Phylum Arthropoda: important evolutionary changes. Class Crustacea: general characterisation. Class Arachnida. Orders Scorpiones and Araneae

At a glance

1. Arthropods have an exoskeleton of chitin, which they moult to grow, jointed appendages specialised for different jobs, and segments fused into body parts, the tagmata. Their coelom is reduced, and blood fills an open body cavity, the haemocoel.
2. Crustaceans have two pairs of antennae and two-branched limbs, mostly breathe with gills and hatch as a nauplius larva. The copepod Cyclops is the intermediate host of the guinea worm and the fish tapeworm; freshwater crabs and crayfish carry the lung fluke.
3. Arachnids have two body parts, the prosoma and the opisthosoma, four pairs of legs, chelicerae and pedipalps, and no antennae or wings. They breathe with book lungs or tracheae.
4. Scorpions have pincers, a broad mesosoma and a tail of five segments ending in a sting. Most stings only hurt; the dangerous species, nearly all Buthidae of North Africa, the Middle East, India and the Americas, cause an autonomic storm through sodium channel toxins.
5. Latrodectus tredecimguttatus causes latrodectism: spreading pain, a rigid abdomen and sweating. Loxosceles causes loxoscelism: a wound in which the skin dies, rarely haemolysis.

01The arthropods: what changed

The Arthropoda are the largest phylum of animals, over 80 per cent of all known living species. Like the annelids they are segmented protostomes, and older books joined the two as the Articulata; DNA places the arthropods instead with the roundworms among the moulting animals, the Ecdysozoa, so their segments may have arisen separately. Set side by side, the two plans still show what a jointed outer skeleton changed.

What each brought:

• An exoskeleton. The cuticle, secreted by the epidermis, is a waxy outer layer over layers of chitin, a polymer of N-acetylglucosamine, and hardened proteins. It protects and keeps water in, and muscles attach to its inside. It cannot stretch, so the animal grows by moulting, ecdysis: it grows a new, soft cuticle under the old, sheds the old one and expands before the new one hardens. Each stage between moults is an instar.
• Jointed appendages. Each segment once carried a pair of limbs, and in every arthropod some pairs have become antennae, mouthparts, pincers, gills or legs.
• Tagmata. Segments are fused into body parts that do different work: a head, thorax and abdomen in an insect, a prosoma and opisthosoma in an arachnid.
• An open circulation. With jointed legs moved by muscle, the coelom is no longer needed as a skeleton and has shrunk. The blood, haemolymph, leaves the heart and bathes the organs in the haemocoel. Many crustaceans carry oxygen with haemocyanin, which holds copper.
• Breathing and excretion on land. Water animals breathe with gills; land arthropods with book lungs or tracheae, branching tubes that take air straight to the tissues. Many land arthropods excrete their nitrogen dry, insects as uric acid and arachnids mostly as guanine, through Malpighian tubules, saving water, while crustaceans lose ammonia through their gills.
• Eyes. Compound eyes, made of many units, and simple eyes, the ocelli. In spiders the main eyes are ocelli that can form images.
• Development. Some hatch as small copies of the adult and grow through their moults; others hatch as larvae, a crustacean's nauplius or an insect's caterpillar, and change their shape on the way to the adult, metamorphosis.

The groups

• Chelicerata: the arachnids, with the horseshoe crabs and sea spiders.
• Myriapoda: the centipedes and millipedes, about 13,000 species, all on land, with many similar segments.
• Crustacea: crabs, shrimps, crayfish, woodlice and copepods.
• Hexapoda: the insects, which arose from within the crustaceans.

02Class Crustacea

The crustaceans are about 67,000 described species, mainly aquatic, from tiny copepods to lobsters and the Japanese spider crab.

• The head and thorax are often fused into a cephalothorax under one shield, the carapace.
• They have two pairs of antennae, the first usually with one branch, the second with two.
• Their limbs are typically biramous, two-branched: a base carrying an inner and an outer branch.
• Larger crustaceans breathe through gills, which are modified parts of their limbs, and lose ammonia through them; small ones such as copepods have no gills and take in oxygen through the body surface. Organs that work as kidneys lie near the antennae.
• Many, copepods among them, hatch as a nauplius, a tiny larva that swims free, with three pairs of limbs, the two pairs of antennae and the mandibles, and a single eye in the middle of its head.

Cyclops

Cyclops is a genus of copepods of fresh water, with over 400 species, common along the weedy banks of ponds and slow streams. A copepod is typically 1 to 2 mm long, with a teardrop-shaped front body of the head and the first five thoracic segments, a slender hind body of the last thoracic segment and the abdomen ending in two caudal branches, and long first antennae. Cyclops has a single eye in the middle of its head, red or black, which gives it its name. After mating the female carries her eggs in two sacs on her hind body until they hatch as nauplii.

Its medical importance is as an intermediate host:

• Of the guinea worm, Dracunculus medinensis. Its larvae develop inside Cyclops, and people are infected by drinking water that holds the copepods. Straining water through a fine cloth removes them, and heating to 60 °C kills them.
• Of the fish tapeworm, Diphyllobothrium latum, as its first intermediate host: copepods swallow its larvae, fish eat the copepods, and people eat the fish raw.

Larger crustaceans matter too: freshwater crabs and crayfish are the second intermediate host of the lung fluke Paragonimus westermani, whose cysts lie in their muscles and infect people who eat them raw or pickled.

03Class Arachnida

The arachnids are over 110,000 named species. The figure shows their plan in two spiders and a scorpion.

• Two body parts: the prosoma, or cephalothorax, covered by a single carapace, and the opisthosoma, or abdomen.
• Six pairs of appendages, all on the prosoma: the chelicerae, the only appendages in front of the mouth, which serve as mouthparts; the pedipalps, for feeding, walking, sensing or mating; and four pairs of walking legs.
• No antennae and no wings, which tells them apart from insects at a glance.
• Breathing with book lungs, stacks of thin plates filled with blood with air between them, or with tracheae, or both.
• Excretion through Malpighian tubules and coxal glands, with guanine as the main waste, which saves water on land.
• No jaws: they digest their prey partly outside the body. A spider floods it with digestive enzymes and sucks up the liquid.

The orders in this course are the Scorpiones, the Araneae, and the Acari, the ticks and mites.

04Order Scorpiones

Scorpions are over 2,500 species, living mainly in deserts and preying mostly on insects.

• The prosoma carries a pair of eyes in the middle of the carapace and usually two to five pairs at its front corners; small chelicerae of three sections, used in feeding; pedipalps ending in large pincers, the chelae; and four pairs of legs.
• The opisthosoma has two parts. The mesosoma, broad, is seven segments, each covered by a hard plate on the back; underneath it carries a pair of comb-like sense organs, the pectines, and four pairs of book lungs. The metasoma, the tail, is five narrow segments and the telson, a bulb, the vesicle, with a pair of venom glands, ending in the curved sting, the aculeus.
• All scorpions give birth to live young, which ride on their mother's back until their exoskeletons harden.
• Their exoskeleton glows under ultraviolet light.

Scorpion stings

Most stings cause pain, tingling and some swelling, and healthy adults rarely need treatment. About 25 species, fewer than one in a hundred, have venom that can kill, and stings cause about 3,000 deaths a year worldwide, mostly where medical care is hard to reach. The dangerous species belong mostly to the family Buthidae; the deadliest live in North Africa and the Middle East (Androctonus, Leiurus, Buthus, Hottentotta), in Mexico (Centruroides), in India (Hottentotta tamulus, the Indian red scorpion, long called Mesobuthus tamulus) and in South America (Tityus).

Their neurotoxins hold the voltage-gated sodium channels of nerves open, prolonging their firing. The result is intense pain and an autonomic storm, and in severe cases pulmonary oedema, heart dysfunction, disordered clotting and failure of several organs. Buthus occitanus, the common yellow scorpion of southern France and Spain, gives a painful but mild sting; its North African relatives, long counted as the same species and now split into several, can kill. Treatment depends on the severity: relief of symptoms, support of vital functions and, for severe stings, antivenom. In India, prazosin, which blocks the α₁ receptors that the flood of noradrenaline drives, has cut the deaths from the red scorpion.

05Order Araneae

The spiders are the largest order of arachnids, with 53,680 species recorded in early 2026, almost all predators.

• The prosoma carries up to four pairs of eyes at the front of the carapace, in patterns that differ between families. All its ganglia are fused into one mass.
• The chelicerae end in fangs, through which most spiders inject venom from glands at their roots.
• The pedipalps are small and leg-like; a male's end in an enlarged section that transfers sperm.
• A narrow stalk, the pedicel, joins the prosoma to the soft, unsegmented opisthosoma, which holds the heart, the gut, the silk glands and the book lungs or tracheae.
• Spinnerets, appendages at the end of the abdomen, spin silk for webs, egg cases and lines.

Almost all spiders are venomous, but most bites cause only mild symptoms where they are given, and most need only pain relief and an antihistamine for itching. Spider bites are often wrongly blamed for other skin lesions. Two genera matter in Europe.

Latrodectus and latrodectism

Latrodectus tredecimguttatus, the Mediterranean or European black widow, lives in steppe and grassland round the Mediterranean and into central Asia, and accounts for most serious spider bites in Europe. The female, 7 to 15 mm long, is black with thirteen spots on the back of her abdomen, usually red but sometimes yellow or orange; the male, 4 to 7 mm, cannot pierce human skin. Bites happen mostly in summer, to men working in the fields.

The venom holds α‑latrotoxin, which acts on presynaptic nerve membranes and causes a massive release of acetylcholine, noradrenaline and GABA. This gives latrodectism:

• Intense pain at the bite 5 to 10 minutes later, then sweating and goosebumps round it within an hour.
• Pain that spreads up the limb to the trunk, with muscle cramps and rigidity. A rigid, board-like abdomen with nausea and vomiting can be mistaken for an acute abdomen.
• Heavy sweating and a fast pulse.
• The illness usually lasts three to six days. In Europe the heart is sometimes involved, with myocarditis, pulmonary oedema or arrhythmias, and these cause most of the rare deaths: in a review of about 500 European cases, about 1.3 per cent died.

Treatment is supportive, with analgesia and benzodiazepines. Antivenom, available in few European countries, was followed by quicker relief in European reports, but in trials elsewhere patients given a placebo also recovered quickly, so how much it helps is debated.

Loxosceles and loxoscelism

The recluse spiders, Loxosceles, are brown spiders 7 to 12 mm long with long, slender legs; L. rufescens, the Mediterranean recluse, is the European species. They are recognised by their eyes: six, in three pairs, with no eyes in the middle of the front row. Many have a dark violin-shaped mark on the carapace, but other spiders have similar marks, so it is not proof.

Their venom holds the enzyme sphingomyelinase D. Recluses are the only spiders proven to cause bites in which the skin dies, loxoscelism:

• Most bites are minor, with little or no necrosis.
• In the cutaneous form the skin round the bite darkens and dies, leaving a shallow ulcer that can take six to eight weeks to heal and leave a scar.
• Rarely, systemic loxoscelism breaks down red blood cells, which can need transfusion and injure the kidneys.

There is no proven specific treatment. Many necrotic wounds blamed on recluses are something else, most often a skin infection such as MRSA, or herpes zoster, a diabetic ulcer or pyoderma gangrenosum, so a firm diagnosis needs the spider to be caught and identified.

Check yourselfAnswer, then say how sure you were: a lucky guess still counts as a weak point.

1How does the body cavity of an arthropod differ from an earthworm's?

It is a true coelom divided by septa, as in the earthwormMost of it is a haemocoel, full of blood, and the coelom is reduced to small spaces round the gonads and excretory organsIt has no body cavity at allIt is a pseudocoelom, as in a roundworm

2Which set of features marks an arachnid?

Three body parts, three pairs of legs, one pair of antennaeTwo body parts, four pairs of legs, chelicerae and pedipalps, no antennaeTwo pairs of antennae and two-branched limbs, breathing with gillsMany segments alike, with one or two pairs of legs on each

3In a village where people drink from an open pond, guinea worm disease is common. Why does straining the water through a fine cloth prevent it?

It removes the eggs of the wormIt removes the copepods, Cyclops, which carry the infective larvaeIt removes the snails that release the larvaeIt kills the larvae by drying them

4A farmer in Greece has severe abdominal pain, a board-like abdomen and heavy sweating an hour after something bit him in a field. What does the black widow's venom do to cause this?

Its enzyme kills the skin and the muscle round the biteIts α‑latrotoxin makes nerve endings release their neurotransmitters in a floodIt blocks the acetylcholine receptors of muscle, as curare doesIt breaks down red blood cells

5Which spiders are the only ones proven to cause bites that kill the skin round them, and how are they recognised?

The black widow, by thirteen red spots on its abdomenThe recluses, Loxosceles, by six eyes in three pairs and often a dark violin on the carapaceThe wolf spider, by its large eyesThe house spider, by its long legs