Lecture 1 : Arthropods
January 12, 2026
Classification of Living Things
Classification is the hierarchical organization of living organisms into groups resembling a "family tree" that reflects their common ancestry and evolutionary relationships based on the following criteria:
Morphology: Study of the appearance and structure of organisms, both external and internal
Behaviour: Observations of how organisms interact and behave in their environments.
Life Cycles: The patterns of growth and reproduction of organisms.
Biochemical Reactions: Focuses on metabolism and the chemical processes occurring within organisms.
Genetics: Involves DNA sequencing to understand hereditary information.
Naming System: Phylum Arthropoda
Hierarchical classification (in order) includes:
Domain
Kingdom
Phylum
Class
Order: Groups with the greatest number of organisms, more broad
Family
Genus
Species: Groups with the least number of organisms, most specific
What is an Arthropod?
Arthropods belong to the phylum Arthropoda, which encompasses a vast diversity of animal life. Key categories include:
Arachnida
Diplopoda
Chilopoda
Entognatha
Malacostraca
Insecta
All arthropods share common characteristics:
Jointed Legs
Adaptation for movement.
Segmented Body
Body divided into segments, allowing for specialization of body parts.
Exoskeleton
Composed of chitin, providing structural support and protection.
Open Circulatory System:
A system where organs are bathed in a fluid called haemolymph instead of blood.
Variable Features across arthropods:
Number of legs can differ.
Presence and number of antennae may vary.
Number of body parts or segments is not fixed.
Types of mouth parts vary.
Identifying Key Classes of Arthropods
Essential classes to know include 6 specific ones:
Arachnida: Includes spiders, scorpions, ticks, and mites.
Diplopoda: Millipedes.
Chilopoda: Centipedes.
Entognatha: Springtails.
Malacostraca: Includes lobsters and shrimp.
Insecta: Other insects such as ants, grasshoppers, bees, etc.
Class Arachnida - Overview
Includes members such as spiders, scorpions, ticks, and mites.
Key Characteristics:
4 pairs of jointed legs attached to the cephalothorax (fused head and thorax).
1 pair of pedipalps, serving various functions.
Two distinct body parts: cephalothorax and abdomen.
Absence of antennae.
Predominantly predatory life style.
Subclass Acari: Mites and Ticks
Characteristics of Mites:
Body typically oval with minimal delineation of regions.
4 pairs of jointed legs attached to the cephalothorax.
1 pair of pedipalps.
Possess mouthparts for piercing/sucking capability.
Lack antennae.
Some species are predatory, others are plant pests or crucial soil organisms.
Notable pests include spider mites, gall mites, and rust mites.
Myriapoda (Subphylum)
Consists of four classes:
Chilopoda: Centipedes.
Diplopoda: Millipedes.
Pauropoda: Small, soft-bodied myriapods.
Symphyla: Small soil-dwelling myriapods.
Class Diplopoda - Millipedes
Greek Terminology:
Diplous: Double
Podos: Foot
Latin Naming:
Millipede: Derived from Milli (thousand) and Ped (foot).
Characteristics of Millipedes:
Number of abdominal segments ranges from 11 to over 300.
Each segment typically hosts two pairs of legs (4 per segment).
Possess a singular pair of short antennae.
May roll into a ball when disturbed.
Generally slow-moving, prefer dark environments.
Function as detritivores, consuming decaying organic matter.
Class Chilopoda - Centipedes
Greek Terminology:
Kheilos: Lip
Podos: Foot
Latin Naming:
Centipede: Derived from Centi (hundred) and Ped (foot).
Characteristics of Centipedes:
Body segment count ranges from 15 to 200.
Each segment contains one pair of legs (2 legs total per segment).
Have one pair of long antennae.
Possess cerci on the last abdominal segment.
Venomous claws for predation on invertebrates and small vertebrates.
Thrive in damp, dark environments.
Class Entognatha - Springtails (Order Collembola)
Greek Terminology:
Entos: Within, inner
Gnathos: Jaw
Characteristics of Springtails:
Composed of usually 6 body segments and 3 pairs of legs.
Internal mouthparts designed for specialized feeding.
Frequently found in moist environments like soil.
Possess a Furcula (jumping device).
A pair of antennae present.
Tubular structure called collophore situated on the underside of the abdomen for water absorption and waste excretion.
Primarily fungivores, but can consume small invertebrates and become crop pests.
Class Crustacea (Subphylum)
Encompasses a variety of classes, including:
Malacostraca
Branchiopoda
Thecostraca
Copepoda
Ostracoda
Class Malacostraca - Woodlice (Order Isopoda)
Other names include pill bugs, sow bugs, potato bugs.
Greek Terminology:
Malakós: Soft
Óstrakon: Shell
Iso: Equal, identical
Podos: Foot
Characteristics of Isopods:
Compressed body structure; thoracic and abdominal segments appear fused.
Two pairs of antennae (the first pair often reduced).
Typically possess 7 pairs of legs.
Commonly found in moist habitats.
They can act as pests but primarily function as detritivores.
Class Insecta - Insects
Insects represent the most diverse animal group within the phylum.
Insect Characteristics
Recognizable anatomical features include:
Three Body Parts:
Head: Contains sensory organs and mouthparts.
Thorax: Comprises three pairs of legs and typically bears wings.
Abdomen: Houses reproductive and digestive organs.
One Pair of Antennae: Important sensory appendages.
Three Pairs of Legs: Essential for locomotion.
Wings: Many insects possess wings; if present, usually characterized by two pairs, but wing absence is also common among certain insects.
Section 1: Identification Quiz
Which class possesses 2 pairs of antennae (though the first pair is often reduced) and typically exhibits 7 pairs of legs?
Answer: Class Malacostraca (e.g., Woodlice/Isopods).
How do you distinguish between Class Chilopoda and Class Diplopoda based on their legs?
Answer: Chilopoda (centipedes) have 1 pair of legs per body segment (2 legs total), while Diplopoda (millipedes) have 2 pairs of legs per abdominal segment (4 legs total).
Which class is characterized by having 0 antennae and 4 pairs of legs attached to a cephalothorax?
Answer: Class Arachnida.
Identify the class that features internal mouthparts, 6 body segments, and a specialized jumping organ called a furcula.
Answer: Class Entognatha (Springtails).
What are the three specific body regions found in Class Insecta, and how many pairs of legs do they possess?
Answer: The three regions are the head, thorax, and abdomen; they possess 3 pairs of legs (6 legs total).
Which class includes organisms that often roll into a ball when disturbed and function primarily as detritivores with short antennae?
Answer: Class Diplopoda.
Section 2: Quick Reference Differentiation Table
Class | Body Regions | Pairs of Antennae | Pairs of Legs | Unique Features |
|---|---|---|---|---|
Arachnida | 2 (Cephalothorax, Abdomen) | 0 | 4 | Pedipalps; predatory. |
Malacostraca | Compressed/Fused | 2 | 7 | Moist habitats; isopods. |
Chilopoda | 15 to 200 segments | 1 (long) | 1 per segment | Venomous claws; flat body. |
Diplopoda | 11 to 300+ segments | 1 (short) | 2 per segment | Round body; rolls into ball. |
Entognatha | 6 segments | 1 | 3 | Furcula; internal mouthparts. |
Insecta | 3 (Head, Thorax, Abdomen) | 1 | 3 | Often have wings (usually 2 pairs). |
Section 3: Key Terminology for Differentiation
Cephalothorax: A fused head and thorax region found in Arachnids.
Furcula: The fork-like jumping appendage unique to Class Entognatha.
Segments: The repeating units of an arthropod body; count is vital for Myriapoda (Chilopoda and Diplopoda).
Detritivore: An organism (like millipedes) that feeds on dead organic material, as opposed to predators (like centipedes or spiders).