Four pairs of "walking" legs,
one pair of pedipalps (like feelers), one pair of chelicerae (claws or
A body divided into two main
regions; prosoma with the legs and head region, and the opisthosoma with the
abdomen plus non-locomotory appendages
A ventral mouth, straight gut
The juvenile stages tend to be
similar to adult forms
Gaseous exchange by book lungs
The blood system circulates
the respiratory gases
Fertilisation tends to be
external in marine species and internal in terrestrial
Greek: chele = talon; cerata = horns
There are almost 100 000 species
described so far, and it is believed there may be as many as one million species. The Chelicerata are placed in three separate classes.
Merostomata - horseshoe crabs
Horseshoe crabs are also known as Xiphosura. From the Cambrian to the Permian they were numerous, but today only five species survive, and these are practically unchanged from those found in the Triassic.
Horseshoe crabs 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.
In the mating season horseshoe crabs 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 98, 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.
These are found amongst litter, on bark and even on the sea shore. Most are just a few millimetres long; with a size range of 0.7 - 12.0 mm, and they resemble scorpions except that they do not have a tail.
They are found world wide, and there are over 3380 described species, and 26 in Britain. Some have 4 eyes, some 2 and some are eyeless. The eyes are simple, and are able to detect light and dark only. 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.