Transcript Chapter 33 Introduction to Phylum Arthropoda

Chapter 33
Introduction to
Phylum Arthropoda
Arthropods as Vectors
Mechanical vector - functioning merely as a passive carrier of
the etiologic agent
Biological vector - the arthropod is used by the disease
producing organism not only as a means or transmission but also
as a vehicle for the development and/or reproduction before it
becomes infective
General Features of
Arthropods
Metamerism
• Display metamerism (=segmentation) and their segments have
appendages
• The evolution of the arthropods was witness to a reduction in
metamerism
Tagmatization
• The arthropods evolved
modified groups of segments
• The fusion of groups of
segments into functional
groups is called
tagmatization
• In so doing, various
appendages on segments
became specialized for
functions other than
locomotion
Exoskeleton and Molting
• The epidermis secretes the exoskeleton
• Advantages to possessing this structure: provides strong support; provides
rigid levers that muscles can attach to and pull against; offers protection; serves as a
barrier to prevent internal tissues from drying out; serves as a barrier to prevent
infection
• The exoskeleton is composed of the polysaccharide chitin and protein
bound together to form a complex glycoprotein
• The outer surface of the cuticle is called the epicuticle
• The thicker portion is called
the procuticle; divided into
the exocuticle and the
endocuticle
• In the exocuticle the
glycoprotein chains are crosslinked - the process is called
tanning
Exoskeleton and Molting cont.
• In order to grow the arthropod must shed its exoskeleton, and
secrete a new and larger one - molting or ecdysis.
Jointed Appendages
• The exoskeleton is divided into a number of plates and numerous
cylinders around the appendages
• At the junction point between plates and cylinders, the arthropod
exoskeleton remains thin and flexible - the joints - hinge-like areas of
the legs and body
• The presence of jointed appendages allows arthropods to move
efficiently and quickly without dragging the body
• Muscles are integral to arthropod movement
• The muscles of the
arthropods attach to the
inner side of the
exoskeleton
• And they often function
as a lever system
Post Embryonic
Development
Insects
There are two general types of metamorphosis: incomplete and complete
1. Incomplete Metamorphosis - Hemimetabolous
Early developmental stages are very similar to the adults
Only the wings and the reproductive structures gradually develop
The immature stages are called nymphs
Thus development is egg----> nymph ----> adult
2. Complete Metamorphosis - Holometabolous
• Each of the developmental stages is structurally and functionally very
different
• The egg develops into an immature larva, that eats voraciously
• Larvae then forms a
transitional stage called the
pupa, that is often
contained within cocoon
• Within the pupal
exoskeleton a
metamorphosis takes place
and emerging from the
cocoon is a sexually mature
adult insect
Crustaceans
• The typical larva hatching from the egg is the nauplius larva
• It has three pairs of appendages: antennules, antennae, and mandibles;
all of which tend to have a locomotor function
• The larvae undergoes several ecdyses and usually adds somites and
appendages with each molt
• Crustaceans vary widely in their developmental patterns
Diversity in
Crustacean
Development
Mites and Ticks
• After the eggs there is usually an immature nymph, which is a tiny
version of the adult
• The number of nymphal instars depends on the group in question
• Among the mites a six-legged larva becomes an eight-legged
nymph after the first molt
• Most mites have 3 nymphal instars: protonymph, deuteronymph,
and tritonymph
Drawing of bulb mite development stages
External Morphology
Crustacea
• Possess 2 pair of antennae; first pair is homologous to those of insects;
second pair are unique to the crustaceans
• Second pair of antennae have various functions: sensory, locomotion, feeding.
• The head usually bears a pair
of compound eyes and 3 pairs
of mouthparts for grinding,
chewing or filtering, including
a pair of mandibles, and 2
pairs of maxillae
• The trunk usually has 4 pair
of walking legs
Crustacea cont.
• The abdomen is also highly variable; primitively there are usually six pairs
of appendages
• Five pairs of structures called swimmerets (=pleopods); used in
locomotion, generating water currents, brooding eggs
• The last pair of abdominal structures are called uropods; together with the
terminal telson they form a tail fan than can serve as rudders during
locomotion
Crustacea cont.
• Primitively many of the appendages of the crustaceans are biramous,
two-branched: exopodite and endopodite); Both borne on a basipodite
that is attached to the coxa (coxopodite)
• Possess a hardened exoskeleton, which is impregnated with calcium
carbonate; forms a protective shell called the carapace
Insects
• Body is divided into 3 parts: the head, thorax and the abdomen.
• On the head are one pair of antennae and a pair of compound eyes and
several sets of simple eyes
Insects cont.
• Mouthparts of primitive insects were adapted for chewing; consist of a pair of
mandibles, and two pairs of maxillae
• One pair of maxillae are fused together to form a lower lip - labium
• There is an upper lip - labrum - formed from an extension of the head
• Throughout evolution other types of mouthparts have evolved:
• The cutting-sponging type
- sharp-bladed mandibles and
long, styletlike maxillae
• The mouthparts of most
nonbiting dipterans are the
sponging type
• The piercing-sucking type
- mandibles, maxillae, and a
hypopharynx modified into a
long, thin, tubular, sharptipped stylet for piercing skin
Insects cont.
• The thorax is composed of 3 segments and each one has a pair of legs;
he last two have a pair of wings.
• Each leg is usually divided into 5 segments; the segments include basal
segment or coxa, trochanter, femur, tibia, and tarsus
Acari
• The main tagmata of this group are the cephalothorax (prosoma)
and the abdomen (opisthosoma)
Acari cont.
• The gnathostoma or capitulum is a small anterior projection bearing the 3
structures that make up the mouthparts: the hypostome, a pair of chelicerae,
and the pedipalps
• The hypostome is usually toothed, medially located and ventral to the mouth
• The chelicerae are used to pierce or tear the skin, then the entire captiulum of
the toothed hypostome can be inserted into the opening
• During feeding, the
pedipalps either bend outward
(soft ticks) as the chelicerae
and hypostome penetrate the
flesh or remain rigidly and
intimately associated with the
hypostome (hard ticks) during
skin penetration
• In either instance, the
pedipalps serve as counteranchors while the tick is
attached to the host