Arthropods

The term arthropods derived from Greek word Arthros means joint and podos means limbs or foot, so arthropods means jointed limbs.

The phylum Arthropoda contains over 80% of all known animal species. Arthropods are bilaterally symmetrical invertebrate organism with segmented body, jointed limbs and their body is covered with hard chitinous exoskeleton.

 So all arthropods possess the following structure-

·         Exoskeleton

·         Segmented body

·         Jointed limbs and jointed mouth parts

·         Bilaterally symmetrical body

·         Ventral nerve cord (in vertebrate present dorsally)

·         Dorsal blood pump.

The arthropods have three main characteristics-

1.      Exoskeleton

2.      Jointed limbs

3.      Segmented body

General characteristics and function:

       External Anatomy

Exoskeleton:

Ø  The exoskeleton is formed by the chitinous structure which is formed by the secretion of underlying epidermal chitinogenous cells, present just beneath the exoskeleton.

Ø  The exoskeleton is notably covers the external surface of the body but also possess from mouth to anus.

Ø  The chitinous structure which the inner portion of mouth is called stomodaeum.

Ø  The chitinous structure which the inner portion of anus is called proctodaeum.

Ø  The exoskeleton is usually present in the form of chitinous plate , called sclaride or sclaron.

Ø  A typical segment of the body having a dorsal chitinous plate or sclaride is called tergum (tergite).

Ø  A typical segment of the body having a ventral chitinous plate or sclaride is called sternum (sternite).

Ø  The chitinous plate which covers the lateral surface and connect the tergum and sternum is called pleuron.

Ø  The casting of exoskeleton is called ecdysis.

Segmented body:

The arthropods are metamerically  segmented animal. When the arthropods are grown and increase in size this chitinous plate shed from the body and this process is called ecdysis.

The whole body of an arthropod divided into three parts

        I.            Anteriorly – head

      II.            Middle – thorax

    III.            Posteriorly- abdomen

Head: It contains one or two pair of antennae, two eyes and feeding apparatus. Antennae are a sensory organ and varies in size.

Eyes: There are two types of eyes – simple and compound.

Simple eye:This is called ocelli.

Compound eye:

In case of male or female eyes are different types -

In case of female, the eyes are distinctly separated and these eyes are called dichoptic eyes.

In case of male, the eyes are closely associated and these eyes are called holoptic eyes.

Thorax: It contains three segments (pro, meso and Meta) and each segment contains one pair of legs. Thorax also contains wings.

Abdomen: Most veterinary important arthropod has no legs in their abdomen.But in aquatic form contain numerous legs in abdomen for swimming in the water. Eg- Cyclops (Crustacean).

Internal anatomy

Digestive system:

It is divided into three parts.

Anterior portion is called stomodaeum (fore gut) which contains pharynx, proventiculus (crop) and gizzard.

The posterior portion is called proctodaeum (Hind gut).

The middle portion is called mesenteron (mid gut) which connects anterior and posterior part.

Respiratory system:

Gill or bronchiole – in case of aquatic type.

Trachea- in case of insects, arachnids they have trachea. It is a fine elastic tube with thin chitinous layer. The trachea branches and ramifies internally to supply oxygen.it opens externally ventral ventral to the abdomen. The opening of the trachea is called stigmata or spiracle. It is external opening by which air enters.

Book Lung – present in spider.

Gill book- present in crabs.

Respiration through the body cuticle occurs in parasitic mite.

Circulatory system:

Arthropods have variation from other animals. It has no coelom. They contain space between gut and body wall filled by blood or haemolymph which is called haemocoel. The blood of this haemocoel bathes the entire internal organ and transmits metabolites.

They have no true heart. The heart or epicardium is an enlarged blood vessel which sucks blood from the body through a small opening called Ostia and blood enters into the haemocoel by a small artery.

Nervous system:

It contains cerebral ganglia which runs ventrally which connects with the sensory organ (setae, antennae, eye, and pulp).

 Excretory system:

It varies in different class of Arthropoda such as

In case of crustacae – they have nephridia which situated at the base of 2^nd antennae.

In case of insecta – they have mulpigian tubule which opens in front of the proctodaeum.

Class Arachnida also found mulpigian tubule in front of the proctodaeum and additional organ is coxal gland which opens in front of the coxae of leg.

Reproductive system:

Both sexes of arthropod are separate.

The male genital organ consists of penis, a pair of testicle, each with the vas deference and common or separate seminal vesicle which store spermatozoa.

The female genital organ consists of ovipositors, a pair of ovary, oviduct, uterus and posteriorly vagina. An important excretory organ of female genital system is spermatheca which stores the fertilized ova after copulation viable often throughout the female’s life.

Most of the arthropods are oviparous.

Arthropod leg 

The arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments (called podomeres) are of Latin origin, and may be confused with terms for bones: coxa (meaning hip, plural coxae), trochanter (compare trochanter), femur (plural femora), tibia (plural tibiae), tarsus (plural tarsi), ischium(plural ischia), metatarsus, carpus, dactylus (meaning finger), patella (plural patellae).

Homologies of leg segments between groups are difficult to prove and are the source of much argument. Some authors posit up to eleven segments per leg for the most recent common ancestor of extant arthropods, but modern arthropods have eight or fewer. It has been argued  that the ancestral leg need not have been so complex, and that other events, such as successive loss of function of a Hox-gene, could result in parallel gains of leg segments.