Sometimes the head and thorax are fused together into a cephalothorax . Each of these body sections still bear the appendages that went with it, though these appendages are often highly modified. Arthropods are very highly cephalized, often with intricate mouthparts and elaborate sensory organs, including statocysts , antennae, simple eyes and compound eyes . Sensitive hairs on the surface of the body can detect touch, water currents, or chemicals. Their nervous systems are highly developed, with chains of ganglia serving various parts of the body, and three fused pairs of cerebral ganglia forming a brain.
Aquatic arthropods respire with gills .
Terrestrial forms rely on diffusion through tiny tubes called trachea . Trachea are cuticle-lined air ducts that branch throughout the body, and open in tiny holes called spiracles , located along the abdomen. Insects can open and close these spiracles, to conserve water that would otherwise be lost to evaporation from the open tubes. Their reliance on diffusion for respiration is one of the reasons that insects are small. Arthropods excrete by means of malphigian tubules , projections of the digestive tract that help conserve water.