What are arthropods? Are most arthropods harmful to humans?

Arthropods compose the largest number and most diverse group of animals on earth. They are invertebrates that arise from eggs and share these three anatomic features:
• Segmented bodies (head, thorax, abdomen)
• Hard outer exoskeleton derived from cutis
• Symmetrically paired and jointed appendages (legs, antennae, mouth parts)

Less than 0.5% of the 1 million named species are injurious to humans. Most are harmless, while others are even beneficial to humans.

Arthropods may be classified as follows:
• Arachnids (spiders, ticks, mites, scorpions): Medically important
• Insects (e.g., bees, lice, fleas, beetles, mosquitos, butterflies, moths): Medically important
• Millipedes
• Centipedes
• Crustaceans