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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