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For educational purposes:--

Information on the basics of Entomology

 

Introduction                                                                                                                                                                                                                                                                           Contents

 

An Introduction To The Study of Entomology 1

Kingdom:  Animalia, Phylum: Arthropoda

Subphylum: Hexapoda: Class: Insecta: Order: Odonata

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Insecta:  Pteragota:  Hemimetabola

  Order:  Odonata (74 Families)

    General Summary

    Fossil Record

    Body Form

    Reproduction

      Mating

      Oviposition

    Sample Examinations

    References      Citations

 

General Summary of Odonata

 

          Odonata --  <Adults> & <Juveniles> -- are predacious insects with biting mouthparts.  They have two similar pairs of wings with characteristic reticulate venation; prominent eyes and small antennae.  The abdomen is elongated with accessory male genitalia on the 2nd and 3rd sterna.  The metamorphosis is Hemimetabolous, the naiads are aquatic and they have a modified labium known as the mask.

 

 

          There are two suborders, the Anisoptera, where the naiads are without external gills but rather have rectal gills, and Zygoptera, where there are three prominent external and terminal gills.

 

          Fossil Record. -- Specimens date back to over 250 million years.  Although present day forms have a wingspread of about 13 cm, extinct forms had a 3/4 meter wingspread. 

 

          Body Form. -- They are all large insects, and in the Carboniferous period genera existed which had a wing expanse of two feet. They are strong and rapid fliers, catching their food, in the form of small insects, on the wing. The forwardly directed legs play an important part in catching the prey and holding it while it is chewed.


          The thorax has an unusual obliquity of form, the pleural sclerites being directed downwards and forwards at each side so that the leg bases are carried forwards towards the mouth and the wing bases backwards.  The adults have a long slender abdomen, and there are prominent cerci on the posterior part of the abdomen.  The legs are modified for grasping, and all thoracic segments have been turned forward so that the legs are also directed forward.


          The wings are of the ancient insect type with a complex netted or reticulate venation, characteristic features being a cuticular thickening of the wing membrane near the apex, a nodus or prominent cross-vein at right angles to the first two longitudinal veins and a complex of veins near the wing base known as the triangle. They have a characteristic stigma on the front part of the wing and a nodus on the costal margin. There is no mechanism to bind the wings together and they operate independently. 

 

          All the mouth appendages are toothed, and the maxillae and labium augment the mandibles in chewing capacity unlike most insects with biting mouthparts. The compound eyes of Odonata are the largest of all insects.

 

Reproduction of Odonata

 

          Mating. -- The process is quite unique for insects.  Although the male pore is on segment 9 of the abdomen, the copulatory apparatus is found in the sternal region of segments 2 and 3.  Before copulation, spermatozoa are transferred to this structure.  The male then grasps the female in the region of the prothorax by means of his posterior abdominal claspers. While in flight in this tandem position the female turns her abdomen down and forwards and receives sperm from the accessory copulatory structure of the male.

 

          Oviposition. -- Dragonfly eggs are laid in water or on waterweeds. The naiads breathe with tracheal gills and are of two kinds: (I) those with external gills in the positions of cerci anales and caudal filaments and caudal filaments-Zygoptera, (2) those with gills on the walls of the rectum- Anisoptera. In the latter case water is pumped in and out through the anus, and this action may be made use of in locomotion-the sudden expulsion of water causing a rapid forward movement on the part of the naiad. However, the naiads on the whole slow-moving creatures, lurking well camouflaged among waterweeds while in wait for their prey. The main difference between the mouthparts of the naiad and imago concerns the labium. In the adult this has normal proportions, but in the naiad the post- mentum and prementum are elongated and capable of being shot out rapidly from the folded resting position, thereby impaling the prey, e.g., a tadpole, on the labial hooks.

 

Details of Insect Taxonomic Groups

 

          Examples of beneficial species occur in almost every insect order, and considerable information on morphology and habits has been assembled.  Therefore, the principal groups of insect parasitoids and predators provide details that refer to the entire class Insecta.  These details are available at <taxnames.htm>.

 

 

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References

 

Introduction                                                                                                                                                                                                                                                                           Contents