Insect




They contain about 70% of known animal species.

General morphology:
The body can be divided distinctly into three parts head, thorax and abdomen.

1. Head:
Ø  It is an oval or gloves structure present in front of the body and is covered by a chitinous structure plate.
Ø  Head contains eyes, antennae, feeding apparatus.
Ø  Eyes- there are two types of eyes holoptic and diochoptic. Eyes are arranged in triangular position, dorsum and ventex.
Ø  Antennae- They has single pair of antennae, present in front or between the compound eyes. The antennae are variable in shape e.g. -Mosquito contain elongated and segmented antennae. Some contain short and stout antennae (house fly). They are frequently haired or may carry bristle eg- arista.

2. Feeding apparatus:
The mouth parts consists of –
                                I.            The labrum or upper lip
                              II.            The labium or lower lip
                            III.            The pair of maxillae and mandible
                            IV.            The hypopharynx bears opening of salivary gland
                              V.            The epipharynx, bears the organ of taste
                            VI.            The labium may be extended (house fly) or it may be piercing (mosquitoes)

3. Thorax:
There are three segments –
·         prothorax,
·         mesothorax and
·         metathorax.
 Adult have one pair of legs in each segment. mesothorax and metathorax contain one pair of wings.
Leg –
each leg consists of
·               basal coxa,
·               trochanter,
·               femur,
·               tibia and
·               tarsus.




Tarsus contains 5 joints and last joint contains claw below the claws there is pulvillus.
In mammals 1 pair and in bird 2 pair claws present.
Wings-
  
normally two pairs of wings occur but in Diptera the last pair is reduced to become halters or balancers, which is sensory structure and have balancing function.
Wings are outgrowth of thoracic teguments supported by hollow tubes called veins which run longitudinally and crosswise, the interveining areas of tegument known as cells. The arrangement of veins and shape of the cells are important in identification of insects.



4. Abdomen:



Abdomen is usually segmented and soft membranous which contain copulatory clasper (penis like structure), ovipositor, external ganglia , stigmata and different kinds of identifying characters.
Insects are oviparous, larviparous, pupiparous, parthenogenesis occur in many insects.

Developmental stages or life cycle:
In insects the sexes are separated and after fertilization either eggs or larvae are produced.
They have two types of life cycle-
Holometabolous:
Development often involves three or more larval stages followed by theformation of a pupa and a marked metamorphosis to the adult, is known as holometabolous life cycle.
ie  egg- larvae- pupa- adult. Eg- all Diptera
Hemimetabolous:
Development occurs from the egg through several nymphal stages which resemble the adult, is known as hemimetabolous life cycle.
ie egg- nymph- adult. Eg- lice









Metamorphosis:
The developmental changes from one stage to another (ie- egg- adult in size, form, structure) in the life cycle of insect or arthropod is known as metamorphosis. There are three types of metamorphosis may occurs-
Simple Metamorphosis (incomplete/ Exopterigota/ hemimetabola):
When newly hatched insect are very similar to that of adult ie nymph are very similar that the appearance of adult but sexually undeveloped. This type of metamorphosis is called simple metamorphosis. Eg- Bee, moth, cockroach, grass hopper.   Egg-nymph- adult. 
Complete Metamorphosis (complex/ compound/ Endopterigota/ holometabola): When newly hatched insect are very dissimilar to that of adult in appearance ie one stage is different from that of other in size and form, this type of metamorphosis is called complete metamorphosis.eg- Fly. Mosquito, butter fly, house fly, tick, flea.

Ametamorphosis:
Without metamorphosis or least change.  Eg- Silver fish.
Terminology related to the development of insects:
Instar: Developmental stage or form of the insect life cycle. Larval instar, pupal instar.
Imago: Immature adult of insect. There is no development of gonad. Eg- butter fly, house fly, tick.
Naiad: When the nymphal development occur in the water.
Cast off: Exoskeleton changing process. It is called ecdysis.
Stedium: The time interval between the moulting of insect . 1st- Nymph, 2nd- Adult.
Chrysalis: in case of butterfly or housefly the form which leaves from the egg is called larvae. Feed and grow another form or phase is called pupa or chrysalis.

Larvae: the form leaves from in complete metamorphosis. There are various form of larvae are found.
Types of larvae:
        I.            Polypod larvae: They have well marked head, thorax of three segments each of which bear one pair of clawed legs, an abdomen of ten segments bear five pair of fleshy hooked legs.  Eg- larvae of butter fly.
      II.            Oligipod larvae: They have well marked head, thorax contains three pairs of leg but abdomen contains no leg. Eg- larvae of beetle.
    III.            Apodous larvae: Their head is reduced and no legs either in thoracic or abdominal region. Eg- larvae of fly, house fly, blow fly and all diptera flies.



Pupa: is the non-feeding stage of insect during which the larval stage is transferred to the adult character. Eg – beetle
There are three types of pupa found in insects.
        I.            Free or exarate pupa: Wings and leg can be seen externally and they are free from the rest of the body. Eg- Beetle
      II.            Obtectate pupa: Wings and leg can usually be seen externally and they are bound tothe body by moulting fluid.eg- Mosquito.
    III.            Coaretate pupa: Enclosed in its cast skin of the last larval phage called puparium. Their skin harden and the insect inside cannot be seen. Eg- House fly.










Ecdysis: as the arthropods grown they become too weak for their chitinous covering, these chitinous covering cast off ie moulting and development of new exoskeleton and these cast off is called ecdysis.
When simple metamorphosis occur the form leaves from the egg are more like to the adult is called nymph whish grow and cast it skin for several time to become adult.
Importance of arthropods or relation of arthropods with the animal to human health:
1.     Direct agent:
Arthropod act as a direct agent of disease or discomfort-
·         Entomophobia . eg- Cockroach
·         Annoyance and blood loss by blood sucking insects. Eg- Lice
·         Accidental injury to the sense organ.
·         Dermatitis and skin disease. Eg- sheep scabies.
·         Myiasis- invasion of tissue by maggot.
·         Allergic reaction. Eg- Beetle.
·         Envenomation. Eg- Ant, bee.
Some viruses are transmit by arthropod those viruses are called Arbovirus.

2.     Vector or Indirect:
Indirectly act as an intermediate host or vector for the transmission of disease.
·                     Mechanical transmitter:
a.                  Indirect: Escherichia coli, vibrio cholera transmitted by leg of fly.
b.                  Direct: transmission of Trypanosome sp. (protozoa) by tabanus flies.
·                     Biological:
a.         Propagative: The organism undergoes no cyclical changes but multiply as a culture tube. Eg Yersinia pestis is transmitted by Xenopsylla cheopis (flea).
b.         Cyclo-propagative: The organism undergoes cyclical changes and multiplies. Eg – Plasmodium sp. is transmitted by female Anopheles mosquito.
c.          Cyclodevelopmental: the organism undergoes cyclical changes but no multiplication occurs. Eg- Stephofilaria assamensis is transmitted by Musca fly.
·                  Trans-ovarian transmission: Babesia is transmitted by Boophilus sp.
·                  Fecal material: Escherichia coli can be transmitted by arthropod.

Arthropod act as intermediate host of helminth-
a.      Dipylidium caninum – Ctenocephalides canis
b.      Dibothriocephalus latum – Cyclops
c.       Moniezia expansa –Ant
d.      Dicrocoelidium dentriticum - Ant
e.      Onchocerca sp. - Fly

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