Considering huge diversity in number and form among various living beings (plants and Animals) on our planet , its rather impossible to study them in detail, unless divided into different categories based on the differences and similarities between them. As many of you are aware that classification of plants and animals is not something of recent times , evidence from the days of Aristotle is available for Classification. ( For more details on classification of animals refer to previous blog post on “Biological classification of Animals”
The scientific study dealing with identifying, naming and grouping of living organisms is called “classification” and the branch of biology that deal with classification is called taxonomy.
Today’s topic is on the classification of the most diverse group of animals on Earth , which are “INSECTS” . But to know about Insects , its important to take a brief look into Arthropods.
Arthropods are animals belonging to the Phylum Arthropoda ,which are identified based on jointed feet or legs. Another important aspect of arthropod body plan is the presence of segments . These organisms can be considered as real rulers of the planet in any sense, be in forms, number or geographical habitat. Arthropods are considered most successful animal forms,as members of this phyla are found in land, sea and air, making up over three-fourths of all currently known living and fossil organisms.
The Phylum Arthropoda is divided into five sub phyla :
1) Trilobites are a group of formerly numerous marine animals that got extinct in the Permian-Triassic extinction event.
2) Chelicerates which includes spiders, mites, scorpions and many other related organisms.
3) Myriapods includes millipedes and centipedes.
4) Crustaceans are primarily aquatic comprising of animals like lobsters, crabs, barnacles, crayfish, shrimp and many others.
5) Hexapods comprises of insects (only winged arthropods) and four small orders of insect-like wingless arthropods : Thysanura, Collembola, Protura, and Diplura.
Go anywhere or look for any environments on the planet , one is sure to find Insects. Belonging to the class Insecta , these organisms forms the biggest class of arthropods , most diverse at the equator and their diversity declines toward the poles. As there are so many species of Insects found in every corner of our planet , so there is even more need to properly classify them , for the possibility of their detailed study. Classification of Insects is rather complicated but here we take a look at simplified version ,which I think is good enough for basic understanding of Insects.
The Class Insecta in the sub-phylum hexapoda is again divided into two sub class:
1) Apterygota : consisting of primitive wingless insects.
Apterygota is further divided in Orders : Thysanura eg: Silver Fish , Collembola eg: Spring tails , Dpilura eg: Campoda Silver fish and Protura eg : Telson tails
2) Pterygota : includes winged Insects
Again subclass Pterygota is divided into two divisions:
Exopterygote and Endopterygote based on the type of metamorphosis observed in these insects.
Metamorphosis refers to the way that insects develop, grow, and change form during their development from egg to adult. Its a rather complex biological phenomenon related to change in physical appearance of animal after its birth. In Metamorphosis a conspicuous and relatively abrupt change in the animal’s form or structure through cell growth and differentiation is observed.
A normal life cycle of Insect involves different stages , starting with embryo getting transformed into larvae or Nymph then to pupal stage (seen only in some groups only ) and finally adult.
There are two main types of metamorphosis in insects: Hemimetabolism (Incomplete) and Holometabolism (complete).
So to come back to differences in Exopterygote and Endopterygote based on type of metamorphosis :
Exopterygotes ( Exo – Outside , Pterygion: little wings – Wings develop externally) undergoes incomplete metamorphosis involving three stages -Egg , Nymph and adult. After the eggs hatched into Nymphs ,which almost resemble the adult except in size. Most important thing to notice here is that there is no PUPAL STAGE in insects with simple or incomplete metamorphosis / Hemimetabolism. Nymphs usually don’t have wings and these tiny adults slowly grow by molting their exoskeleton. The insects stop molting when they reach their adult size and wings can also be seen by this time.
Examples of Exopterygotes : aphids, whitefly, cicadas, leafhoppers, grasshoppers ,crickets, etc.
The division Exopterygota is sub divided into 16 orders:
1. Order Ephemeroptera (May flies)
2. Order Odonata (Dragon flies and Damsel flies)
3. Order Orthoptera (Grasshoppers)
4. Order Dictyoptera (Cockroaches and Mantids)
5. Order Phasmida (Leaf insects and Stick insects)
6. Order Plecoptera (Stone flies)
7. Order Isoptera (Termites)
8. Order Zoraptera (Zorotypus)
9. Order Embioptera (Web-spinner)
10. Order Dermaptera (Earwigs)
11. Order Psocoptera (Book lice )
12. Order Mallophaga (Bird lice)
13. Order Anopleura (=Siphunculata) (Sucking lice)
14. Order Thysanoptera (Thrips)
15. Order Hemiptera (Bugs, Hoppers, Cicadas)
16. Order Grylloblattoidea (Grylloblatta)
Endopterygota : Endo – Inside , Pterygion: little wings – Wings develop externally ) includes insects which exhibit Holometabolism, also called complete metamorphism. In this kind of development Insects life cycle show fours stages in Egg , larvae, pupae and adult. When eggs get transformed into larvae and they don’t resemble the adult at all .They usually have a worm-like shape. After few molts larvae gets into pupal form ,where it stops eating and in this pupal case it gets transformed into adult shape with proper wings , legs and other structures. Most of these adult structure’s come from special tissue called imaginal discs . An imaginal disc is flat sheet of epithelial cells present in holometabolous insect larva that will get transformed into a structure of the adult insect during the pupal transformation. There are imaginal discs for Wings, legs, eye and other structures in larva of holometabolous insects.
Examples: Flies, Beetles, Butterflies, Ants, bees, sawflies, wasps, moths etc.
The division Endopterygota is sub divided into 9 orders:
1. Order Neuroptera (lacewings)
2. Order Coleoptera (Beetles)
3. Order Strepsiptera (stylopids)
4. Order Mecoptera (Scorpion flies)
5. Order Trichoptera (Caddis flies)
6. Order Lepidoptera (Butterflies and Moths)
7. Order Hymenoptera (Wasps, Bees, Ants)
8. Order Diptera (True flies)
9. Order Siphonaptera (Fleas)