These are the horseshoe crabs. From the Cambrian to the Permian they were numerous, but today only five species survive, but these are practically unchanged from those found in the Triassic. They are relatively large and grow up to 50 cm long. Females are larger than males. They have a thick horseshoe-shaped carapace covering the prosoma and hiding the legs, and a long tail/telson ending in a spine. The pedipalps and first three pairs of legs are chelate (end in pincers). They breathe through external gills and have a pair of dorsal compound eyes as well as a pair of simple eyes, they have photo-receptors in their tail spikes. The larvae resemble trilobites.

They usually inhabit shallow waters and feed at night on worms, mussels and other small molluscs. In the mating season they mate on the shore at high tide. The female digs a burrow in the sand and lays her eggs. The males approach the burrow and add their sperm to the eggs. More than one male may creep up behind the female to add his sperm. The eggs are then covered, and in this position are protected from the waves and warmed by the sun. The larvae hatch at high tide.

Limulus sp., above and left is found in shallow waters along the North American Atlantic coast. It is 50 cm long.

About 75 000 species have been described so far, but there are thought to be as many as one million mites awaiting discovery. And 1m³ of soil may contain more than 1 000 000 mites. Arachnids have 6 pairs of appendages - 4 pairs of legs, 1 pair of fangs (chelicerae) and 1 pair of palps. Most are predators and their chelicerae and pedipalps often have fangs (see the Tarantula with a bird on the left); they may also have poison glands and stings. For more details click on the links above.

Order Pseudoscorpiones

These are found amongst litter, on bark and even on the sea shore. They are just a few millimetres long, and they resemble scorpions except that they do not have a tail. They are found world wide, and there are over 2000 described species, and 26 in Britain. Some are eyeless. They have poison glands in their relatively huge pedipalps (claws). They feed on other small animals, e.g. collembolla and mites, poisoning their victim to kill it. Before each moult a silken nest is built for security and protection. A silken nest is also built in cold climates for overwintering. They are most often seen by us when we find them clamped on to the leg of another animal, such as a harvestman or a fly. What they are doing is hitching a ride to a new location. This is called phoresy.

Maturity can take 1 - 2 years, and their lifespan is 2 - 5 years. Courtship and mating is varied, complex and depends on the species. The female lays her eggs either in a silk nest where she remains, or she carries them around with her on her body.

Left and right are Izodes ricinus, the castor bean tick or sheep tick. In the photograph on the right the bars are in mm. The top tick had just been taken off my dog where it had been sucking her blood, and the bottom tick is either male or had not yet started to suck. This is one of the most common ticks in the UK, and will suck the blood of anything warm blooded. The mouthparts are a little like a harpoon with backward-curving teeth making it difficult to pull out once it has pierced the skin. Only the female has the hugely extendible skin enabling her to swell up with a large blood meal.

Lifecycle. The lifecycle takes 3 years. When the egg hatches the larva has just 6 legs. Next spring the young tick climbs up grass or other vegetation to wait for a passing host. The tick dries out quickly, so if a host doesn't pass soon it will drop to the moist ground before trying again. After feeding it drops to the ground where it moults into an 8 legged larva/nymph resembling the adult. Next spring it climbs vegetation and attaches to a host yet again for a blood meal. It feeds for several days, then again it drops to the ground and moults into an adult. Next spring it again climbs vegetation to suck blood. The female takes in a huge blood meal increasing her size greatly. Mating may take place on the host or after dropping to the ground. The males die after mating. The eggs are laid in the summer in crevices in the ground. Each egg is covered with a waterproof layer of wax. After the female lays eggs she, too dies.