This category is for sites devoted to the largest of all animal phyla, Phylum Arthropoda. This phylum includes the insects, spiders, mites and ticks, crustaceans, centipedes, millipedes, isopods, and trilobites.
The arthropods typically share a common body plan. They have an outer tough cuticle and they are divided into segments, each of which may carry jointed appendages. Often there is a head, thorax or trunk and an abdomen. They are an amazingly successful group of creatures and have been around since the Precambrian, 600 million years ago.
Arachnids are arthropods, comprising some 70,000 named species, and include spiders, scorpions, harvestmen, ticks and mites. Nearly all are terrestrial.
There is no separate head and the body is organized into two parts called the prosoma, covered by a carapace-like shield, and the opisthosoma, or abdomen, which may be segmented. There are 6 pair of appendages. The first pair, the chelicerae, serve in feeding and defense. The next pair, the pedipalps have been adapted for feeding, locomotion, and reproductive functions. The other 4 pairs serve as limbs.
Chilopods are commonly known as centipedes. Each trunk segment has one pair of legs. The first trunk appendages are modified as fanglike forcipules equipped with poison glands. Centipedes are nocturnal raptors.
Most millipedes are elongate, often worm-shaped, mandibulates, although some are short and resemble woodlice. The body consists of a head and a trunk of many segments, most of which bear paired appendages.
The Merostomata includes two distinct groups of marine organisms, the eurypterids (an extinct group that lived 200 to 500 million years ago) and the horseshoe crabs. The horseshoe crabs are also an ancient group; only 4 or 5 species (depending on who is counting) exist today.
The Flora_and_Fauna category is primarily for websites about living organisms, which means it is mostly about the biology of horseshoe crabs. Websites about eurypterids should be sent to Science/Earth_Sciences/Paleontology/Invertebrates.
Order of sea spiders that have adopted an underwater life, with spindly legs and a small, vestigial abdomen. Although they resemble true spiders in appearance there are considerable anatomical differences. They date back to the Devonian era and there are about 85 genera and 1000 living species.