Introduction to the Arthropods


Include nearly one million species.

Adapted for life in all areas of land, water and air.


Main features that reflect the annelidian ancestry of arthropods.


  • Bilaterally symmetrical.
  • Triploblastic.
  • Coelomate protostomes.
  • Segmented. (segmentation is evident in the embryonic development of all arthropods)
  • They have paired appendages on each body segment.
  • The nervous system consists of a dorsal brain & ventral nerve cord containing ganglion.

Structure of generalised arthropod.
 


Segmentation & Tagmatisation


Segmentation is reduced through loss of segment, by fusion.

Tagmatisation is a specialisation of the group of segments.

Some arthropods like insect their body is divided into 3 tagmata.

  1. Head
  2. Thorax
  3. Abdomen


Cuticle (chitinous exoskeleton)


They consist of hard, chitinous cuticle.

Cuticle secreted by the underline epidermis.

The cuticle consists of two main layers.

1.Epicuticle.

2.Procuticle.

Arthropod Cuticle


Epicuticle


Thin out layer.

Composed mainly proteins.

Arachnids, insects epicuticle contains wax.


Procuticle


Thick layer.

Inner to the epicuticle layer.

Procuticle consists of 2 layers.

1.Exocuticle(outer)-strengthened by the formation of cross-linkages between protein.

2.Endocuticle(inner)

Procuticle composed of protein & chitin.


Exoskeleton limitation on the growth of the arthropod.

Cuticle reduces the flexibility of arthropods.

Exocuticle part of the cuticle absent at joints to give flexibility & allow movement.

Weight of the exoskeleton limits arthropods to small size.


Moulting of the exocuticle


An old cuticle is shed and a new cuticle is formed at periodic intervals. That called Moulting.

This mechanism allows the arthropod to grow and increase the size.

Whole moulting process under hormonal control.

(some hormones- Ecdysone, Juvenile)


The function of the arthropods cuticle


1. A hard protective covering against predators.

2. Support for locomotion.

3. Terrestrial forms cuticle(wax layer) reducing water loss/water barrier.

4. Aquatic forms cuticle serves for gas exchange.


Jointed appendages and musculature

(pic)


Characteristic features of Arthropoda.


Jointed limbs.

Each limb is made of a number of joints connected by flexible areas of chitin & attached \

to the body by a basal segment.

They can rapid movement. Because muscles in arthropods are entirely striated.


2 types of limbs.

They are, 

1. Uniramous- Unbranched appendages.

  seen in Centipedes, Millipedes and Insect.

Uniramous appendage


2. Biramous- branched appendages.

Biramous appendage


Centipede each body segment carries a pair of appendages.

But insects only three pairs of walking legs.


Arthropods appendages serve a variety of functions

Sensory antenna of a honey bee

Abdominal mating appendage of a lobstar

Chewing mandible of a  cockroach

Swimming thoracic leg of a water beetle

Walking thoracic leg of a grasshopper

Pinching thoracic leg of a lobstar



Coelom and haemocoel

Body cavity of arthropods

They reduced coelom.

The coelom is restricted to the cavities in the excretory & reproductive organs.

The body cavity in which the organs lie is a haemocoel.


Circulatory system


Included -

  • Dorsal heart with openings in the wall. That called Ostia.(Characteristic feature)
  • Blood vessels
  • Haemocoel

The whole circulatory system

Transverse section through thorax


Blood in the heart leaves through closed vessels into the haemocoel.

Blood returns back to the heart through the Ostia directly from the haemocoel.

Therefore arthropods have an Open Circulatory System.

Their blood contain Haemocyanin as a respiratory pigment, Some have Haemoglobin.


Cilia

They lack cilia anywhere in the body, even in the larval stage.


Digestive system

The digestive tract is lined with a chitinous cuticle in the anterior and posterior ends.

An anterior end makes up the Foregut.

The function of the foregut- Ingestion, Tituration, Food storage.

The posterior end makes up Hindgut.

Functions-water absorption, Formation of faeces.

Between the foregut and hindgut lies Midgut.

The function of the midgut- Enzyme production, Secretion, Digestion, Absorption.


Excretory system


Arthropods possess 2 types of excretory organs.

Many of them possess paired blind saccules(end sac) 

Saccule

                                           

The excretory organ takes the name of the appendage which is associated.


Another excretory organ is the Malpighian tubules.


Malpighian tubules 


These are found in centipedes, millipedes, arachnids, insects.

Located at the junction between the mid and hindgut.


Nervous system and sense organs


Arthropods are more cephalised than annelids.

Head region well developed.

Their brain shows a high degree of development.

Also, they have well-developed sense organs(such as compound eye, antennae, sensory bristles & hairs).


Respiration


By gills, Book lungs, Tracheae or body surface.


Reproduction and development


Sexes are separate.

They have modified appendages for sperm transfer.

Fertilization is always internal in terrestrial forms.

The eggs are rich in yolk and cleavage is commonly superficial.

Also,

They have a larval stage.

The adult stage reached through metamorphosis.

Metamorphosis involves moulting of the exoskeleton.

The stage between moults is known as instars.