Introduction to

PHYLUM ARTHROPODA

   The arthropods are an enormous group of animals that outnumber all other animals put together. The largest group of arthropods is the insects, followed by the arachnids and then crustaceans.

General characters:

• Arthropods have a cuticle containing chitin; in almost all of them this cuticle forms a strong exoskeleton. Thinner, flexible cuticular joints articulate the rigid parts of this exoskeleton with each other. The appendages are generally composed of several such articulations, hence the name from the Greek (arthron “joint”+ podos “foot”). Beneath the cuticle is the soft hypodermis, in which there is muscle bundles attached to the inner surface of the cuticle.
• Molting is a process characteristic of arthropods. It is because the animal outgrows the nonexpansible cuticle covering it. The arthropod thus goes several molts during its life cycle. At the time of molting the old cuticle splits in some weak lines, and the organism climbs out of its old clothes.
• There is a body cavity or hemocoele surrounding the internal organs and continuous through body segments. It is filled with the arthropod blood or hemolymph.
• The circulatory system is present in large arthropods in the form of a dorsal tubular heart, through which the blood goes in one direction by the aid of valves. Sometimes there is arteries coming out of the tubular heart and distributing the hemolymph to the body compartments.
• Respiration of the arthropods is variable. Aquatic arthropods like Crustacea use gills for respiration. Insects and Acari use tracheal system for breathing air. It is a network of tubes lined with cuticle, opens to the body surface by spiracles
• Nervous system is composed of a dorsal ganglionic mass and outcoming nerves.
• Digestive system is divided into foregut, midgut and a hindgut. The foregut is composed of esophagus, croup, proventriculus and a pair of salivary glands opening into the buccal cavity. The midgut is tubular stomach, which may have two or more ceca emerging from its anterior part. The hindgut is divided into intestine and rectum; its function is evacuating food waste as well as regulating water and ions in insect’s body.
• Excretory system differs according to classes.
• Reproductive system differs according to the class and will be discussed later.
• Development of arthropods or metamorphosis is the change in the body form from larva to adult. This goes through either one stage hatches from the egg with segmentation and appendages more or less like adults and are are called nymphs (the juvenile), or several stages or instars which do not resemble the adults on hatching are called larvae or larval instars. The first case is considered incomplete metamorphosis (hemimetabolous), and the other is complete metamorphosis (holometabolous).
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Classification of Arthropods

Class insecta (true insects): there are four orders of medical importance:

1. Order Diptera (fliers).
2. Order Siphonaptera (fleas).
3. Order Anoplura (lice).
4. Order Hemiptera (bugs).

1. Class Arachnida: one order in this class, Order Acarina, has medically important members (ticks and mites).
2. Class Crustacea: this also has two genera only of medical importance, Cyclops and Diaptomus.Disease Transmission by ArthropodsParasitic diseases are transmitted by arthropods in the following ways:
3. Arthropods can transmit many diseases to man and are considered vectors of many microorganisms as well as many parasites.
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1. Mechanical transmission, this is by:
   1. Carrying the infective stage (helminth eggs or protozoan cysts) on the body hairs from soil or garbage to human food or drinks.
   2. Transporting the infective stage in their gut during feeding and its regurgitation or defecation on food or drinks.
   3. Carrying the blood parasites in the residual fluid in their proboscis (wet proboscis).
2. Biological transmission, this is the active participation in the life cycle of the parasite during its transmission from one host to another. It is either: 
   1. Propagative, in which there is increase in the number with developmental changes of outcoming parasites
   2. Cyclical transmission, in which the parasite develops to the infective stage within the arthropod, it is of the following types:
      1. Cyclopropagative, in which there is increase in the number with developmental changes of outcoming parasites. This is considered either:
         1. Anterior station transmission with the infective stages enter the host during blood sucking or biting through salivary glands.
         2. Posterior station transmission in which the stages come out with feces of the arthropod and contaminate the bite wound.
      2. Cyclodevelopmental, in which there is no reproduction or increase in number, but the parasite develops into the infective stage.
   3. Transovarian transmission in which the parasite undergoes reproduction and is transmitted through the arthropod egg to its offsprings.
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CLASS INSECTA

Class Insecta or true insects is the most numerous of arthropods. It is characterized externally by:

• The body is divided into head, thorax, and abdomen and is covered by tiny hairs or bristles.
• The head is carrying two compound eyes, several simple eyes or ocelli and head appendages.
  1. One pair of antennae carrying sensory hairs or bristles.
  2. One pair of maxillary palps.
  3. The mouthparts, which are a tube like structure formed of a ventral labium, a dorsal labrum epipharynx, a central hypopharynx surrounding the salivary duct, and lateral mandible and maxilla on each side. The shape of mouth parts differs according to their function.
• The head appendages are:
• The thorax is composed of three segments: prothorax, mesothorax and metathorax. From each thoracic segment a pair of jointed legs comes out, and a pair of wings originates from lateral sides of meso- and metathorax. The leg is composed of a coxa connected to thoracic wall, a trochanter, a long femur, a tibia and a tarsus formed of 5 segments ending with a pair of small tarsal claws, and sometimes a small pillow like soft structure, the pulvillus.
• The abdomen is composed of 10 segments originally, may be fused together to be 8 or less. The terminal segments carry appendages which are the external genitalia. The male external genitalia are in the form of a pair or several pairs of claws (claspers) and the male genital organ, the aedegus.
• The female genitalia are in the form of an ovipositor, and two rounded cerci one on each side.

ORDER DIPTERA

 (The double-winged insects)

• Members of this order are collectively called the flies.
• The first pair of wings is membranous and is armed by an anterior thick chitinous end (costa), and chitinous linear ridges (veins).
• The second pair of wings is replaced by a small, pinhead like structure called halter, which is a sensory organ used during flying.
• Development in this order is holometabolous (egg, four larval instars, pupa and adult).
• Many members are considered intermittent parasites as they are blood suckers.
•  According to the way of emergence from the pupa they are divided into two large suborders:

1. Orthorrapha, in which the imago comes out from a linear or T-shaped suture in the dorsal part of the pupa. Only females of this subfamily are bloodsuckers. This is subdivided into two sections:
   1. Section Nematocera, with long antenna composed of more than 10 segments. Members of this section are Families Culicidae (mosquitoes), Psychodidae (sand flies), Culicoides (midges), Simuliidae (black flies).
   2. Section Brachycera with few antennal segments. Medically important members of this section are of Family Tabanidae (horse flies and deer flies).
2. Cyclorrapha, in which the imago comes out from a rounded suture in one pole of the pupa. Both males and females of some families are bloodsuckers.
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CLASS ARACHNIDA

Ticks, Mites, Spiders, Scorpions

General characters:

• Differ from insects in the absence of wings, antennae, and compound eyes.
• There are 4 pairs of legs in the adult.
• Head and thorax may be fused together to form cephalothorax and abdomen, or all body segments are fused in one body sac (ticks and mites).
• Development is by eggs (laid in batches or singly), then larva, and then several nymphal stages to adult. 
• Classification:
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1. Order Acari (ticks and mites).
2. Order Araneida (spiders).
3. Order Scorpionida (scorpions).
4. Order Pentastomida (tongue worms).
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SUPERCLASS CRUSTACEA

CLASS COPEPODA

General characters:

• Body is composed of: cephalothorax, a free wider thorax and a tapering abdomen.
• The cephalothorax (sometimes the whole body) is covered by a carapace, a crust- like chitin.
• The first appendages arising anteriorly are 2 pairs of antennae; the first is smaller than the second. There is a median eye.
• Feeding appendages arise anteriorly bilaterally , they are mandibles, maxillules and maxillae, followed by a thoracic pair called maxillipods.there is biramous thoracic pereopods and abdominal pleopods.
• The abdomen terminates by a telson, which may be surrounded by the last pair of pleopods called uropods.
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