Spiders and their Relatives


  • horseshoe crabs, ticks, scorpians
  • belong to the subphylum Chelicerata
  • Chelicerates are arthropods that are characterized by a two part body and mouth parts called chelicerae
  • lack sensory "feelers", found in the head
  • body devided into two parts, cephalothorax and abdomen
  • the anterior end of the cephalothorax contains the brains, eyes, mouth, mouth parts, and esophagus
  • posterior end contains front part of the digestive system and several pairs of walking legs
  • abdomen contains most of the internal organs
  • chelicerates have two pairs of appendages attached near the mouth that are adapted as mouth parts; the chelicerae and pedipalps
  • both mouth parts are made to serve different purposes in feeding different species

Horseshoe Crabs

  • oldest chelicerates - 430+ million years ago, ordovian period
  • up to 60 cm long
  • five pairs of walking legs, long spiked tail, heavily armor-plated


  • includes spiders, scorpians, ticks, mites
  • adults have four pairs of walking legs
  • pedipalps are adapted for capturing and holding prey
  • chelicerae is adapted for biting and sucking out their soft parts


  • usually feeds on insects
  • tropical spiders are capable of capturing small vertebrates such as a hummingbird
  • Capturing - ensnare their prey in webs, stalk and pounce, camouflage and hide underground and then leaps out to grab its prey
  • once captured, it uses its hollow fang like chelicerae to inject paralyzing venom into it
  • when the prey is paralyzed, the spiders mouth introduces enzymes into the wounds made by the chelicerae, which then breaks down the prey's tissues, making it easier for the spider to suck up the liquified tissues with its esophagus and a specialized pumping stomach
  • all spiders produce a strong flexible protein called silk
  • produced in special glands located in the abdomen (5x stronger than steel)
  • spins silk into webs, caccoons for eggs, wrappings for preys, and other structures by forcing liquid silk through organs called spinnerets
  • web-spinning seems to be programmed into their brain as soon as they hatch

Mites and Ticks

  • small arachnids - parasites on humans, farm animals, and agricultural plants
  • most species are smaller than 1mm, but some ticks can be as big as 3cm
  • chelicerae are needle like structures, used to pierce the skin of their hosts, also helps the parasite to keep a form hold on its host
  • pedipalps are often equipped with claws for digging in and holding on
  • some species, such as spider mites, damage house plants and are major agricultural pests on crops such as cotton
  • chiggers, mange, and scabies mites cause painful itching rashes in humans, dogs, and other mammals
  • tick bites are dangerous, can spread Rocky Mountain Spotted Fever and Lyme Disease


  • widespread in warm areas
  • carnvivores, prey on other invertebrates, usually insects
  • pedipalps on scorpians are enlarged into a pair of claws
  • abdomen - long and segmented, terminates in a venomous barb used to string prey
  • scorpions grab prey with its pedipalps, then whips its abdomen over head and stings its prey, thus killing or paralyzing it
  • chews on its meal with its chelicerae
  • most North American scorpions have venom powerful enough to cause about as much pain as a wasp sting
  • the venom of one genus of scorpions that lives in Mexico and Arizona has killed small children who were stung accidentally