This document includes a brief introduction to the phylum Arthropoda. The notes cover the subphylla of Crustacea, Chelicerata (focusing on spiders, spiders, and ticks) and Uniramia (myriapods). The notes include descriptions, characteristics, pictures, and species of medical significance.
Characteristics
Hard exoskeleton made from protein and chitin – flexible, lightweight and protective. It
prevents desiccation.
They undergo moulting – the process of shedding old exoskeleton when it becomes too
small
They have specialised appendages for locomotion, sensing and feeding
They have specialised body segments – namely head, thorax, abdomen, &
cephalothorax
They have special tubes called trachea which supply oxygen directly to body tissues.
This allows for a high metabolic rate and sustained activity, e.g. flight.
They have compound eyes which gives them a wide angle of vision. Many allow for
motion perception.
They undergo metamorphosis – larva focus on feeding and growth. Adults focus on
dispersion and reproduction.
Why are they so successful?
Hardened exoskeleton, jointed legs, fused and modified segments, specialised
respiratory and sensory structures, metamorphosis
Evolution of flight
Behavioural patterns such as mimicry
Subphylum Crustacea
There are roughly 52 000 species of crustaceans
Examples: Crabs, lobster, prawn, shrimp, crayfish, and barnacles.
Most are aquatic – both marine and freshwater.
Terrestrial examples:
o Crabs
o Terrestrial hermit crab
o Wood lice
Adaptations of Crustaceans
The body covered by a hard, shell-like calcified exoskeleton.
The body is elongated and divided into cephalothorax and abdomen.
The cephalothorax is covered by carapace.
The thorax bears 5 pairs of walking legs – first pair may be modified pincers/claws.
They have two pairs of antennae.
They have two pairs of maxillae, and one pair of mandibles.
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, Abdominal segments have modified, paddle-like appendages called swimmerets.
In males the first pair of swimmerets is used to transfer sperm to the female.
Last 2 segments are for the uropods and telson = fan like tail used for steering.
Barnacles
All barnacles are hermaphrodites.
Subphylum Chelicerata
Class Arachnida
Order Araneae
This order specifically refers to spiders
Spider Venom
Cytotoxic
o Tissue destroying – supresses cell function and causes cell death.
Haemotoxic
o Kills red blood cells and prevents the blood from clotting – results internal and
external bleeding.
Neurotoxic
o Agent/process destructive to nerve cells – kills the receptors.
New world tarantulas
Originate from the Americas.
Defence is primarily using urticating hairs
o These hairs can cause severe irritation and other symptoms in humans.
o They rub their legs vigorously releasing the hairs from the opisthosoma.
Their bite is generally harmless, may cause localised pain.
Old World tarantulas
Originate from Asia, Africa, Australia, and Europe
Defence is primarily biting and injecting venom.
Far less docile than new world – much more likely to bite.
Their venom and bite can be significantly dangerous to humans.
Sac spider – Cheiracanthium sp.
Genus of araneomorph spiders
Up to 10mm in length. Yellow-white in colour, with a bulbous abdomen.
They construct sac-like, silken tubes in foliage.
They are aggressive and will bite if threatened.
Several species thought to be responsible for 90% of dangerous spider bites in SA.
Bites induce an intense stinging pain, itchiness, swelling, and necrotic lesions.
Lesions heal in 6-8 weeks.
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