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Metamorphosis in Insects

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Metamorphosis 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”. 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) Gradual metamorphosis or Paurometabolous development (3) Incomplete metamorphosis or Hemimetabolous development (4) Complete metamorphosis or Holometabolous development and (5) Hypermetamorphosis or Hypermetabolous development. (1) Ametabolous Development or Direct Development: The insects undergo little or no metamorphosis. The young ones emerged from the eggs resemble 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. The young which emerges from the egg resembles a miniature adult, called juveniles. In these the reproductive organs are undeveloped, and after several moults they become adult. This type of development is seen in apterygote (wingless) insects e.g. Silver fish (Lepisma), Spring tails (Order- Collembola, etc.). Fig. 1. Ametabolous development in Silverfish 2. Gradual Metamorphosis or Paurometabolous Development: This type of metamorphosis is seen in less primitive forms like cockroaches, grasshoppers, Bugs, Preying mantis, white ants, etc. The newly hatched young ones which comes out of the egg closely resembles the adult in general body form, habits and habitat but many adult features, i.e., wings and reproductive organs are undeveloped and their relative proportions of the body also differ. The young ones are called nymphs. The wings develop as wing pads in the second and third thoracic segments at early age and gradually increase in size by mitosis 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 or paurometabolous development because the young undergoes slow but steady change in each moult, the genitalia development is gradual. Life stages include, egg, nymphs and adults. Fig. 2. Paurometabolous development in Grasshopper In the life cycle of these insects there are three stages, e.g., egg → nymph → imago (adult). There is no pupal stage. 3. Incomplete Metamorphosis or Hemimetabolous Development: These insects also have three stages in their life namely, egg, young one and adult. e.g., dragonflies, mayflies and damselflies, the young ones are called naiads which are aquatic and respire by ex- ternal gills but the adults are terrestrial. They are different from adult in both habit and habitat. When these naiads are ready to become 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. In the naiads of hemimetabolous insects there are 3 pairs of thoracic legs, a head with compound eyes, antennae and small abdomen with posterior tracheal gills. From the egg → nymph/naiad→ adult Fig. 3. Hemimetabolous development in Dragonfly 4. Complete Metamorphosis or Holometabolous Development: Complete metamorphosis or holometabolous development is a kind of rapid morphological change during post embryonic transformation in some forms of insects where larva has no similarity with the adult and there is always a pupal stage. Complete metamorphosis takes place in majority of insects, viz., beetles, caddis- flies, butterflies, moths, mosquitoes, flies, bees and wasps. Larvae of butterfly are called caterpillar. Larva differs greatly in form from adult. Compound eyes are absent in larva instead stemmata are the visual organs, mouth parts and food habits differ from adults. When the larval growth is completed, it transforms into pupa. During the non-feeding pupal stage, larval tissues disintegrate and adult organs are built up. Fig. 4. Holometabolous development in Butterfly Fig. 5. Holometabolous development in Fly 5. Hypermetamorphosis or Hypermetabolous Development: 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 life cycle. There are more than five larval instars. This type of metamorphosis is seen in blister beetles. Note: A larva is a motile, immature feeding stage in arthropods which is morphologically different from the adult stage. The larvae of hemimetabolous insects are called nymphs. The adult stage of holometabolous insects is called imago. Ecdysis or moulting is the periodic shedding 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. Hormonal control of Metamorphosis The postembryonic process occurs in all insects, which involves moulting in the young ones, controlled by hormones of endocrine glands. The prothoracic glands are a pair of small glands in the first thoracic segment, they produce a hormone called ecdysone which brings about moulting and development of imaginal discs and reproductive organs. The neurosecretory cells in brain secrete brain hormone which reaches corpora cardiac, a neurohormonal organ. Through this. The brain hormone is passed into circulating haemolymph and exerts its influence on prothoracic glands (PTG) and corpora allata (CA). PTG when stimulated produces ecdysone which is a moulting hormone. CA is under the control of brain hormone and produces juvenile hormone (JH), which controls PTG and also decides what type of cuticle has to be laid, (whether next larval instar or pupal stage). If the quantity of JH is large at the time of moulting, larval cuticle is formed, if the quantity is less then pupal cuticle is formed, while its absence results in the formation of adult cuticle. Thus, the juvenile hormone of corpora allata controls growth and moulting up to the end of the larval period. So long as the juvenile hormone of corpora allata is produced the final moulting into a pupa or into an adult cannot take place. Fig. 6. Principle endocrine organs and their site of action Metamorphosis of Amphibians Metamorphosis may be defined as “a rapid differentiation of adult characters after a relatively prolonged period of slow or arrested differentiation in a larva”. According to Duellman and Trueb (1986) Metamorphosis can be defined as “a radical transformation from larval life to the adult stage involving structural, physiological, bio- chemical and behavioural changes”. "Amphibian" means dual (amphi-) life form (-bian) and refers to the typical life history of these animals, in which an aquatic larva metamorphoses into a terrestrial adult. The reptiles, such as turtles, lizards, and snakes, is another class of vertebrates whose species superficially resemble adult amphibians, but they do not undergo metamorphosis. Metamorphosis differs in many different amphibian species. In frog development, the eggs hatch and give rise to tadpoles, small aquatic larvae that have external gills and are mainly vegetarian. As the tadpole grows, internal gills and limbs form. Several significant changes occur during metamorphosis into the adult, including growth of a large mouth and tongue, loss of gills, formation of lungs, growth of the front legs, and resorption of the tail. Numerous biochemical changes accompany these morphological changes, such as synthesis of a new visual pigment in the eyes and a new oxygen-binding hemoglobin protein in the blood. The adult is mainly insectivorous and partly terrestrial. The sexually mature adults of some amphibians, such as the axolotl, have a larval morphology. The retention of larval or juvenile characteristics in adulthood is defined as neoteny. Neoteny is apparently caused by a genetic mechanism which uncouples development of body cells and the development of the sexual organs. Neoteny is most apparent in amphibians, because they are normally metamorphic. Metamorphic changes of amphibians: Etkin (1968) have divided three stages: a. Premetamorphic stage: The stage is characterized by the considerable growth and development of larval structures but metamorphosis does not occur. b. Prometamorphosis: The stage is characterised by the continuous growth specially the development of limbs and initiation of metamorphic changes. c. Metamorphic climax: The stage is characterised by the radical changes in the features of the larva, and climax is considered by the loss of most larval features. Structure of a freshly hatched tadpole larva: 1. A freshly hatched tadpole larva has a limbless body. 2. The body is divided into an ovoid head, a short trunk and a slender tail. 3. A small opening situated ventrally at the root of the tail is known as anus. 4. An adhesive sucker is present on the ventral side of the head by which the tadpole larva attaches itself to the aquatic 5. The mouth is lacking and as a result it cannot take anything from outside. 6. The yolk material provides the nutrition. 7. The respiratory organs comprise of three pairs of highly vascular and branched feathery external gills. 8. After a few days the mouth is formed near the sucker. 9. A pair of horny jaws surrounds the mouth. 10. The tail becomes more elongated and develops a dorsal and a ventral fin. 11. V-shaped myotomes develop on both the sides of the tail. 12. At this time this free-swimming tadpole larva ingests aquatic weeds, as a result of which the alimentary canal becomes extremely elongated. 13. To accommodate such a long alimentary canal inside the cavity of the short trunk, it becomes spirally coiled like the spring of a watch. Structure of an advanced tadpole larva: 1. In the advanced stage, the pharynx of the tadpole larva becomes perforated by gill- slits. 2. External gills disappear and the internal gills are formed between the gill slits. 3. The gills and the gill-slits are covered by the operculum (or gill-cover). 4. Thus the tadpole larva has three pairs of external gills at the start which are subsequently replaced by three pairs of internal gills.
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  • Arthropods of Public Health Significance in California

    Arthropods of Public Health Significance in California

    ARTHROPODS OF PUBLIC HEALTH SIGNIFICANCE IN CALIFORNIA California Department of Public Health Vector Control Technician Certification Training Manual Category C ARTHROPODS OF PUBLIC HEALTH SIGNIFICANCE IN CALIFORNIA Category C: Arthropods A Training Manual for Vector Control Technician’s Certification Examination Administered by the California Department of Health Services Edited by Richard P. Meyer, Ph.D. and Minoo B. Madon M V C A s s o c i a t i o n of C a l i f o r n i a MOSQUITO and VECTOR CONTROL ASSOCIATION of CALIFORNIA 660 J Street, Suite 480, Sacramento, CA 95814 Date of Publication - 2002 This is a publication of the MOSQUITO and VECTOR CONTROL ASSOCIATION of CALIFORNIA For other MVCAC publications or further informaiton, contact: MVCAC 660 J Street, Suite 480 Sacramento, CA 95814 Telephone: (916) 440-0826 Fax: (916) 442-4182 E-Mail: [email protected] Web Site: http://www.mvcac.org Copyright © MVCAC 2002. All rights reserved. ii Arthropods of Public Health Significance CONTENTS PREFACE ........................................................................................................................................ v DIRECTORY OF CONTRIBUTORS.............................................................................................. vii 1 EPIDEMIOLOGY OF VECTOR-BORNE DISEASES ..................................... Bruce F. Eldridge 1 2 FUNDAMENTALS OF ENTOMOLOGY.......................................................... Richard P. Meyer 11 3 COCKROACHES ...........................................................................................