Introduction

Arthropoda is the largest phylum with about nine lakh species. They may be aquatic, terrestrial or even parasitic. They have jointed appendages and a chitinous exoskeleton.

This phylum includes several large classes and contains the class Insecta which itself represents a major portion of the animal species in the world. They possess the ability to survive in every habitat.

Characteristics of Arthropoda

• It is the largest phylum in the animal kingdom, including 900,000 species. The largest class is insecta with 750,000 species. General characters are
• They are triploblastic coelomate and bilaterally symmetrical animals.  
• The body cavity is full of haemolymph (blood) and it's known as haemocoel. The true coelom is restricted to gonals.  
• The body is covered by chitinous cuticle, which forms the exoskeleton. Strengthen by deposition of minerals (Cal. Phosphate & Carbonate).
• They have a segmented body, each segment bearing a pair of jointed appendages covered by a jointed exoskeleton.
• Exoskeleton is made of chitinous cuticle that is shed at intervals.  
• The process of casting off of skin or integument is known as ecdysis or moulting. Chitnnous exoskeleton is secreted by the underlying epidermis.  
• The body is divided into head, thorax and abdomen. In some cases, the head and thorax is fused to form cephalothorax. In insects the thoracic segments have legs and wings, the abdomen has no legs in insects.           
• Muscles are striated.
• Respiratory organs are gills, book gills, book 1^st time developed in Arthropoda.  lungs or tracheal system.
• Excretion takes place through green glands or malpighan tubules coxal gland.
• Excretory matter = Ammonia (Aquatic) ; Uric Acid (Terrestrial) Nephridia (Perpatus)
• Sensory structures in arthropods are antennae for perceiving odour, eyes, statocysts or balance organs and sound receptors (in chirping crickets and cicadas). Eyes are compound. In honey bees, butterflies and months and insects, the gustatory receptors are present on their feet.
• The heart is dorsal pulsatile, many-chambered and the circulatory system is open (Haemocoel). Blood haemolymph colourless. 
• The central nervous system consists of a circumentric ring formed by paired pre-oral ganglia connected by commissures to a solid, dorsal ganglionated, ventral nerve chord.
• In land arthropods, the fertilization is always internal. Few aquatic has external fert.
• Arthropods are oviparous. In some like the scorpion, the eggs hatch within the female body. They bring forth the young ones alive. They are viviparous.

Advancement Over Annelida

• Distinct-head in all species.
• Jointed appendages serving a variety of functions.
• Jointed exoskeleton for protection and muscle attachment.
• Striped muscles arranged in bundles for moving particular parts.
• Special respiratory organs such as gills, trachea, book lungs in the majority of cases.
• Well developed sense organs such as compound eyes, statocysts auditory organs, taste receptors etc.
• Endocrine glands and pheromone secretion for communication

Classification of Arthropoda

• The phylum arthropoda is divided into five classes.

Class 1. Crustacea

• The body is divisible into cephalothorax (head + thorax) and abdomen.
• Dorsally, the cephalothorax is covered by a thick exoskeletal carapace.
• There are present two pairs of anternnae and a pair of stalked compound eyes.
• Respiration is carried out either by body surface or by gills.
• Excretion takes place usually by antennary glands ( =  green glands)
• Sexes are usually separate. Sexual dimorphism is also seen.
• Development is usually indirect
  Examples : Palaemon (Prawn), Macrobrachium (Prawn), Atacus (Cray fish), Palinurus (Lobster), cancer (Crab), Lucifer (Shrimp), Eupagurus (Hermit crab), Oniscus (wood louse), Daphnia (Water-flea), Cyclops, Balanus (barnacle). Tiny crustaceans such as Daphnia and Cyclops act as zooplankton which form important link in the food chain in water.

Class 2. Chilopoda

• Body is long and segemented, which is divisible in to head and trunk.
• Each trunk segment bears a pair of legs. The first pair of legs are modified in to poison claws.
• There is a single pair of antennae and ocelli.
• There are present many legs.
• Respiration occurs by tracheae.
• Excretion takes place by Malpighian tubules.
• Development is direct
  Examples :  Scolopendra (Centipede)

Class 3. Diplopoda

• Body is divisible into head, thorax and abdomen.
• There is a single pair of antennae and ocelli.
• Except first thoracic segment, (It does not have legs) each thoracic segment bears a pair of legs, however each abdominal segment has two pairs of legs
• Respiration occurs by tracheae
• Excretion takes place by Malpighian tubules
• Development is generally direct e.g. julus (Millipede)

Class 4.  Insecta (Hexapoda) [Largest number of species]

–    Body is divisible into head, thorax and abdomen.

–    There is a pair of antennae, and a pair of compound eyes.

–    The thorax consists of three segments with three pairs of legs and usually two pairs of wings. Mesothorax has thick and leathery false wings called as tegmina and membranous metathoracic wings. 

–    The abdomen may consist of ten segments.

–    Respiration usually takes places by tracheae. (No respiratory pigments).

–    Heart is tubular and  divided into chambers

–    Malpighian tubules are the excretory organs. Uric acid is chief excretory waste.  

–    Sexes are separate.

–    Development may be direct or indirect e.g. silverfish, cockroach, bedbug, locust, termites, butter flies, rat flea, beetle , wasp, aphid, glow worm etc.

–    Maggot is the larva of Housefly

Insects communicate with each other by ectohormone called as Pheromones. Pheromones are chemicals secreted to the outside of the body and perceived (as by smell by other individuals of same species). They help in communication amongst the organisms of same species.

Ex. 

1. Musa domestica        –          House fly
2. Tachardia lacca (Lacifer) –      Lac Insect
3. Dactylopis –     Cochineal bug
4. Lytta   –    Blister Beetle  
5. Apis     –     Honeybee
6. Bombyx   –    Silkworm  
7. Locusta   –     (Locust)                                              

The insects may be divided into four groups on the basis of their mode of development:

–    Insects without Metamorphosis (Ametabolous Development). Certain insects, such as silver fish, do not undergo metamorphosis. These insects are most primitive and wingless.

There are three stages present in the life history; egg,  young and imago (adult)

–    Insects with gradual Metamorphosis (Paurometabolous Development). In this type of metamaophosis, the life history includes egg, nymph (young) and imago (adult). The nymph resembles the adult in its mode of life but differs in structure, the young being without wings. Gradual metamorphosis. Occurs in cockroaches, grasshoppers, locusts, termites, stick insects, praying mantis, bed bug and lice.

–    Insects with incomplete Metamorphosis (Hemimetabolous Development). In this type of metamorphosis the life history includes egg, naiad (young) and imago (adult). The naiad differs from the adult in both mode of life and structure. Incomplete metamorphosis occurs in dragon flies and may flies.

–    Insects Complete Metamorphosis (Holometabolous Development). In this type of metamorphosis the life history includes egg, larva, pupa and imago (adult). Complete metamorphosis occurs in butterflies, moth, beetles, house flies, mosquitoes, fleas, honey bees, ants, wasps. The larvae of butterflies and moths is called caterpillar. The larva of houseflies is known as maggot. The larva of beetles is termed grub and the larva of mosquito is called wriggler. The young one formed after every moulting is called as instar. The period between two molting is stadium.

–    Hypermetabolous. Each larval stage differs from the others in habits, food and mode of living e.g. Blisterbeetle

Class 5. Arachnida

–    The body is usually divisible into cephalothorax and abdomen.

–    The cphalothorax bears simple eyes and six pairs of appendages (One pair of chelicerae, one pair pedipalpi and four pairs of legs)

–    Antennae are absent.

–    Respiratory organs are book  lungs or trachea or both

–    Excretion takes place by Malpighian tubules or coxal glands or both

–    Development is generally direct.

Examples :  Scorpion, spider, Tick, Mite, Aranaeus (garden spinder), Palamneus (Scorpion), Buthus (Scorpion)

–    Spiders spins the web by means of a secretion of abdominal glands.

–    There are two more classes in the phylum Arthropods. These classes are Onychophora and Merostomata.
Peripatus is an important examples of the class Onychophora, which has characters of Phylum Annelida and Arthopoda. Hence it is called "connecting link" between annelida and arthropoda .  It breathes by trachea.

– *Limulus. (the king crab or horseshoe crab) is good example of class  Merostomata which respires with book gills. The king crab are called "living fossils". A living fossil is a living animal of ancient origin
with many primitive characters.

–    Larvae of different Arthropods

(a) Bombyx (Silkworm)  - Caterpillar / Silkworm

(b)  Beetles, honey bee - Grub   

(c)  Musca (Housefly) - Maggot

(d)  Culex, Anopheles -  Wriggler

(e)  Pennaeus (Marine prawn) - Mysis, nauplius, protozoea

(f)  Cancer (Crab) -  Megalopa metanauplius, zoea 

–    Arthopoda is the biggest phylum. About 9,00, 000 species are there . Largest class is insect.

–    Von Siebold established phylum Arthoropoda.

–    Arthopods are Aquatic (Fresh water/marine), Terrestrial, burrower, parasitic.

Type of Mouthparts in Insects

– Generally mouth parts of an insect are one labrum, one labium, one hypopharynx, two mandibles and two maxillae. Following type of mouth parts are found is insects.

–    Biting and chewing type e.g., Grass-hooppers, cockroaches and crickets.

–    Piercing and sucking type.e.g. mosquitoes, bed-bugs

–   Chewing and lapping type. Lap means to drink by scooping with tongue and lapper is that which laps liquid e.g. honey bees.

–    Sponging type. eg. Housefly (Musca domestica.) It lacks mandibles. Feeds on sugar by dissolving in saliva and sucking.

–   Siphoning type e.g. Butterflies and moth. Here the proboscises formed by the modification of maxilla.

Uses of Arthropoda

Lac Insect

–    Lac is produced commercially by an insect Tachardia lacca (Laccifer lacca).           

–    Lac is actually secreted for its protection and not for the food of the insect.

–    Male insects are winged and structurally complete, but females are degenerated.

–    Lac or shellac is an exuviate (secretion) of mainly female.

Lac is resinous substance 

 Cochineal Bug

–    Dactylopis cocccus lives upon cactus. Dead and dried bodies are used for making a dye called cochineal dye.

Blister Beetle

–    Lytta is a genus of blister beetle. The drug cantheridine is prepared from its blood.

–    Cantheridine is widely used for healthy growth of hair

Red Ants

–    Red ants are used for the production of formic acid.

Glow worm

Wingless female and larva of certain beetles like Lymphyris notiluca which emits greenish light. Also yields luciferin

Honey Bee : A hive consists of a vertical sheet of wax with a number of hexagonal cells for rearing young bees (brood cells), storing food (storage cells), royal chamber and chamber of themselves.

Bee is a social, polymorphic and colonial insect. It has three main castes – queen, drones and workers.

Queen is the only fertile female of the colony which continues to lay eggs for 2–5 years.

Drones are male honey bees. They develop from unfertilized eggs. Drones and virgin queens take part in nuptial flight. After copulation the drones are not allowed to come back into hive.

Workers are sterile females.

Scout bees search for food and intimate the same to worker bees by dances-round dance for less than 75m and tail wagging dance for longer distances.

Prof. karl von Frisch got Nobel prize 1973 for decoding the language of bee dances. Workers have pollen collecting apparatus, honey storing mechanism and wax secreting glands.

Young workers secrete royal jelly. Royal jelly is given to queen or potential  queens.

Honey contain simple sugars (fructose and levulose), Vitamins and minerals. It is a tonic, laxative and sweetening agent. True product of honey bee is bees wax.