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Metamorphic Patterns in Insects

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METAMORPHIC PATTERNS IN INSECTS Metamorphosis can be defined as “a rapid and complete transformation from an immature larval life to a sexually adult form involving morphology, function and habitat changes”. Ecdysis or moulting is the periodic shed- ding off the old exoskeleton. The duration of the period between two successive moults of a developing insect is called stadium. The form of the developing insect between two moults is called instar. Types of Metamorphosis: On the basis of degree of changes there are 5 basic types of metamorphosis seen in insects. 1. Ametabolous development or Ametamorphic 2. Metabolous development or Metamorphic 1) Ametabolous Development or Direct De- velopment: Ametabolous type of development occurs when the insects undergo little or no metamorphosis. Here the young ones called nymph emerges from the eggs and resembles the adults in all respects except in size and sexual structures. It grows only in size by replacing its old skin through a process, called moulting. In nymph the reproductive organs are undeveloped, and after several moults the nymph becomes an adult. This type of development is seen in apterygotan (wingless) insects. Eg. Lepisma and spring tails (Collembola). 2) Metabolous Development or Indirect Development: This is a group of insects where metamorphosis occurs. Metamorphosis may be simple or complex; based on this, metabola is divided into 4 types as follows, i. Gradual Metamorphosis or Paurometabolous Development: Here the newly young which comes out of egg closely resembles the adult in general body form, habits and habitat. The young’s are called nymphs. Nymph is different from adult in many features, i.e., wings and reproductive organs are undeveloped. 1 The wings develop as wing pads in the second and third thoracic segments at early age and gradually increase in size in each moult. The external genitalia develop gradually at each moult. These nymphs lead an independent life and attain adult form through several moults. This type of metamorphosis is called gradual metamorphosis t because the young undergoes slow but steady change in each moult and attains the adult form. There is no pupal stage. Eg. cockroaches, grasshoppers, mantis and white ants. Sometimes the gradual metamorphosis or paurometabolous development is included under hemimetabolous development. Egg → Nymph → Imago (Adult) ii. Incomplete Metamorphosis or Hemimetabolous Development: In this group, the metamorphosis is partial. Different stages of the life cycle resemble to paurometabolous development except the nymphs are called naiads which are aquatic and respire by external gills but the adults are terrestrial. When these nymphs are ready to be adult they come out of water and adult winged forms are released. The wings and genitalia develop externally but are not fully formed until adulthood Eg: dragonflies, mayflies and damselflies. Egg → Larva → Adult iii. Complete Metamorphosis or Holometabolous Development: In this group metamorphosis is complete. Egg hatches into a larva. The larva transformed into pupa. The pupa is the third stage in the life of holometabolous insects and usually immobile and often remain within a protective covering from predators, called cocoon.The pupa is converted in to adult.Thus there are four stages in the life cycle. Eg: 2 beetles, caddis- flies, butterflies, moths, mosquitoes, flies, bees and wasps Egg → Larva → Pupa → Imago (adult) iv. Hypermetamorphosis or Hypermetabolous Development Here the metamorphosis is complete. It is a kind of metamorphosis in which there are two or three distinct types of larval instars with different habits and structures found in certain insects. This type of metamorphosis is seen in blister beetles 3 4 NEURO ENDOCRINE REGULATION OF METAMORPHOSIS Ecdysis or moulting is the periodic shed- ding off the old exoskeleton. The duration of the period between two successive moults of a developing insect is called stadium. The form of the developing insect between two moults is called instar, It has been well established that the moulting and metamorphosis in insects are controlled by hormones (Fig. 18.141). The secretions of three organs are related to this process. (i) The brain (protocerebrum) (ii) The prothoracic gland and (iii) The corpora allata i) The Brain (protocerebrum): In the brain there are four groups of neurosecretory cells. Of these two groups lie on the midline and another two group’s lie on the sides-one group on each side. The neurosecretory cells secrete a kind of protein hormone, called prothoracotropic (PTTH) or brain hormone that activates the prothoracic glands which in turn produce moulting hormone. The protocerebrum sends the neurosecretory axons to the corpora cardiaca, a pair of small glands which lie posterior to the brain. Prothoracotropic hormone passes to the corpora cardiaca along the axons where it is released to the blood. (ii) Prothoracic Glands (PG): There are a pair of glands located in the prothoracic region. These are also called moult gland or ecdysial gland. Each gland appears V-shaped and is a mass of glandular tissue of non- nervous origin. It produces a hormone, known as ecdysone. 5 It is steroid in nature. Butenandt and Karlson (1951) first isolated from the pupa of silkworm. The hormone stimulates growth and initiates the process of moulting and shedding the old cuticle of the larva and the new cuticle is formed beneath the old cuticle. (iii) Corpora Allata: They are paired non- nervous secretory cells and situated behind the brain posterior to corpora cardiaca. The corpora allata secrete another fat soluble hormone, called juvenile hormone (JH). Chemically it is a steroid in nature. The juvenile hormone keeps the larval cells active and also controls the qualitative changes in the body during metamorphosis. As long as the juvenile hormone is secreted from the corpora allata, the pupa and imago (adult) stages are not developed. After a certain period the production of ecdysone instructs to stop the flow of juvenile hormone on one hand and on the other hand triggers the imaginal buds to be active. The absence of juvenile hormone causes the death of larval cells and they are used as nutrients for the growing imaginal buds. If the amount of juvenile hormone (JH) becomes lower in the blood, the moulting from larva to pupa takes place and absence of juvenile hormone in the blood, there occurs from pupa to adult moult. So it has been determined that the process of moulting is under hormonal control. 6 CONTROL OF INSECT PESTS 1. Chemical Control The most common method of pest control is the use of pesticides. Pesticides are chemicals synthetic chemicals that either kill the pest or inhibit their development. Pesticides are classified according to the pests the used to control Fungicides- Used to control fungal infections Insecticides- Used to control insect pests Herbicides- Used to control weeds Rodenticide- Used to control rodents (rat, mice, squirrels) Insecticides: Are chemicals used to kill insect pests. Insecticides are of three types 1. Stomach poisons: Kill the insects when the chemicals are swallowed. They are applied on the insect’s food. Eg: Lead arsenate, Calcium arsenate. 2. Contact Poisons: They penetrate the insect’s body through the cuticle or spiracle and cause death. They are sprayed on the infected plant part. Eg: DDT, BHC. 3. Fumigants: Are poisonous gases. They enter the body through spiracles and kill insect. They are used in closed rooms. Eg: Aluminium phosphide, Naphthalene Advantages of Chemical control 1. They are fast acting and effective 2. Most of them are broad-spectrum chemicals that can kill different species of insects 3. They can kill insect pests of all life stages (egg, larva and adult) 4. It is readily available in the market, easy to use and have more shelf life compared to other pest control methods like botanical and microbial pest control methods. Disadvantages of Chemical control 1. They causes bioaccumulation (accumulate in side biological tissue ) and biomagnifications 2. Affects human beings health 3. Insects develop insecticide resistance to several chemical insecticides 4. Kills useful insects like pollinators and natural enemies. 2. Biological control Definition: The action of biocontrol agents (natural enemies) against the target species to keep the pest below economic threshold level (ETL). Biological control includes reduction of pest population by natural enemies along with active human role. It is the control of pests and parasites by the use of other organisms, In other words, it is a practice in which an organism is used against another organism.e.g., of mosquitoes by fishes which feed on their larvae. Under this practice, there are four types of pest control: (i) Classical biological control or importation, in which a natural enemy from another geographical area, often the area in which the pest originated from, is introduced to contain the pest below the economic injury level, EIL. (ii) Inoculation, in which the periodic release of a control agent is required so that it is available throughout the year. (iii) Augmentation, which involves the release of an indigenous natural enemy in order to supplement an existing population, and is therefore carried out repeatedly usually to coincide with a period of rapid growth of pest population iv) Inundation, which s the release of large numbers of natural enemy, with the aim of killing those pests present at the time. The natural enemies of insects pests include 1. Predators: They are free living species which consume a large number of prey during their life time. Example, a small predaceous ladybird beetle, Rodolia cardinalis, commonly called vedalia, has been used to control the cottony- cushion scale insect (Icerya purchasi), a pest of citrus trees. 2. Parasitoids: are species whose immature stages develop on or within the body of the host insect-Egg parasitoids, larval parasitoids and adult parasitoids based on which stage of the host is attacked by that particular parasitoid Eg: Trichogramma .Apanteles 3.
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  • The Biodiversity and Systematics of the Entomophagous Parasitoid Strepsiptera (Insecta)

    The Biodiversity and Systematics of the Entomophagous Parasitoid Strepsiptera (Insecta)

    The Biodiversity and Systematics of the entomophagous parasitoid Strepsiptera (Insecta) Jeyaraney Kathirithamby, Department of Zoology and St Hugh’s College, Oxford. [email protected] [email protected] ABSTRACT Strepsiptera are small group of entomophagous parasiroids of cosmopolitan in distribution. They parasitize seven orders of Insecta and the common hosts in Europe are Hymenoptera, Hemiptera and Thysanura. INTRODUCTION Strepsiptera are obligate endoparasites the hosts of which include Blattodea, Diptera, Hemiptera, Hymenoptera, Mantodea, Orthoptera, and Thysanura, and 33 families. The name of the group is derived form the Greek words: twisted ( Strepsi-) and wing (pteron ), and refers in particular to the twisted hind wing of the male while in flight. Representatives of the suborder Mengenillidia show more primitive characteristics and parasitise Thysanura (Lepismatidae), the only known apterygote to be parasitized. Strepsiptera are cosmopolitan in distribution and are difficult to find: often the host has to be located to find the strepsipteran. To date about 600 species have been described, but many more await description and some could be cryptic species. The group is relatively well known in Europe (Kinzelbach, 1971, 1978), where details of Strepsiptera life history have been studied in Elenchus tenuicornis Kirby (Baumert, 1958, 1959), a parasite of Delphacidae (Homoptera) and in Xenos vesparum (Christ) (Hughes et al ., 2003, 2004a, 2004b, 2005), a parasite of polistine paper wasps (Hymenoptera: Vespidae). While most strepsipterans parasitize single taxa (leafhoppers or halictid bees), the males and females in the family Myrmecolacidae parasitize hosts belonging to different orders: (Formicidae and Orthoptera, respectively) (Ogloblin, 1939, Kathirithamby and Hamilton, 1992).