Insecta By Assist. Lecturer Maytham A. Alwan
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, depend ng on Salivary duct and feeding habits, with i adaptations for chewing biting, slashing spongingand 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 hereas the highly host specific larvae spend a considerable time feeding and w 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 yellow orange hairs with a broad band of black hairs around the middle.
Larvae: The mature larvae are thick and somewhat barrel shaped, apering anteriorly; when mature they t 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 (L 1), 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 L 2 occurs there. In February and March migration is resumed, and the L 2, arrive under the skin of the back where they moult to the L 3, which can be palpated as distinct swellings (warbles). A cutaneous perforation is made by the L 3 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. 3/17/2018 18
Pathogenic significance The L 3, 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 L 1, 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 L 2 crawl into the frontal sinuses where the final moult to L 3 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 L 1 and L 2 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 L 3 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. 3/17/2018 23
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 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 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. 3/17/2018 28
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 100 200. Adult flies can live for about 30 days. 3/17/2018 30
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 some times bserved only on close examination. Where death o occurs, it is often due to septicaemia. 3/17/2018 31
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. 3/17/2018 32
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