Arthropods are the most diverse and abundant organisms on Earth, and comprise an estimated 90% of animal species living today. The defining features of arthropods include possession of a hard, chitinous exoskeleton (part of a tri-layered cuticle), body segmentation, paired jointed appendages, specialisation of body regions into functional units ('tagma') and growth through periodic moulting of exoskeletal plates, triggered by cycles of ecdysteroid hormone. These features, and body tagmosis in particular, have brought extreme adaptability and ecological success, allowing arthropods to diversify into all available marine, freshwater and terrestrial habitats. Four major groups of living arthropods are recognised, namely the Chelicerata (spiders, mites, scorpions and horse-shoe crabs), Myriapoda (primarily centipedes and millipedes), Crustacea (crabs, shrimps, lobsters etc.) and Hexapoda (insects and their allies). Precise inter-relationships of these groups are contentious, but strong evidence suggests that sea-spiders (pycnogonids) are basal to chelicerates, and the remaining three groups form a derived clade of 'mandibulate' arthropods, with hexapods emerging from within the crustaceans.

Origins

The closest living relatives of arthropods include velvet worms (onychophorans) such as Peripatus and tardigrades ('water bears') both of which are known as fossils from the Cambrian onwards. Onychophora are predators that walk on segmental but un-jointed 'lobopods', whereas tardigrades are tiny creatures (typically 200-500 micrometres long) with a clearly segmented body and four pairs of legs with claws. Tardigrades inhabit moss, lichens, soil and sand grains in terrestrial and aquatic environments, eating fungi, algae, plant cells and minute animals (e.g. rotifers and nematodes). Some (e.g. Milnesium) are capable of forming dormant cysts, and in this state of 'cryptobiosis' can survive the most harsh extremes of temperature, dessication and pressure known for any animal. The features of these arthropod-like taxa, as well as enigmatic Cambrian forms such as Hallucigenia, Ayshaeia, Kerygmachella and Anomalocaris indicate that the common ancestor of the arthropods probably had fairly uniform trunk segments with lobopod-like, unbranched limbs and anterior sensory appendages.

True arthropods first appear as a diverse assemblage of marine animals in the Lower Cambrian (around 520 Ma ago). They included the lace-crab Marrella, various 'arachnomorph' taxa (e.g. the famous trilobites and horseshoe crab-like Aglaspis) and crustaceans. In the Silurian (443-417Ma), terrestrial arachnids, myriapods and crustaceans appear, while hexapods first appeared in the fossil record in the Early Devonian (around 400Ma, Rhynie Chert) with the collembolan Rhyniella praecursor and bristle-tail Leverhulmia mariae.

Among the chelicerates, myriapods and crustaceans we find numerous excellent examples of convergence, a few of which, ranging from mimicry to eusociality and bioluminescence are named below...

Chelicerates

Certain predatory spiders mimic their prey by resembling ants, and spider crabs (crustaceans) are masters of camouflage, fixing algae, sea urchins and other marine organisms to their exoskeleton. Spiders, spider mites and scorpions all produce silk, as do many insects (e.g. ants, wasps, flies, lepidopterans). Silk is used not only for webs but also a range of other functions, such as trip wires, escape lines, cocoons, egg covers and in scytodid spiders as an ingredient of toxic saliva. The spitting spiders' mode of hunting is similar to that of onychophorans, which squirt a venomous slime containing adhesive threads (collagen-like, rather than silken) at its prey. Silk threads have been independently recruited for 'ballooning' in various spiders, spider mites and moth larvae. A few spiders offer nuptial gifts (e.g. nursery web spider Pisaura and Paratrechalea) wrapped in silk, similar to the silken offerings made by male dance flies. Male nursery web spiders feign death if threatened by a female during courtship, and this 'thanatosis' behaviour has also appeared independently in insects (e.g. fire ants and Claviger testaceus beetles) as well as fish (e.g. cichlids) and mammals. Also notable in terms of convergence, adhesive pads (scopula) at the ends of spiders' legs are hairy, dry and remarkably similar in structure to the adhesive toe pads of geckos.

Myriapods

Myriapods teach us much about convergence, especially through their independent evolution of an insect-like tracheal system, allowing them to breathe effectively on land. Arthropods and other animals (e.g. early tetrapods, evolved from lobopodian fish) made the move from sea to land (terrestrialisation) as soon as the plants had colonised it in the Silurian, and myriapods, chelicerates and crustaceans did so independently. Several insect groups, most famously the butterflies and moths produce toxic defence compounds such as alkaloids, and interestingly, many millipedes and centipedes also secrete alkaloids, making them equally distasteful to predators.

Crustaceans

Crustaceans offer a rich window into convergence, most strikingly perhaps in the evolution of eusociality in alpheid shrimps. True eusociality has evolved independently elsewhere in several insect groups (e.g. ants, bees, termites) and among mammals, in the naked mole rats. Mantid shrimps are notable in having a complex compound eye and the capacity for colour vision, as well as raptorial limbs that are convergent with those of many insects - mantids and mantaspids being the most well known. Barnacles are intriguing crustaceans with shell plates in one taxon structured like the lamellar bone of vertebrates, while their vestigial abdomen is formed through the same genetic mechanism as abdomen-less mites and ticks. Fire-flies, fungus gnats and marine organisms including squid, fish and jellyfish emit bioluminescent signals, as do certain myodocopid ostracodes, that produce luminescent flashes to confuse deep sea predators. To keep their balance most crustaceans use statoliths (also found in cephalopods and cnidarians), but some crabs have a semi-circular canal system that is astonishingly similar to that of vertebrates. Finally, male fiddler crabs, with their huge dimorphic claw, display to females through lekking, a behaviour evolved independently in other animals, from birds to frogs, deer and even squid.