Ch 28-2: Chelicerates - Spiders and Their Relatives

Chelicerates Two-part body system Mouthparts called chelicerae Lack sensory “feelers” on head

Two Part Body Cephalothorax - contains brain, eyes, mouth, beginnings of digestive system and several pairs of walking legs Abdomen – contains most of the internal organs

Chelicerae Specialized appendages for eating In spiders, they are sharp for piercing the prey, and the poison glands empty through the ends

Pedipalps A second pair of specialized appendages for eating In spiders, they act together with the chelicerae to puncture and hold the prey

Wolf Spider

Horseshoe Crabs NOT TRUE CRABS “living fossils” – have not changed since 430 mya Have book gills for respiration (unique to horseshoe crabs)

Arachnids Spiders, scorpions, ticks and mites Have 4 pairs of walking legs on their cephalothorax Pedipalps capture/hold prey Chelicerae bite, poison, then suck out prey’s soft parts

Spiders Usually eat small insects Capture prey in webs, or pouncing on them Hollow chelicerae inject venom to paralyze prey

Digestive enzymes are injected into prey through wounds Pumping stomach sucks liquefied tissues into digestive system

Momma wolf spider carries Her babies on their back until They can feed themselves

Back to spiders If a yellow crab spider lays her eggs on a white- flowered plant, her offspring will be white, not yellow. Furthermore, if a white crab spider lays her eggs on a yellow-flowered plant her offspring will be yellow, not white.

Silk All spiders can produce silk from gland in abdomen -liquid silk is forced through and pulled out of spinnerets - this silk is a flexible protein (5x stronger than steel) - Silk is used for webs, cocoons, wrapping prey

Building webs is an innate behavior

Internal Structures of the Spider

Mites and Ticks Small Many are parasites Chelicerae and pedipalps may be modified to dig in and hold on to the prey Can also be spreaders of disease

Engorged Ticks

Mighty-mites

Scorpions Live in warmer climes Carnivores (of mostly insects) Pedipalps modified into claws End of abdomen ha venomous barb to sting prey

Pedipalps (claws) capture prey, then abdomen stinger kills or paralyzes it. Chelicerae are used as teeth to chew Most scorpion’s sting is like a wasp sting – one species is strong enough to kill a small child

Crustaceans Most are aquatic - two pairs of antennae Hard exoskeleton- mouthparts called mandibles

Main Structure Head, thorax and abdomen Some (crayfish and others) have cephalothorax covered in covered by tough shell called carapace

Large crustaceans have calcium carbonate in their exoskeletons to make them stronger

First two pairs of appendages are antennae (feelers) Have many sensory hairs on them Other crustaceans use their antannae for filter feeding or even as oars (water fleas)

Third pair of appendages are mandibles (mouth parts) Often for biting and grinding food Some use mandibles for filter feeding, as feelers, or to suck blood from host

Appendages on Thorax Barnacles have feathery appendages for filter feeding

Some thorax appendages are used as legs Other are used for internal fertilization, carrying eggs, capturing prey, etc