🕷️ Classification of Phylum Arthropoda

Insects are only one class within a much larger arthropod lineage. To understand why insects are placed separately from spiders, crustaceans, centipedes, and millipedes, we first need the core characters of Arthropoda and then the comparative features of its major classes.

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Core Characters of Phylum Arthropoda

Insects belong to phylum Arthropoda. In older course language, insects are often placed under subphylum Uniramia.

The name Arthropoda comes from:

• arthro = joint
• poda = foot

Major characters of arthropods:

• segmented body
• segments grouped into two or three body regions, called tagmosis
• renewable chitinous exoskeleton
• growth by moulting
• bilateral symmetry
• body cavity largely represented by haemocoel
• complete alimentary canal with mouth and anus
• dorsal heart with ostia
• dorsal brain with ventral nerve cord
• striated muscles
• absence of cilia
• paired segmented appendages

Segmented appendages, chitinous exoskeleton, haemocoel, and moulting are the most important identifying features of arthropods.

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Major Classes of Arthropoda

The lesson groups Arthropoda into seven major classes:

1. Onychophora - claw-bearing forms such as Peripatus
2. Crustacea - prawn, crab, and wood louse
3. Arachnida - scorpion, spider, tick, and mite
4. Chilopoda - centipedes
5. Diplopoda - millipedes
6. Trilobita - an extinct group
7. Hexapoda or Insecta - insects

This classification is useful because insect characters become clearer when compared with the neighbouring arthropod groups.

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Position of Insects in the Animal Kingdom

Within Arthropoda, insects are distinguished by their body plan and appendages. The most familiar identifying characters are:

• body divided into head, thorax, and abdomen
• one pair of antennae
• three pairs of legs attached to the thorax
• usually two pairs of wings on meso- and metathorax
• mostly mandibulate mouthparts
• tracheal respiration

These features explain why insects are recognized as Hexapoda, meaning six-legged arthropods.

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Relationship of Insects with Other Arthropods

The comparative table below organizes the main differences among the major arthropod groups discussed in this lesson.

Character	Onychophora	Crustacea	Arachnida	Chilopoda	Diplopoda	Insects (Hexapoda)
Habit	Terrestrial	Mostly aquatic, few terrestrial	Terrestrial	Terrestrial	Terrestrial	Mostly terrestrial, few aquatic
Body regions	Not distinctly separated	Cephalothorax and abdomen	Usually two or three regions depending on group	Head and multisegmented trunk	Head and multisegmented trunk	Head, thorax, and abdomen
Antennae	One pair	Two pairs: antennule and antenna	Absent	One pair	One pair	One pair
Visual organs	Not distinct	Stalked compound eyes	Simple eyes	Simple eyes	Simple eyes	Compound eyes with or without simple eyes
Locomotory organs	Many pairs of unjointed legs	At least five pairs of biramous legs	Four pairs of legs	One pair of legs per segment; first pair modified as poison claws	Two pairs of legs per segment	Three pairs of thoracic legs; usually two pairs of wings
Mouthparts	Non-mandibulate	Mandibulate	Non-mandibulate, with chelicerae	Mandibulate	Mandibulate	Mandibulate
Respiration	Cutaneous	Gills	Book lungs or tracheae	Tracheal	Tracheal	Tracheal
Body fluid	Haemolymph	Haemolymph	Haemolymph	Haemolymph	Haemolymph	Haemolymph
Circulatory system	Heart with ostia	Heart with ostia	Heart with ostia	Heart with ostia	Heart with ostia	Heart with ostia
Development	Anamorphosis	Anamorphosis	Variable; absent in some, present in others	Metamorphic development	Metamorphic development	Metamorphosis
Feeding habit	Organic matter feeding	Herbivorous and carnivorous	Phytophagous and predatory forms present	Carnivorous	Herbivorous	Phytophagous, predatory, and parasitoid forms
Special note	Link between Annelida and Arthropoda	Calcification strengthens exoskeleton	Chelicerae and special body organization	Opisthogoneate reproductive opening	Progoneate reproductive opening	Genital opening associated with abdominal segments 8 and 9

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Summary Cheat Sheet

• Arthropoda means jointed appendages and includes animals with segmented body, chitinous exoskeleton, haemocoel, moulting, and paired appendages.
• The seven classes listed here are Onychophora, Crustacea, Arachnida, Chilopoda, Diplopoda, Trilobita, and Hexapoda/Insecta.
• Insects are separated from other arthropods by head-thorax-abdomen body division, one pair of antennae, three pairs of legs, and usually two pairs of wings.
• Crustacea usually have two pairs of antennae and are mainly aquatic.
• Arachnida lack antennae and possess four pairs of legs.
• Chilopoda have one pair of legs per segment, while Diplopoda have two pairs per segment.
• Insects respire mainly through a tracheal system and show great feeding diversity, including phytophagous, predatory, and parasitoid forms.

References

2 sources • [1] [2]

[1]

Fundamentals of Entomology

[2]

Insect Morphology and Systematics