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Zoology Notes Page 7 Arachnids (spiders and their relatives)
General Information
The most disliked and feared by humans of all the animals. Includes spiders, ticks, mites, daddy longlegs,
scorpions, sea spiders. They are a major predator of garden pests so are liked by gardeners. Almost all have
venom (poison) intended to immobilize/kill their prey. The 2 most dangerous ones in the United States are
also found in Alabama—The brown recluse and the black widow.
Body Form
Body is divided into2 main segments: Cephalothorax and Abdomen
Cephalothorax- on the face region they have appendages. Pedipalps and Chelicera. Pedipalps are
appendages for grasping and holding prey. The Chelicera are appendages than terminate in “fangs” that have
poison glands that inject poison and digestive enzymes into the prey. The “thorax” region has 8 walking legs.
Abdomen – Hold the reproductive organs (testes or ovaries). Also, silk glands are found in the abdomen of
spiders (a type of arachnid). The silk they produce is extruded out of openings at the end of the abdomen
called Spinnerettes.
Muscles attach to the inside of the exoskeleton. Arachnids also use blood pressure to move limbs.(8 walking
legs and “face appendages”. They mold like insects also as they grow.
Silk
An extraordinary characteristic of one type of arachnids, spiders, is the production of silk. Silk can be used to
build webs (funnel, cob, sheet or orb) that are homes/nets to catch prey. Some use them to “balloon”(fly) on
the wind.(usually these are small and light weight)
Like all arthropods they have an exoskeleton made of chitin, an open circulatory system, a digestive tract, a
brain/ganglia, and malpighian tubules that get rid of nitrogen waste from the blood. Arachnids may have
tracheal tubules (like insects) or book lungs to breathe oxygen with.
Ways of Living and Ways of Surviving
Many spiders build webs that they stay in and maintain and weight for prey to eat. Others are “coursers” that
wander around and hunt their prey. Some spiders build shelters they use to ambush their prey (trap door
spiders) Scorpions dig a burrow and live inside it till they come out to hunt. Mite and ticks are often parasites.
Reproduction
They are dioecious and have internal fertilization. Spiders will build a “nest/cocoon” with their silk and keep
their eggs safe inside it till they hatch. With spiders, often the female is much larger than the male and will kill
him after mating. (what use is he after she gets what she wants!!) *often, the male “penis” is on one of his
pedipalps.
Significance to mankind
Mainly, arachnids are pest to most people. They are respected and appreciated by gardeners, but few others.
The significance of many (spiders, scorpions, ticks, and many mites) is their bites/stings, some of which are
harmful and possible lethal. Black Widows and Brown Recluse are potentially lethal to human and are found
locally. Scorpions have venom in their “stingers”. Some species of scorpions can be lethal.
Other “yucky” relative of spiders?
Ticks and mites. Ticks are ectoparasites that suck blood of their hosts. Mites called “chiggers” are similar to
ticks in that they suck blood. Their presence causes itching. Another mite causes “scabies” that result in
similar itching. Most people reading these notes have follicle mites on their hairs and eyelashes right now!!
Some mites are thought to be the cause of dust allergies.
*Fact: over 10 years a mattress on your bed doubles in weight. Why? Dust mite poop, dust mites, and your
dead skin cells they are eating on.
tick
mite
scorpion