin and
flexible, non-waxy and permeable.
Phylum Onychophora
 Onychophorans
do not have jointed legs,
but possess other arthropod
characteristics including a reduced coleom
and a hemocoel.
 The
14-43 pairs of legs are large and
conical with terminal pads and claws.
They move using their legs and by
extending and contracting the body.
Phylum Onychophora
 Most
are predators and use glue which
they can spray from a pair of oral papillae
to catch their prey.
 They
can spray two streams of glue up to
50cm and this hardens almost immediately
trapping the prey in a net of sticky threads.
Phylum Onychophora
 Peripatus
video 3 minutes
 https://www.youtube.com/watch?v=nvjBO
oWSILE
Phylum Arthropoda
 The
Arthropoda (from the Greek Arthron,
joint and podus, foot) are the largest group
of organisms and they occur in all
environments on earth.
 The
group includes: spiders, ticks, mites,
centipedes, millipedes, crustaceans,
insects and others.
Characteristics of the Arthropoda
 Bilaterally
symmetrical with a segmented
(metameric) body divided into head, thorax
and abdomen; cephalothorax and
abdomen; or fused head and trunk.
Rove beetle
http://members.fortunecity.com/nrbq1/gal4arthropod9.jpg
Goliath bird-eating spider
http://animal-world.com/encyclo/reptiles/spiders/images/
GoliathBirdEatingSpiderWHSp_AP7I.jpg
http://static.howstuffworks.com/gif/tick-1.jpg
Tick
http://repairstemcell.files.wordpress.com/2009/09/tick-bird.jpg
Characteristics of the Arthropoda
 Jointed
appendages. Primitively one pair
per segment, but number often reduced.
 Appendages
often greatly modified for
specialized tasks.
Centipede
http://www.butterflyrevolt.com/wp-content/uploads/2009/04/centipede.jpg
http://garpga.org/issues/animals-used-for-food/870-2
Lobster
Characteristics of the Arthropoda
 Exoskeleton.
 Exoskeleton
secreted by underlying
epidermis. Made of chitin, protein, lipid
and often calcium carbonate.
 Exoskeleton
is shed periodically (ecdysis)
as the organism grows.
Sea spider (Pycnogonid)
https://nsf.gov/news/mmg/mmg_disp.jsp?med_id=77817&from=
Horseshoe crab
http://www.pbs.org/wnet/nature/
crash-a-tale-of-two-species-video-blue-blood-at-15000-a-quart/614/
Characteristics of the Arthropoda
 Muscular
system is complex and muscles
attach to the exoskeleton.
 Striated
muscles for voluntary movement
and smooth muscles for the viscera.
 Coelom
is reduced. Most of body cavity is
a hemocoel (sinuses or spaces) filled with
blood.
Characteristics of the Arthropoda
 Complete
digestive system. Mouthparts
are specialized being modified from
appendages and specialized for dealing
with various types of food.
 Open
circulatory system with a dorsal
contractile heart arteries and hemocoel.
Characteristics of the Arthropoda
 Respiration
occurs in multiple possible
ways




across the body surface
via a system of tracheal tubes (e.g. beetle)
Gills (e.g. crabs)
book lungs (spiders)
Horseshoe crab book gills
http://library.thinkquest.org/26153/marine/sketch/719a.jpg
http://www.dwm.ks.edu.tw/bio/activelearner/44/images/ch44c3.jpg
Characteristics of the Arthropoda
 Nervous
system is similar to that of
annelids. A dorsal brain connected via a
ring around the pharynx to a double
ventral nerve cord.
http://images.google.com/imgres?imgurl=http://cas.bellarmine.edu/tietjen/Laboratories/Bi
o%2520Pix%25204%2520U/Image117.gif&imgrefurl=http://cas.bellarmine.edu/tietjen/Et
hology/simplenervoussystpix.htm&usg=__hKuhulliD2AlFWkHh8AA6kRX5eo=&h=645&w
=1016&sz=42&hl=en&start=4&um=1&tbnid=ORNjwuvPSCNDM:&tbnh=95&tbnw=150&prev=/images%3Fq%3Darthropod%2Bnervous%2Bsys
tem%2Bimages%26ndsp%3D18%26hl%3Den%26sa%3DG%26um%3D1
Characteristics of the Arthropoda
 Sexes
are usually separate with internal
fertilization.
 Can be oviparous or ovoviviparous.
 Offspring often go through process of
metamorphosis.
 Parthenogenesis (reproduction without
fertilization) occurs in a some species (e.g.
aphids).
Aphid giving birth
http://nathistoc.bio.uci.edu/hemipt/28.jpg
http://anordinarymom.files.wordpress.com/2009/02/metamorphosis-of-monarchbutterfly.jpg
Classification of Phylum
Arthropoda
 Subphylum
Chelicerata: horseshoe crabs,
spiders, tick, mites, scorpions.
 Subphylum Crustacea: crabs, lobsters,
copepods, barnacles, pill bugs.
 Subphylum Myriapoda: millipedes and
centipedes.
 Subphylum Hexapoda: springtails, insects.
Classification of the Arthropoda
 Subphylum

Chelicerata
Class Merostomata
• Subclass Xiphosurida: Horseshoe crabs


Class Pycnogonida: sea spiders
Class Arachnida: spiders, scorpions, ticks,
mites, harvestmen, whip scorpions.
Subphylum Chelicerata


Body is divided into two tagmata (fused
segments) head and abdomen.
Six pairs of appendages:




a pair of chelicerae, [has 2 or 3 segments] first pair of
appendages used for feeding (e.g. fangs in spiders).
a pair of pedipalps [6 segments] not in horseshoe
crabs)
4 pairs of walking legs (5 in horseshoe crabs).
No mandibles or antennae.
Cheilicerae
"Kaldari Phidippus audax 01" by Kaldari Own work. Licensed under CC0 via
Commons https://commons.wikimedia.org/wiki/File:Ka
ldari_Phidippus_audax_01.jpg#/media/File:
Kaldari_Phidippus_audax_01.jpg
Pedipalps
Scorpion
http://a.fotg.xyz/scorpion/
Pedipalp is modified into a claw.
Pedipalp
Chelicerae
Whip scorpion
http://colinhuttonphoto.deviantart.com/art/Tailless-whip-scorpion-Amblypygi-508501240
Subphylum Chelicerata
 The
chelicerates are a very ancient group
that includes the extinct Eurypterids (giant
water scorpions (200-500 mya), which
were the largest known arthropods
reaching up to 3m long.
Eurypterus (left) from the Silurian of New York State. Eurypterus remipes was voted
the New York state fossil in 1983.
Mixopterus (right) from the Devonian reached about one meter in length
Subphylum Chelicerata
 Subclass
Xiphosurida: Horseshoe crabs.
These are an ancient group that date from
the Cambrian (543-495 mya).
 Five
living species. Limulus, which lives in
shallow waters on the Atlantic coast of the
U.S. is almost unchanged since the
Triassic (251-206 mya).
Figure 18.02
12.2
Subphylum Chelicerata: Horseshoe
crabs
 Horseshoe
crabs have an unsegmented
carapace, which is sort of horseshoe
shaped, hence the name.
 There
is also a wide abdomen with a long
spinelike tailpiece (telson).
 Flat
leaf-like gills, called book gills, are
present on the underside.
Horseshoe crabs

Horseshoe crabs feed on foods such as worms and
molluscs, such as clams, which brings them into conflict
with fishermen.

The synchronized breeding of horseshoe crabs which
come to spawn on the beaches of the mid-Atlantic coast
of the U.S. at the lunar high tides is a striking sight in
summer.

The eggs produced in the millions are an important food
source for migrating shorebirds such as Red Knot, which
flock to areas such as the Delaware Bay to fatten up for
migration.
Horseshoe crabs
 Horseshoe
crabs are harvested
commercially for bait and also for their
blood, which is used in laboratory testing
for endotoxins in medical products.
 Currently,
the horseshoe crab is at the
center of a series of legal battles about the
size of harvests and their sustainability.
Horseshoe crab blood
 https://www.youtube.com/watch?v=e8KlA
mtIu1E
Subphylum Chelicerata: Class
Pycnogonida: sea spiders

Another name used for these animals is
Pantopoda (“all legs”), which is an excellent
description.

The body is greatly reduced in size, whereas the
legs are long and clawed. In some species
modified legs called ovigers are used by males
to carry egg masses.

There is a long proboscis on the head, which the
pycnogonid uses to feed on soft-bodied
invertebrates, especially cnidarians.
http://www.mbari.org/news/feature-image/pycnogonid-450.jpg
Figure 18.03
12.3
Class Pycnogonida: sea spiders
 Most
pycnogonids are small being only a
few millimeters long, but a few reach 70cm
measured by leg spread.
 Pycngonids
are exclusively marine
(occuring from the intertidal to the deep
seas) and there are about 1000 species.
Subphylum Chelicerata: Class
Arachnida: Order Aranae: spiders

Spiders are a very large group with more than 35,000
described species that occur worldwide except in
Antarctica.

The body is divided into a cephalothorax and an
abdomen.

Spiders breathe using book lungs or tracheae. Book
lungs are unique to spiders and consist of a large
number of air pockets extending into a blood-filled
chamber. Tracheae are tubules that carry air directly
from outside to tissues. They can be closed to prevent
excessive water loss.
Spiders

Spiders and insects have an excretory system
that uses structures called Malphigian tubules.

Potassium, other salts, and other waste solutes
drain into these tubules, which empty into the
intestine. Rectal glands selectively reabsorb
most of the potassium and water leaving behind
nitrogenous wastes in the form of uric acid,
which requires little water for its excretion.
http://www.earthlife.net/chelicerata/images/spider-guts.gif
Spiders
 Most
spiders possess eight eyes (some
have a few as two), but their image
forming ability is limited.
 Hairlike
setae, however, provide a lot of
information about the environment sensing
e.g. vibrations and air currents
http://www.eurospiders.com/spider_eyes.jpg
Spiders

All spiders are predators and their chelicerae
function as fangs.

The fangs are connected via ducts to venom
glands that produce a lethal venom the spider
uses to dispatch its prey.

After killing a prey item the spider injects
digestive fluid into the organism and sucks up
the resulting soup.
Figure 18.05
12.4
Spiders
 Spiders
use a variety of techniques to
catch prey.
 Some
chase their prey or leap on it, some
ambush prey often using trip wires, and, of
course, many use webs.
 Trapdoor
spider
 http://www.youtube.com/watch?v=7yJgPV
xS5aI&list=PLr96eNfxmgJRv9AlzVs35Qm
ybWaKUlPsb
 One
small group, the bolas spiders,
capture their prey by deception.
 http://www.youtube.com/watch?v=2UfMJJ
AzvbI&list=PLr96eNfxmgJRv9AlzVs35Qm
ybWaKUlPsb
 Bolas
spider 2’55”
Bolas spiders

Bolas spiders at night spin a line of thread with a sticky
globule the bolas (akin to the bolas used by South
American Gauchos to hunt) at the free end.

The bolas contains pheromones which attract certain
male moths which think they are approaching a female.
When the spider senses the vibrations in the air caused
by a nearby flying moth it twirls its bolas in response.

Once the bolas strikes the moth it sticks and the moth is
hauled in.
Gladiator spider
 The
gladiator spider throws a net of silk to
catch its prey
 http://www.youtube.com/watch?v=rLSg_O
KMi0I&list=PLr96eNfxmgJRv9AlzVs35Qm
ybWaKUlPsb
2
min
Silk-spinning

Spiders spin silk, which they use to make webs
to trap prey.

Webs come in a variety of shapes and sizes and
contain and are coated with an adhesive that
holds prey.

Webs are typically placed in insect flyways and
the spider sits and waits for an insect to become
entangled. The vibrations of the struggling prey
signal the spider to emerge and dispatch it.
Silk-spinning

The silk is produced by two or three spinnerets,
which are connected to abdominal silk glands.
The silk is formed from a protein secretion that
hardens on contact with air.

The silk is extremely strong (stronger e.g. than
steel of equivalent weight) and is being used as
a kevlar substitute. Unlike steel, silk can stretch
which makes it an extremely useful material.
Web building
 http://www.youtube.com/watch?v=eb6bqI
WdQao
3
minutes
Silk-spinning
 The
silk is used to make webs, but also for
a variety of other purposes:




Line nests
Form egg sacs
For dispersal by ballooning
To wrap prey
Reproduction in spiders
 Courtship
rituals are a major feature of
spider mating in which the (usually much
smaller) male attempts to mate without
being eaten by the female.
 Males
produce a sperm packet wrapped in
silk which he holds in a cavity in one of his
pedipalps (second pair of appendages).
Reproduction in spiders

The male, if he lives long enough, inserts a
pedipalp into a female’s genital opening and she
stores the sperm in a seminal vesicle.

The female later fertilizes eggs when she is
ready to lay them. Eggs are laid in a silk cocoon
where the young hatch and remain for a short
time and molt before departing for an
independent life.
 Argiope
spider mating
 http://www.youtube.com/watch?v=xPvCIrY
rs5k&list=PLr96eNfxmgJRv9AlzVs35Qmy
bWaKUlPsb
Toxic spiders
 The
vast majority of spiders are harmless
to humans, but a handful are toxic and
potentially deadly.
 In the U.S. there are a few venomous
spiders: several black widow species
(genus Latrodectus), the brown recluse
(Loxosceles reclusa) and the hobo spider
(Tegenaria agrestis) introduced from
Europe to the Pacific Northwest.
Toxic spiders
 Spiders
eat a liquid diet by injecting
digestive juices into their prey and some
spider venoms contribute to this process
by destroying tissue.
 Bites
of such spiders can lead to tissue
necrosis. This is the type of venom
possessed by the hobo and brown recluse
spiders.
Hobo spider
Brown recluse spider
Toxic spiders
A
bite from one of these spiders results in
a bite site that develops a painful ulcer
where tissue dies (necrosis) and in the
case of hobo spiders severe headaches.
 One
component of brown recluse venom
is sphingomyelinase D which attacks and
dissolves cell membranes.
“Many medical conditions are mistaken for brown recluse bites.
The wound on the left is from a brown recluse spider, the one
on the right from a bacterial infection.”
http://www.ca.uky.edu/entomology/entfacts/ef631.asp
Toxic spiders

Sphingomyelinase D and other components in
the venom apparently also turn on the patient’s
inflammatory defenses against his or her own
tissues.

White blood cells destroy themselves releasing
other enzymes that attack the victim’s own flesh
and blood clots form in tiny vessels cutting off
the blood supply to the bite area and causing
necrosis.
Toxic spiders
 Generally,
bites are not fatal but if the
venom gets into the bloodstream it may
destroy red blood cells or attack the bone
marrow, which may lead to fatal
complications.
Toxic spiders
 Black
widow spiders and the funnel web
spider (one of the world’s most dangerous
species [naturally it’s from Australia!])
produce potent neurotoxins that affect the
nervous system.
Female Black Widow spider with egg sac
Toxic spiders
 Venom
is complex and many components
only affect invertebrates, but one
substance in black widow venom called
alpha latrotoxin stimulates cells to release
neurotransmitters such as acetylcholine.
 Nerves contract uncontrollably and this
can cause small muscle rigidity and
intense, agonizing pain.
Toxic spiders
 Additional
symptoms include nausea and
vomiting, increased blood pressure, and
the heart may begin racing or slowing
significantly.
 Brain
functions may also be affected
producing anxiety, amnesia, and even
psychosis.
Toxic spiders

Black widow bites were historically often
associated with outhouses, but now usually
occur when working in the fields or clearing junk
in outbuildings.

Fatality rates have been estimated at 1-5%, but
this figure certainly is an overestimate as many
people bitten do not seek medical attention.
Those most at risk are small children and the
elderly.
Toxic spiders

Funnel web spiders (especially the Sydney
funnel web spider) are extremely dangerous.

Funnel web spiders inject a venom whose lethal
component is called atrotoxin.

The toxin travels in the lymphatic system and
binds to nerve endings all over the body where it
causes nerves to discharge wildly, especially
those of the autonomic nervous system.
Funnel web spiders
Funnel web spider burrow site
 http://www.youtube.com/watch?v=EC11G
xIMURc
 Sydney
funnel web spider 4’30”
Toxic spiders
 The
constant discharging of the autonomic
nervous system results in fever, irregular
heart rhythm and wild changes in heart
rate and blood pressure that can cause
respiratory failure, coma and cardiac
arrest.
 Death
may occur anything from 15
minutes to 6 days after a bite.
Toxic spiders
 Work
by Dr. Struan Sutherland and
colleagues in Melbourne has led to
success in treating funnel web spider bites
and greatly reduced mortality.
 Dr
Sutherland pioneered the use of
pressure bandages to prevent the
spreading of the venom and led the team
that developed successful antivenins.
Subphylum Chelicerata: Order
Scorpionida: Scorpions
 There
are about 1,500 species of
scorpions and the group includes some of
the largest of the arachnids as some
scorpions reach up to 18cm in length.
 Scorpions
are best known for their
prominent front claws, which are modified
pedipalps and their stinging tail, with which
they inject venom.
Scorpions
 Scorpions
have a body that consists of a
quite short cephalothorax, which bears 4
pairs of walking legs and a long,
segmented abdomen which is divided into
a thick preabdomen and a thinner
postabdomen that carries the stinging
apparatus.
Giant desert hairy scorpion
http://www.photographytips.com/page.cfm/31994
by Rhonda Spencer
Scorpions
 Scorpions
glow when a black light (UV
light) is shone on them. This is because
the cuticle contains a variety of fluorescent
compounds.
 This
characteristic makes nocturnal
surveys of scorpions quite easy.
http://fireflyforest.net/firefly/2006/11/13/fluorescent-scorpion-in-uv-light/
Scorpions
 Scorpions
occur on all continents except
Antarctica.
 They
are commonest in tropical and
subtropical areas, but some occur in
colder areas including high mountain tops.
Scorpions

Scorpions are generally secretive and hide from
potential predators during the day in crevices
before emerging to feed at night on insects and
similar prey. The largest species may kill small
reptiles and mammals.

Scorpion’s liking for dark places to hide is why
it’s important to shake out your shoes in the
morning in scorpion country.
Feeding

Scorpion pincers are covered in highly sensitive
hairs and respond quickly to prey that brush
against them.

Prey is gripped with the pincers (pedipalps) and
either crushed or stung to death. Scorpions
consume only liquids so the prey is torn into tiny
pieces using the jawlike chelicerae and
predigested in a pre-oral cavity below the
chelicerae until it liquifies and is then consumed.
Scorpion toxicity

The venom of most species of scorpions is not
harmful to humans, but that of some African
species and some found in Mexico and the
southwestern U.S. can be fatal.

In all about 25 species are potentially dangerous
to humans. The venom is a mix of compounds
that includes neurotoxins and enzyme inhibitors.
One short chain scorpion toxin is known to block
K+ channels in cell membranes.
Scorpions
 Because
scorpions have a low metabolic
rate and are relatively inactive they can go
for long periods between meals.
 Some
species can survive 6-12 months of
starvation.
Scorpion courtship
 Most
scorpions reproduce sexually
although a few species are known to
reproduce parthenogenetically.
 There
is a complex courtship ritual in
which the male grips the females claws
and dances with her as he attempts to
maneuver her over a spermatophore he
has produced.
Scorpions
 Scorpions
are viviparous (give birth to live
young) and these stay with their mother
and ride on her back until they undergo
their first molt. They depend on their
mother for protection and to regulate
humidity levels.
 The
young go through 5-7 molts (instars)
before reaching maturity.
Figure 18.10a
12.10 a
Order Uropygidi: whip scorpions
and Order Amblypygi: tail-less whip
scorpions
 The
members of these two groups are
similar in appearance to scorpions, but
lack the stinging apparatus in the tail.
 The
abdomen of these organisms either
ends in a long tail [which has a sensory
function] or there is no tail (hence the
common names of the two groups).
Uropygid (left)
Amblypygid
(bottom left and below)
Order Uropygidi: whip scorpions
and Order Amblypygi: tail-less whip
scorpions

Uropygids are often called vinegaroons because
they can spray an acidic solution from their tails.

Both groups are predatory and there are about
100 described species of Uropygids and about
130 species of Amblypygids. Both groups are
found in tropical and subtropical areas and a few
species occur in the southern U.S.
Uropygids and Amblypygids
 Both
groups use 6 legs for walking the first
pair having been modified into antennaelike sensory organs.
 Both
groups are carnivorous nocturnal
hunters that feed mostly on insects and
other arthropods.
Uropygids and Amblypygids
 Many
behaviors of Uropygids and
Amblypygids are similar to those of
scorpions.
 They
avoid light as do scorpions, hide in
crevices and the young travel on their
mother’s back when small
Subphylum Chelicerata: Order
Opiliones: harvestmen
 Harvestmen
or “daddy longlegs” are
common and easily separated from
spiders by a number of features:




Their abdomen and cephalothorax are not
separated by a narrow “waist” or pedicel as
they are in spiders.
They usually have extremely long thin legs.
They cannot spin silk.
They have only two eyes rather than then the
eight of spiders.
Harvestmen
 The
chelicerae of harvestmen are
pincerlike and they use them to catch
small insects and also commonly
scavenge dead items.
 Unlike
spiders they can eat solid food
 There
are about 6400 described species.
Harvestmen

Most harvestmen reproduce sexually although
pathenogenesis occurs in some species

In some species the male guards the female
after mating and in others the male defends a
territory where females come to lay eggs. The
male tends the eggs until they hatch and
defends the eggs against predators including
females who come to lay.
Subphylum Chelicerata: Order
Acari: ticks and mites
 The
members of the Acari differ from the
other Arachnids in that their cephalothorax
and abdomen are completely fused with
no evidence of a division between the
segments.
 The
mouthparts are mounted on a small
projection called a capitulum.
Figure 18.11a
Wood tick
Wood tick
Ticks and mites

An estimated 25,000 species of Acarines occur
worldwide in a wide variety of habitats on the
ground, in vegetation, in freshwater and marine
habitats often as ectoparasites.

The group includes a number of major pests
including spider mites, which are serious
agricultural pests, chiggers and ticks which are
ectoparasites and spread a variety of unpleasant
diseases.
Disease carrying ticks

Ticks are usually bigger than mites and feed on
the blood of vertebrates.

They wait on vegetation until they sense the
presence of a suitable host and then grasp on.

They will attach by inserting their chelicerae and
a feeding tube called a hypostome and generally
feed for several days (increasing enormously in
size) and eventually drop off.
Disease carrying ticks
 Once
a tick drops off it will digest its meal
and molt before feeding again.
 Because
they feed on blood, ticks, like
mosquitoes, are major vectors of disease.
Disease carrying ticks
A
wide variety of diseases are tick borne,
with ticks being second only to mosquitoes
in the variety and seriousness of the
conditions they spread.
 Diseases
spread include Rocky Mountain
spotted fever, tularemia, red-water fever
and best known of course Lyme Disease.
Lyme Disease

Lyme Disease (named for Lyme Connecticut
where it was first described from) is caused by a
bacterium Borrelia burgdorferi.

Symptoms of Lyme disease, which is spread by
the bite of a deer tick, include fever, headache,
fatigue, and a diagnostic skin rash called
erythema migrans, which has a bull’s eye
appearance.
Lyme Disease

If not treated, an infection with Borrelia
burgdorferi can spread to the joints producing
arthritis-like symptoms as well as to the heart,
and nervous system, which may result in facial
palsy.

Lyme disease can be treated with antibiotics if
caught early enough.

Prompt removal of ticks from clothing and of
attached ticks is important in avoiding infection.
Commensal mites
A
variety of mites live commensally with us
including tiny species that live in hair
follicles especially around the nose and
eyes.
 These
mites (<0.4mm long) are vaguely
wormlike having quite long bodies and live
head down in follicles feeding on dead
skin cells and secretions.
Eyelash mites. Above left: three
embedded head down in a follicle; above
right an individual Demodex folliculorum
and right close up of the anterior end.
Commensal Mites
 In
addition to follicle mites, dust mites are
extremely common and feed on the dead
flakes of skin that we shed every day
(that’s what most dust is).
 Your
pillow is home to thousands of them!
Figure 18.12
Dust mite
Chiggers

Chiggers are tiny mites (< 0.05 mm long), but
cause skin irritations out of all proportion to their
size.

Chigger larvae climb on to vegetation and wait
to hitch a ride on a passing animal.

The larva pierces the skin and injects a salivary
secretion of powerful, digestive enzymes that
break down skin cells into a liquid, which the
chigger ingests.
Chiggers

The digestive enzymes also causes the
surrounding tissue to become harder, which
generates a feeding tube (called a stylostome)
of hardened flesh through which more semidigested skin cells are consumed.

After about four days the larva finishes feeding
and drops off its host, leaving behind a red welt
that itches severely. Although very irritating U.S.
chiggers do not spread disease.