Bio 434 - Arthropods
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Chapter 33 – Introduction to Phylum Arthropoda
Significance of Arthropods as Vectors
Disease producing organisms transmitted to humans by arthropods have significantly
influenced the history and demography of the human race
The manner in which the various organisms parasitic to humans are transmitted dictates
the types of association arthropods establish with parasites
The simplest relationship is one in which the arthropod is a mechanical vector,
functioning merely as a passive carrier of the etiologic agent
As a 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
Once the etiologic agent is infective to humans, there are several means by which it gets
from arthropod to human host
For example, among some of the blood-sucking flies, infective forms leave the insect’s
mouthparts during the blood meal and enter the human skin through the puncture
General Features of Arthropods
Metamerism
Arthropods are segmented animals that probably evolved from annelids or from a
segmented protostome that was a common ancestor of annelids and arthropods
Thus arthropods are said to display metamerism (=segmentation) and their segments
have appendages
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 (recall that the annelid appendages were used primarily for locomotion)
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Exoskeleton and Molting
Among the arthropods the epidermis secretes an external skeleton called the exoskeleton
There are several immediate advantages to possessing this structure:
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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; important because
many arthropods live on land
serves as a barrier to prevent infection
The exoskeleton or cuticle 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 - contains waxes making the
cuticle an inpenetrable barrier
The thicker portion is called the procuticle - divided into the exocuticle and the
endocuticle
Exocuticle differs from endocuticle because in the exocuticle the glycoprotein chains are
cross-linked - the process is called tanning
In order to grow the arthropod must shed its exoskeleton, and secrete a new and larger
one.
This kind of a process is called molting or ecdysis.
Jointed Appendages
Despite the armor of exoskeleton, arthropods can move rather well because 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
These areas are the joints, which are the hinge-like areas of the legs and body
The presence of jointed appendages allows arthropods to move efficiently and quickly
above the ground or sea floor without dragging the body on the ground with legs that
could not be bent or flexible
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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
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 (e.g., hard ticks have
one nymphal stages, while the soft ticks have eight)
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
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External Morphology
Crustacea
Differ from other arthropods because they possess 2 pair of antennae
First pair is homologous to those of insects, but the second pair is unique to the
crustaceans
The second pair of antennae has various functions, including sensory, locomotion or
feeding.
In addition to the antennae, 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 varies considerably among the various classes
However, there is usually 4 pair of walking legs
Some groups (e.g. shrimp, lobsters) have a pair of modified pincerlike claws
The abdomen is also highly variable, but it is primitively a large structure as in the
lobsters
In the groups that have a well-developed abdomen there are usually six pairs of
appendages
Five pairs of structures called simmerets (=pleopods)
These are used in locomotion, generating water currents, brooding eggs
The last pair of abdominal structures are called uropods, these are wide and together
with the terminal telson they form a tail fan than can serve as rudders during locomotion
Primitively many of the appendages of the crustaceans are biramous, two-branched
The lateral branch is the exopod (exopodite) and the medial one is the endopod
(endopodite)
Each of the branches may contain several segments, varying by appendage and according
to species
The endopod and exopod are borne on a basis (basipodite), and the basis is attached to
the coxa (coxopodite)
They usually have an extremely hardened exoskeleton, which is impregnated with
calcium carbonate
The exoskeleton 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
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The mouthparts of primitive insects were adapted for chewing and many extant species
retain that adaptation (e.g., grasshoppers, bees, ants, roaches)
The primitive mouthparts 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
In more advanced insects, the major modification of the feeding apparatus is from the
chewing type to one of several cutting and/or piercing types
The cutting-sponging type, characteristic of horseflies, features sharp-bladed mandibles
and long, styletlike maxillae
The mandibles cut and tear the skin of the host, and the spongelike labium collects the
blood and conveys it to the esophagus via a tube formed partially by the hypopharnyx
The mouthparts of most nonbiting dipterans are the sponging type (mandibles and
maxillae are non functional)
The piercing-sucking type is characteristic of mosquitoes, flies, lice
They feature mandibles, maxillae, and a hypopharynx modified into a long, thin, tubular,
sharp-tipped stylet for piercing skin
The narrow tube is enclosed by the labrum to form a stylet bundle that is held in a
groove on the labium
Together the stylet bundle and the labium male up the proboscis
During feeding, the stylet bundle pierces the skin of the host like a hypodermic needle
The thorax is composed of 3 segments and each one has a pair of legs
The last two have a pair of wings.
Each leg is usually divided into 5 segments: coxa, trochanter, femur, tibia, and tarsus
Acari
The main tagmata of this group are the cephalothorax (prosoma) and the abdomen
(opisthosoma)
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