Arachnids
Arachnids are a class of joint-legged invertebrates in the subphylumChelicerata. They are
mainly terrestrial, but are also found in freshwater and in all marine environments, except
for the open ocean. They comprise over 100,000 named species, including spiders (Figure
12.25), scorpions (Figure12.26), daddy-long-legs, ticks, and mites (Figure12.27) and there
may be up to 600,000 species in total, including unknown ones.
Figure 12.25: A daddy-long-legs with a captured woodlouse. ( 24 )It is commonly understood that arachnids have four pairs of legs and that they may be
easily distinguished from insects on this basis (insects have three pairs of legs). Arachnids
also have two additional pairs of appendages, the first pair, thechelicerae, serve in feeding
and defense. The next pair, thepedipalps, are adapted for feeding, locomotion, and/or re-
productive functions. Arachnids are further distinguished by the fact they have no antennae
and no wings. Their body is organized into thecephalothorax, derived from the fusion of
the head and thorax, and the abdomen.
Arachnids are also well adapted for a terrestrial existence. They have internal respiratory
surfaces in the form of trachea or a book lung. They also have appendages modified for more
efficient locomotion on land, internal fertilization, special sensory organs, and structures for
water conservation, such as more efficient excretory structures and a waxy layer covering the
outer layer of the exoskeleton.
Arachnids are mostly carnivorous, feeding on the pre-digested bodies of insects and other
small animals. Several groups are largely venomous and they secrete venom from specialized
glands to kill prey or enemies. Several mites are parasitic and some of those are carriers of
disease. Arachnids usually lay eggs, which hatch into immature arachnids that resemble the
adults. Scorpions, however, bear live young.