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L.3.
Medical Entomology
MSC IZDIHAR A. ALWAN
Anatomy of insects :
* Digestive System :- A tube that extends from the mouth to the anus , there are three
sections :1- foregut :
Pharynx ( throat )
Esophagus ( gullet )
Crop ( storage )
Proventriculus ( may be muscled , gizzard – like ) .
2- Midgut :
Gastric caeca " blind sacs " ( food storage and enzymes )
Ventriculus ( most digestion , absorption here )
3- Hindgut :
Anterior intestine ( excertory organs empty in ) , Ileum and colon .
Rectum ( reabsorption of water )
Anus
In embryonic development the forgut and hindgut are formed from ectoderm their surfaces
are shed during molting . The midgut is formed from the endoderm its surface is retained
during molting .
Digestion : some insects use external digestion in addition that which occurs internally .
Leafhoppers inject saliva into plant tissues .
House flies regurgitate salivary enzymes on to the surface of food .
Diving beetles inject prey with salivary juices .
Most digestive action is in the midgut , gastric caecae are rich in enzymes . Enzyme diversity
varies with the range of food that different insects eat more proteolytic enzymes in blood
feeders , cellulase in wood-boring beetles , etc .
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* Excretory system : Purpose of excretion
Removal of nitrogenous wastes .
Maintenance / regulation of salts and water balance .
Primary excretory organs Malpighian tubules and the Rectum .
Malpighian tubules "float" in the hemolymph active transport moves wastes ( uric acid salts
from the fat body ) into tubules . Malpighian tubules empty into the hindgut and water
reabsorbed . Excretory and fecal wastes are combined .
* Nervous system :
The brain – the supraesophgeal ganglion ( nerve gland mass above the esophagus )
Optic lobes ( paired ) : the largest lobes of brain , each protrudes from the protocerebrum .
Protocerebrum ( paired ) : innervates compound eyes and ocelli .
Deutocerebrum ( paired ) : innervates antennae .
Tritocerebrum ( paired ) : connect to the visceral nervous system .
Circumesophageal connective ( paired ) : from the dorsal brain to the ventral nerve cord .
The ventral nerve cord - connects segmental ganglia ( nerve cell bundles )
-1- Thoracic ganglia the numbers are three found in pro , meso and meta thorax , it is large
ganglia give nerves to each side of body , legs and wings .
-2- abdominal ganglia the numbers are six , it is smaller than thoracic ganglia except last .
Respiratory system ( tracheal system ) :
- Spiracles :- External openings on each side of most body segments .
- Tracheae :- large tubes that run the length of the body on each side . Smaller tubes are
called tracheal branches and tracheoles .
- Air sacs :- that store air ( air , not just oxygen ) may be located in the abdomen and / or the
thorax .
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Circulatory system :
Insects have what is termed an " open circulatory system " it is comprised of a dorsal vessel
with a posterior " heart " and an anterior aorta .
The heart tube consist of 13 chamber , found in thorax 2 and 11 in abdomen . between each
chamber found two opening for enter the blood called Ostia .
The main body cavity of arthropods is thus a secondary space (the hemocoel ) filled with
fluid ( hemolymph ) containing a variety of cell types are bathed in this fluid , which is
circulated through an open circulatory system by means of a dorsal tubular heart .
Hemolymph enters the heart through pairs of the lateral openings (the ostia ) when the heart
contracts ostia close forcing hemolymph anteriorly into arteries and finally into a system of
tissue spaces .
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Order :- Diptera
1- Family :- Muscidae
Genus :- Musca domestica ( house fly )
The house fly is always found in association with humans or activities of humans . This is
most common species found in the world .
* Life cycle : The house fly has a complete metamorphosis with distinct egg , larva or
maggot , pupa and adult stages :1- Egg : its white color about 1.2 mm in length are laid single but pile up in small masses on
batches ( 75-150 egg ) in feces of animals and humans also in organic materials .
2- Larva : The egg hatched through 12 – 24 hour to Vermiform larvae , its legless ,
immediately feeding and develop in the material where the eggs were laid The mature larva
is 3 - 9 mm in length , creamy whitish in color and cylindrical shape but tapering toward the
head . The head contains one pair of dark hooks . The spiracles are slightly raised and the
spiracular openings are sinuous slits which are completely surrounded by an oval black
border .
3- Pupa : " Coarctate pupa " The pupae 8 mm in length the pupa stage is passed in a
pupal case formed from the last larval skin which varies in color from yellow , red , brown to
black depending on the pupa ages .
The emerging fly escapes from the pupa case through the use of swelling and shrinking sac
called the ptilinum on the front of its head .
4- Adult : The house fly is 6 – 10 mm in length , the female usually larger than the male .
Head has simple and compound eyes are reddish , the mouth parts are sponging and type of
antennae are aristate .
The thorax bears four narrow black stripes and there is a sharp upward bend in the fourth
longitudinal wing vein . The fly has a complex leg " Walking upside down legs " with
sense organs that allow it to walk on the ceiling called Pulvillus that has Arolium and
Empodium .
The abdomen is gray or yellowish with dark midline and irregular dark markings on the sides
, the underside of the male is yellowish .
The sexes can be readily separated by noting the space between the eyes which in female is
broad while in male is smaller .Adults usually live 15 to 25 days and they are sucking
liquids containing sweet or decaying substances .
* Medical important of Musca domestica :
The house fly not only is a nusiance pest but also The population of house flies transmit the
disease agents by means of different parts of their bodies ( hairs body , appendages and
mouth parts ) and secretions ( feces ) . Pathogenic organisms are picked up by flies from
garbage , sewage and other sources of filth and then transferred on their mouth parts and
other body parts through their vomitus , feces and contaminated external body parts to
human food .
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House fly acts as an important mechanical vector for lots of pathogenic microorganism
agents including :- Bacteria such as ( Escherichia coli ,Staphylococci aureus , Pseudomonas spp and Proteus
spp ) .
- Protozoa , Worms , Fungi and Viruses amongst humans and animals .
- There are more than 100 pathogens associated with the house fly may cause disease in
humans and animals including typhoid , cholera , bacillary , dysentery , tuberculosis and
infantile diarrhea .
* Biological Control :
The more common measures involved with the control of house flies are sanitation, use of
traps and insecticides but in some instances integrated fly control has been implemented the
use of biological control in fly management is still at a relatively early stage .
With increasing incidence of insecticide resistant house fly populations rising costs of
insecticides and a growing public concern about actual or potential problems associated with
insecticides , interest in alternative house fly control strategies has increased . The house fly
has many natural enemies and among the more important in poultry facilities are the wasps (
Hymenoptera : Pteromalidae ) Muscidifurax raptor and Spalangia cameroni .
2- Family :- Glossinidae
Genus : Glossina ( tsetse fly )
Tsetse flies are blood sucking most of their time at rest in shaded places in forested areas
the preferred sites are the lower woody parts of vegetation , many tsetse flies hind in holes in
the trunks of trees and between roots . They search for food only for very short periods the
day . The flies often rest near to food sources .
* Life cycle :
The female tsetse fly does not lay eggs but produces larvae ,one at a time the larva develops
in the uterus over a period of 10 days and is then deposited fully grown on moist soil or sand
in shaded places usually under bushes .
It buries itself immediately and turns into a pupa and the fly emerges 22-60 days later
depending on the temperature . Tsetse flies adults are 6-15 mm in length , females mate only
once in their life and with optimum availability of food and breeding habitats can produce a
larva every 10 days .
* Medical important of Glossina :
Tsetse flies occure only in tropical Africa and are important as vectors of African
trypanosomiasis in both humans and animals . Sleeping sickness as it is commonly called , is
generally fatal in humans if left untreated . Sleeping sickness occurs in scattered foci
throughout Africa south of the sahara .
There are two different types of human sleeping sickness are caused by different subspecies
of Trypanosoma parasites
- gambiense sleeping sickness is mainly transmitted by tsetse flies belonging to the
G.palpalis group caused by Trypanosoma brucei gambiense is generally considered to be a
chronic disease and is found mostly in West and central Africa .
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- rhodesiense sleeping sickness is transmitted by savanna species belonging to the G.
morstians group caused by Trypanosoma brucei rhodesiense is an acute disease that occurs
mainly in East Africa .
Tsetse flies transmit trypanosomes in two ways mechanical and biological transmission :* Mechanical transmission requires tsetse fly feed on an infected host and acquire
trypanosomes in the blood meal and then within a relatively short period for tsetse to feed on
an uninfected host and regurgitate some of the infected blood from the first blood meal into
the tissue of uninfected host . Other flies such as horse flies also can cause mechanical
transmission of trypanosomes .
* Biological transmission requires a period of incubation of the trypanosomes within the
tsetse host . In this mode of transmission trypanosomes reproduce through several
generations , changing in morphology at certain periods . This mode of transmission also
includes the sexual phase of the trypanosomes . Transmitted biologically by tsetse cannot be
transmitted in this manner by other insects .
* Life cycle of T.gambiense and T. rhodesiense :
During a blood meal on the mammalian host an infected tsetse fly injects metacyclic
trypomastigotes into skin tissue . The parasites enter the lymphatic system and pass into the
blood stream .
Inside the host, they transform into bloodstream trypomastigotes are carried to other sites
throughout the body , reach other blood fluids (e.g. lymph , spinal fluid ) and continue the
replication by binary fission .
The entire life cycle of African Trypanosomes is represented by extracellular stages . The
tsetse fly becomes infected with bloodstream trypomastigotes when taking a blood meal on
an infected mammalian host .
In the fly midgut , the parasites tansform into procyclic trypomastigotes multiply by binary
fission , leave the midgut and transform into epimastigotes . The epimastigotes reach the fly
salivary glands and continue multiplication by binary fission . The cycle in the fly takes
approximately 3 weeks .
* Prevention and control :
The current strategy for sleeping sickness control is based on active and passive case
detection ( surveillance ) , treatment of infected people and when appropriate tsetse control .
Surveillance aims to reduce the human reservoir of infection and to make treatment less
hazardous through early detection .
- Control measures :
A variety of methods can be used to control tsetse flies before suitable insecticides became
available control efforts mainly involved the removal of the woody vegetation forming the
fly habitat . In areas of rhodesiense sleeping sickness the primary food sources of the flies ,
wild game animals were killed or removed . The tsetse flies then eventually disappeared
because of food shortage . These methods have largely been abandoned and today insecticide
spraying is used along with traps and insecticide impregnated targets .
- Traps and insecticide – impregnated screens : Traps and screens are an effective means
of tsetse control . They are cheap , easy to transport and completely safe for the user and the
environment .
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