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Insecta
By
Assist. Lecturer Maytham A. Alwan
4/13/2015
Class: Insecta
General morphology and life cycle
A. The head of an insect generally comprises six fused segments with a
single pair of antennae. There is great variation in the structure of the
mouthparts, depending on Salivary duct and feeding habits, with
adaptations for chewingbiting, slashing-sponging and piercing-sucking.
(1) The labrum or upper lip is a hinged plate attached to the face or clypeus.
(2) The paired mandibles and maxillae or jaws have areas of their surfaces
adapted for cutting, slashing or grinding. The maxillae may also carry
maxillary palps which are sensory in function and used in the monitoring
of food.
(3) A hypopharynx which arises from the floor of the mouth, bears the
external opening of the salivary glands and is similar to a tongue.
(4) A labium or lower lip, which may be extensively modified, especially in
the flies, and sometimes bears two sensory labial palps.
Fig. 1 - Biting mouthparts on females of Culicidae.
an: antenna; cl: clypeus; ip: hypopharynx; li: labium; ls:
labrum (epipharynx); md: mandibles; ms: maxillae; oc:
eye; pd: pedicel; plp: palpus.
Fig. 2 - Biting mouthparts on a bloodsucking
female of Tabanidae.
an: antenna; cl: clypeus; ip: hypopharynx; li:
labium; ls: labrum (epipharynx); md: mandibles;
ms: maxillae; oc: eye; pm: maxillary palpus.
labium
Insecta
B. The three segments in the thorax (pro-, meso- and meta-thorax) each bear a pair
of jointed legs. The thorax of many insects also bears two pairs of wings, but in
the winged insects of veterinary significance, i.e. the Diptera, only one pair is
functional, the second being reduced to small knob-like sensory structures, called
halteres, which apparently have a balancing function.
C. The abdomen of insects consists of up to 11 segments with terminal
modifications to form the genitalia.
Life cycle: in insects the sexes are separate and after fertilization either eggs or
larvae are produced. Development often involves three or more larval stages
followed by the formation of a pupa and a marked transformation or
metamorphosis to the adult stage as in all the flies and fleas, i.e. a
holometabolous life cycle. In other insects development occurs from the egg
through several nymphal stages which resemble the adult, as in lice, i.e. a
hemimetabolous life cycle. The different stages in the life cycle are known as
instars.
Order: Diptera
This order of insects contains all of the flies of veterinary
importance. These are generally characterized by having a
single pair of membranous wings and a pair of halteres.
Some are important as external parasites, while in others the
larvae parasitize the tissues of the host. Many members of
this group are also important as vectors of disease.
The Diptera can be conveniently divided into three suborders,
namely, the Nematocera, Brachycera and Cyclorrhapha.
Diptera
order
Suborder
Brachycera
Nematocera
Tabanidae
(horse flies)
family
Ceratopogo-nidae(midges)
Cyclorrhapha
Psychodidae
(Sandflies)
Simuliidae (blackflies)
Muscidae
(house &
stable flies)
Culicidae
Mosquitoes
Calliphoridae
(blowflies)
Hippoboscidae
(forest flies&
keds)
Oestridae
(botflies)
Suborder: Brachycera
These are large flies with stout antennae often consisting of only
three segments, the last segment frequently bearing annulations.
cross-veins are present on the wings .
Using their slashing-sponging mouthparts, the females feed on blood
and the eggs, which are laid on vegetation overhanging mud or
shallow water, develop into large carnivorous larvae with ill
defined but usually retractile heads. Like the Nematocera, both
larvae and pupae are mobile and aquatic, and are usually found in
mud.
Suborder: Cyclorrhapha
These are small to medium sized flies with short, three-segmented
antennae, the last of which often bears a feather-like attachment,
the arista . The maxillary palps are small and the wings show
cross-venation.
Both males and females may feed on animals,
but many members of this group are not parasitic as adults and have
either vestigial or sponging mouthparts. Eggs or larvae are laid
on it’s hosts. The larvae have a poorly defined head, and are
mobile and worm-like, often being referred to as `maggots'.
Suborder: Nematocera
These are small flies and the adults are characterized by having a
pair of long, jointed antennae and segmented maxillary palps. The
wings generally have few cross-veins. Only the females are
parasitic and have piercing-sucking mouthparts.
Eggs are laid in or near water and develop into mature larva pupates
on the ground within a hard pupal case or puparium, which is
completely immobile.
Family OESTRIDAE
This is an important family consisting of several genera of large, usually hairy, flies whose larvae
are obligatory parasites of animals. The adults have primitive, non-functional mouthparts and
are short lived whereas the highly host-specific larvae spend a considerable time feeding and
developing in their animal hosts.
Three genera which behave similarly are considered here, namely Hypoderma, Oestrus and
Gasterophilis
(the latter is currently often classified in a separate family, the Gasterophilidae).
Hypoderma
The members of this genus are the `warble flies '
Hosts:
Cattle; the larvae occur erratically in other animals including equines, sheep and, very rarely,
man.
Species: Hypoderma bovis, H. lineatum.
Hypoderma
Morphology
Adults:
H. bovis and H. lineatum resemble bees, but, being Diptera, have
only one pair of wings; the abdomen is covered with yelloworange hairs with a broad band of black hairs around the middle .
Larvae:
The mature larvae are thick and somewhat barrel
shaped, tapering anteriorly; when mature they
are 2.5 cm long, and most segments bear short
spines . The colour is dirty white when newly
emerged from the host, but rapidly turns to
dark brown; the pupa is almost black.
Life cycle
The adult flies are active only in warm weather. The females attach their
eggs to hairs, H. bovis singly on the lower parts of the body on the
legs and above the hocks, and H. lineatum in rows of six or more on
individual hairs below the hocks . Below 18°C there is no fly activity.
The first stage larvae (L1), which are less than 1 mm long, hatch in a
few days and crawl down the hairs, penetrate the hair follicles and
migrate towards the region of the diaphragm. Migration is aided by
the use of paired mouth hooks and the secretion of proteolytic
enzymes, and the larvae feed as they travel to the resting sites where
they will spend the winter, H. lineatun to the submucosa of the
oesophagus and H. bovis to the epidural fat in the spinal canal.
Life cycle
These sites are reached in late autumn, and the moult to the second
stage L2 occurs there. In February and March migration is resumed,
and the L2, arrive under the skin of the back where they moult to
the L3, which can be palpated as distinct swellings (warbles). A
cutaneous perforation is made by the L3 and the larvae breathe by
applying their spiracles to the aperture. After about 4-6 weeks in
this site they emerge and fall to the ground where they pupate under
leaves and loose vegetation for about 5 weeks. The adults then
emerge, copulate and the females lay their eggs and die, all within
1-2 weeks.
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Pathogenic significance
The L3, under the skin damage the adjacent flesh and
this necessitates trimming from the carcass the
greenish, gelatinous tissue called `butcher's jelly', also
seen in the infested oesophageal submucosal tissues.
Finally, if larvae of H. bovis should die in the spinal
canal, the release of a highly toxic proteolysin which
they contain may cause paraplegia, while the death of
H. lineatum larvae in the oesophageal wall may cause
bloat through oesophageal stricture and faulty
regurgitation.
Oestrus
Larvae of this genus spend the parasitic period in the air passages of the hosts
and are commonly referred to as `nasal bots or bot flies '.
Hosts: Sheep and goats.
Species: Oestrus ovis.
Morphology:
Adults: Grey flies about 1.0 cm long, with small black spots on the abdomen
and a covering of short brown hairs .
Larvae:
Mature larvae in the nasal passages are about 3.0 cm long, yellowish-white,
tapering anteriorly with a prominent `step' posteriorly. Each segment has a
dark transverse band dorsally .
Life cycle
The females are viviparous and infect the sheep by
squirting a jet of liquid containing larvae at the
nostrils during flight, up to 25 larvae being delivered
at a time. The newly deposited L1, are about 1.0 mm
long, and migrate through the nasal passages to the
frontal sinuses feeding on mucus whose secretion is
stimulated by their movements. The first moult
occurs in the nasal passages, and the L2 crawl into
the frontal sinuses where the final moult to L3 takes
place. In the sinuses, the larvae complete their
growth and then migrate back to the nostrils.
Life cycle
Where flies are active throughout the year, but in cool or
cold weather the small L1 and L2 become dormant and
remain in recesses of the nasal passages over winter; they
move to the frontal sinuses only in the warmer spring
weather, and then complete their development, the L3
emerging from the nostrils and pupating on the ground to
give a further generation of adults. The females survive
only two weeks, but during this time each can deposit 500
larvae in the nasal passages of sheep.
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Pathogenic significance
Most infections are light, sheep showing nasal discharge, sneezing, and
rubbing their noses on fixed objects. In the rare heavier infections, there
is unthriftiness and sheep may circle and show incoordination, these
signs being often termed 'false gid'. If a larva dies in the sinuses there
may be secondary bacterial invasion and cerebral involvement.
The most important effects, are due to the activity of the adult flies. When
they approach sheep to deposit larvae the animals panic, stamp their
feet, bunch together and press their nostrils into each others' fleeces and
against the ground. There may be several attacks each day, so that
feeding is interrupted and animals may fail to gain weight.
Oestrus can occasionally also infect man. Larvae are usually deposited
near the eyes, where a catarrhal conjunctivitis may result, or around the
lips, leading to a stomatitis. Such larvae never fully develop.
Family: Calliphoridae
This family together with the Oestridae contain the species
responsible for the most important myiasis of domestic
animals and man.
myiasis is defined as the infestation of living animals with the
larvae of dipteran flies. It may be facultative (optional), as
in the calliphorids, or obligatory, as in the oestrids. It also
may be cutaneous (e.g. Lucilia), nasal (e.g. Oestrus) or
somatic (e.g. Hypoderma).
A common term for myiasis caused by members of the
Calliphoridae is `blowfly strike', the laying of eggs by the
fly being termed the `blow' and the development of the
larvae (maggots) and the damage they cause the `strike'. '
Screw-worm myiasis
Screw-worm myiasis
The name screw-worm is given to the larvae of certain species of
[Cochliomyia (syn. Callitroga) including C. hominivorax and C.
macellaria], and to that of a single species of Chrysomya, C. bezziani,
which cause myiasis in animals and occasionally man.
These bluish-green flies have longitudinal stripes on the thorax and
orange-brown eyes. They occur primarily in tropical areas and lay
their eggs on wounds, the larval stages characteristically feeding as a
colony and penetrating the tissues creating a large and foul-smelling
lesion.
Blowfly myiasis
It used to be thought that Chrysomya was confined to the Old World,
Africa and Asia, while Cochliomyia was only present in the New
World. However, in 1988, Cochliomyia hominivorax was introduced
into Libya apparently through imported infested livestock from Latin
America. In a less spectacular fashion various species of Chrysomya
have been described from various South American countries since
1975.
Hosts:
Mainly sheep, but any other animal may be affected. It is important to
note that only the larvae are responsible for myiasis.
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Blowfly myiasis
Life cycle
The gravid female blowfly lays clusters of yellowish cream eggs on
wounds, soiled fleece or dead animals, being attracted by the odour of
the decomposing matter.
In temperate areas under summer conditions, the eggs hatch into larvae
in about 12 hours; the larvae then feed, grow rapidly and moult twice
to become fully mature maggots in 3-10 days. These then drop to the
ground and pupate. The pupal stage is completed in 3-7 days in
summer and the emergent female fly, after a protein meal, reaches
sexual maturity.
The fertilized female can lay up to 3000 eggs, usually in batches of 100200.
Adult flies can live for about 30 days.
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Pathogenesis
• after the eggs are deposited on the wool by the primary adult fly, the
larvae emerge and crawl down the wool on to the skin, which they
lacerate with their oral hooks, and secrete proteolytic enzymes which
digest and liquefy the tissues. Secondary blowflies are then attracted
by the odour of the decomposing tissues and their larvae extend and
deepen the lesion. The situation is often complicated by secondary
bacterial infection.
• The irritation and distress caused by the lesion is extremely
debilitating and sheep can rapidly lose condition. The latter is often
the first obvious sign of strike as the lesion occurs at the skin surface
and is sometimes observed only on close examination. Where death
occurs, it is often due to septicaemia.
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Control
• This has been based largely on the prophylactic treatment of
sheep with insecticides. Any insecticide used must not only kill
the larvae, but persist in the fleece. In this respect the
chlorinated hydrocarbon, dieldrin, proved particularly
effective and gave protection for at least 20 weeks. However
this product has been withdrawn on safety grounds and replaced
mainly by organophosphorous compounds and synthetic
pyrethroids which have much shorter periods of persistence.
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