The phylum Arthropoda contains almost a million species of free-living or parasitic animals and includes crustaceans, spiders and insects, as well as many smaller groups. Humans and arthropods have been living and evolving together for all of our history. According to one study, household pests frequently found in homes are German cockroaches, subterranean termites (28% of houses), and fleas (10% of houses). Larger cockroaches, such as smoky brown and American cockroaches. However, the American cockroach (which is the only of the two considered a true pest) is only recovered from some homes. Some pest species are less common than other such as pillbugs (78%) and springtails (78%).4
The arthropod's exoskeleton, which is jointed to allow movement, is hardened and does not grow with the animal. As a result, it must be periodically shed (a process known as ecdysis) in order to allow growth. The arthropod body is usually divided into head, thorax and abdomen although in some groups there may be no clear distinction between the three regions. The head and thorax may be fused, forming a cephalothorax, or the abdomen may be reduced in size. The head often carries feeding and sensory appendages. Each segment of he body usually has a pair of jointed appendages, which are modified for specific functions in different species.
Arthropods have a body cavity (coelom), but it is small and contains only gonads and excretory organs. The other internal cavities form a hemocoele (blood cavity). The circulatory system is open, with a dorsally situated heart. The digestive system consists of a gut that runs from the mouth to the anus. The foregut and hindgut are lined with chitin and are shed at ecdysis. The nervous system has a dorsally positioned brain and a ventral nerve cord which has branches called ganglia. Tactile bristles are also a common feature. Eyes, which may be simple or compound are unique to arthropods and allow the formation of a highly-defined image.
The animals in the class Chilopoda are commonly known as centipedes and those in class Diplopoda as millipedes. Their body is also segmented, but is usually cylindrical. Millipedes and centipedes are found mainly in damp conditions such as rotting logs or in leaf litter, because they do not possess a waxy cuticle with which to reduce water loss. Both groups have separate sexes, and the female lays eggs which are fertilized by the male. Some species lay in the eggs in a "nest" where they are guarded by the female, but others, such as the centipede Lithobius, lay one egg at a time and then leave it.
Centipedes have one pair of antennae on the head, a pair of mandibles and two pairs of maxillae, in contrasts to millipedes which have at least the front pair of maxillae modified to form a lower lip. Centipedes have eyes that may be simple ocelli (a cluster of photoreceptors) or modified compound eyes. Centipedes are flattened and divided into a large number of segments, each of which, apart from the first, carries a pair of long, slender, walking legs. Millipedes are vegetarian scavengers.
Each segment of a centipede's body possesses a single pair of legs. Emanating from the head is a pair of sensorial antennae, and its three pairs of mouthparts, consisting of paired mandibulae with teeth, maxillae, and palps. Located behind its head, this arthropod's first pair of legs has evolved into a pair of fangs. Although a rare occurrence, when it does bite its victims, the centipede ejects its venom, which is not generally toxic, although it may cause extreme pain to humans.
Despite their name, horseshoe crabs (subclass Xiphosura) are not crabs at all, but primitive marine arthropods. They are the only living group of the class Merostomata, of which only five species remain. The body is divided into three main regions: the horseshoe-shaped prosoma (fused head and thorax), covered by a thick shell, or carapace, and ending in a long, pointed tail called a telson. On the ventral surface of the prosoma is a mouth with a pair of pincerlike feeding appendages called chelicerae, and five pairs of walking legs. The nervous system is well-developed, but the eyes are not; they can detect movement, although they cannot form an image.
The class Pycnogonids contains about 600 species of carnivorous marine animals known as sea spiders. They feed on corals and sponges, and are found in all marine waters. The similarity of these animals to true spiders is only superficial. Pycnogonids have a segmented abdomen and legs which end in claws, whereas spiders do not have these chracteristics.