🦋 Where Do Insects Fit In? Classification & Relationships in the Animal Kingdom
Welcome back to our series on Entomology! Today, we are zooming out to look at the "big picture." We know insects are everywhere, but biologically speaking, where exactly do they stand?
Based on the provided text, let's explore the Phylum Arthropoda, how it is classified, and how our six-legged friends compare to their cousins like crabs, spiders, and centipedes.
📍 Position of Insects in the Animal Kingdom
Before we dive into the details, let's pin down the exact address of insects in the biological classification system:
Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Uniramia
Class: Hexapoda (or Insecta)
Etymology Alert! 📝
Hexapoda: Hexa (six) + poda (legs)
Insecta: In (internal) + sect (cut) — referring to their segmented body.
🦀 Characterizing Phylum Arthropoda
Insects belong to the Phylum Arthropoda, a name derived from Arthro (joint) and poda (foot). Here are the defining characters of this group:
Segmented Body: The body is divided into segments.
Tagmosis: Segments are grouped into 2 or 3 distinct regions (e.g., Head, Thorax, Abdomen).
Exoskeleton: A renewable chitinous outer covering.
Moulting: They grow by shedding their exoskeleton.
Bilateral Symmetry: The left and right sides of the body are mirror images.
Haemocoel: The body cavity is filled with blood.
Digestive System: Tubular alimentary canal with a mouth and anus.
Circulatory System: Dorsal heart with ostia.
Nervous System: Dorsal brain with a ventral nerve cord.
Muscles: Striated muscles.
** appendages:** Paired and segmented.
Absence of Cilia.
📂 The 7 Classes of Arthropoda
The Phylum Arthropoda is divided into 7 distinct classes. Here is the lineup:
Onychophora (Claw bearing):
Example: Peripatus
Note: Considered a link between Annelida and Arthropoda.
Crustacea (Crusta - shell):
Example: Prawn, crab, wood louse.
Arachnida (Arachne - spider):
Example: Scorpion, spider, tick, mite.
Chilopoda (Chilo - lip; poda - appendage):
Example: Centipedes.
Diplopoda (Diplo - two; poda - appendage):
Example: Millipedes.
Trilobita:
Status: An extinct group.
Hexapoda or Insecta:
Example: Insects.
🆚 Relationship of Insects with Other Arthropods
How do Insects (Hexapoda) differ from their relatives? The table below highlights the key differences based on body structure, organs, and habits.
1. Onychophora (e.g., Peripatus)
Body: Not distinct regions.
Antenna: 1 pair.
Locomotion: Many pairs of unjointed legs.
Respiration: Tracheal / Cutaneous.
Special Feature: Acts as a connecting link between Annelida and Arthropoda.
2. Crustacea (e.g., Crab, Prawn)
Habitat: Mostly aquatic, few terrestrial.
Body: Two regions (Cephalothorax and abdomen).
Antenna: 2 pairs (Antennule and Antenna).
Locomotion: Minimum 5 pairs of biramous legs.
Respiration: Gill breathing / Cutaneous.
Special Feature: Calcium deposits strengthen the exoskeleton.
3. Arachnida (e.g., Spider, Scorpion)
Body: Two regions (Prosoma/Cephalothorax and Opisthosoma/Abdomen).
Antenna: No antenna.
Locomotion: 4 pairs of legs.
Mouthparts: Non-mandibulate (Chelicerae present).
Respiration: Book lungs (Scorpion) and Tracheal (Spiders).
4. Chilopoda (e.g., Centipedes)
Body: Two regions (Head and multisegmented trunk).
Antenna: 1 pair.
Locomotion: 1 pair of legs per segment (First pair modified as poison claws).
Habit: Carnivorous.
Special Feature: Opisthogenital (Gonopore present in the terminal segment).
5. Diplopoda (e.g., Millipedes)
Body: Two regions (Head and multisegmented trunk).
Antenna: 1 pair.
Locomotion: 2 pairs of legs per segment.
Habit: Herbivorous.
Special Feature: Progoneate (Gonopore in the 3rd segment).
6. Insecta / Hexapoda (e.g., Insects)
Body: Three distinct regions (Head, Thorax, and Abdomen).
Antenna: 1 pair.
Visual Organs: Both simple and compound eyes (1 pair).
Locomotion: 3 pairs of legs on the thorax and usually 2 pairs of wings.
Respiration: Tracheal system.
Development: Metamorphosis is present.
Special Feature: Genital structures on 8th and 9th abdominal segments.
🧠 Knowledge Check: MCQs
Ready to test your knowledge on Arthropod classification? Try these questions!
1. Which class of Arthropoda possesses two pairs of antennae (Antennule and Antenna)?
A. Arachnida
B. Crustacea
C. Chilopoda
D. Insecta
2. The term "Arthropoda" is derived from words meaning:
A. Internal cut
B. Claw bearing
C. Jointed foot
D. Six legs
3. Which class is considered a connecting link between Annelida and Arthropoda?
A. Onychophora
B. Trilobita
C. Diplopoda
D. Hexapoda
4. How many pairs of legs do Arachnids (e.g., spiders, scorpions) typically have?
A. 3 pairs
B. 4 pairs
C. 5 pairs
D. 1 pair per segment
5. In which class are the first pair of legs modified into poison claws?
A. Diplopoda
B. Crustacea
C. Chilopoda
D. Onychophora
6. Which of the following is an extinct group of Arthropoda?
A. Uniramia
B. Trilobita
C. Onychophora
D. Diplopoda
Answers:
B (Crustacea)
C (Jointed foot)
A (Onychophora)
B (4 pairs)
C (Chilopoda)
B (Trilobita)
