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Biology of spiders D. L. A. Underwood Biology 316 - General Entomology A. Order Araneae (spiders) 1. A few fun notes about spiders in society 2. Taxonomic overview a. There are about 38,000 described species of spider worldwide. (How many mammals are there?) b. The text recognizes two suborders based on the articulation of the chelicerae. More on this in lab. c. Infraorder Mygalomorphae (tarantulas and trap-door spiders) - 8 families d. Infraorder Araneomorphae ("true" spiders or modern spiders) - 68 families 3. Spider external morphology Spiders - Page 1 of 8 4. Aranid internal morphology 5. Team Project - D takes notes, A speaks a. Identify three morphological attributes of spiders that allow them to eat liquid food. Be sure to make clear how the morphological feature allows for a liquid food diet. 6. Silk production and webs a. Silk is a fibrous protein composed of the amino acids glycine, alanine, and serine. b. It is produced in a liquid, water-soluble form that dries into an insoluble form as it leaves the body. c. Silk is produced by special glands and is secreted through specialized structures called spinnerets. d. Each spinneret is composed of a combination of small and large tubes called spools and spigots, which open to the outside. e. The different spools and spigots allow the spider to spin silk into threads of different thickness. f. The spinnerets are thought to be derived from opisthosomal appendages as they possess rather complex musculature. g. There are about six different silk glands producing six different kinds of silk. These different types can be interwoven to produce a variety of silk threads. Spiders - Page 2 of 8 h. Some spiders also have a cribellum, a spinning organ that bears up to 40,000 spigots that produce silk in extremely fine threads that are combed by the spider using Spiders - Page 3 of 8 specialized structures on the fourth walking legs called calamistra. i. Silk is used as safety lines and climbing lines, for the construction of nests, cocoons, and traps, to wrap prey for storage, as egg sacs and sperm platforms, and to line burrows. Young spiders also produce parachute-like structures to ride the wind as they disperse from their natal area. Finally silk is used in prey capture in formation of webs. 7. Team Project - C takes notes, B speaks a. Describe two methods of studying the evolution of silk production and use in spiders. In other words, how could you gain insight into how this complex morphological and behavioral suite of characters evolved? 8. Silk production continues: An example of web construction a. One of the most well known types of web is the type constructed by orb weaver spiders. b. The spider initially spins a thread that is carried by the wind until it attaches to something. The spider secures its side of thread, moves to the center of the line and drops, forming a Y. Radial threads are then placed attached to frame threads and a temporary spiral is spun. c. The second stage of construction involves the placement of the sticky spiral, which is always coated with glue that resembles many small beads along the threads. d. Some orb weavers also produce a thick mesh of silk called the stabilimentum. It is thought that this alerts birds to the location of the web so that they will not fly into it causing damage. The actual function is still debated. Spiders - Page 4 of 8 9. Sensory structures a. Most of the "hairs" on a spider are mechanoreceptors. Some spiders also have very thin tactile hairs on the legs called trichobothria (see Fig 5-7c), which are extremely sensitive to even airborne vibrations (beating insect wings, air currents, sound frequencies, etc.) b. Chemoreceptors are found associated with small hairs surrounding the mouth, on the pedipalps, and are most abundant on the tips of the legs where contact with the substrate is made c. Vision is relatively unimportant for spiders that depend upon capturing prey within webs. These spiders use patterns of vibration on the web to distinguish among prey, predator, or mate. d. Among the spiders that capture prey without webs, vision is exceedingly important. e. Spiders have single-lensed rhabdomeric eyes. The individual sensory units are simple ocelli in a cluster. f. The main eye has the light-sensitive portions of the sensory cells directed toward the lens. The secondary eyes are inverted with the light-receptor elements directed away from the lens. The secondary eyes are thought to serve under low light conditions. 10. Prey capture a. Nearly all spiders produce proteinaceous neurotoxins in the poison glands associated with the fangs of the chelicerae. b. There are basically two types of spiders, those that capture their prey using a silken web or trap and those that hunt down their prey and capture them without the use of silk. c. Aside from the normal web-based prey capture, numerous spiders use silk to catch prey in rather novel ways. i A sheet web is produced by spinning a dome-shaped sheet supported by a network of threads. Insects get entangled by the supporting threads. The spider shakes the entire web, knocking the bug into the sheet below where it is pulled through the web by the spider. Spiders - Page 5 of 8 ii Bolas spiders use sticky threads to throw onto prey to capture them much like small lassos. iii Funnel-web spiders build a web resembling a funnel; the spider can be found at the base of the web in the hole. iv A primitive family of spider lives in a burrow and trails "fishing lines" or trip threads to alert the spider when a prey item is nearby. v Purse-web spiders spin a silken tube that lies upon the ground. The spider hides inside and when it detects the vibrations of prey on the surface it cuts through the webbing with its chelicerae and captures the prey. vi Net-casting spiders employ a small web that they throw onto their prey much like casting a net for fish. d. Spiders that are analogous to cheetahs include the lycosids and the salticids. They run down their prey or lie in ambush. Crab spiders typically are camouflaged and catch insects that visit flowers. Spiders - Page 6 of 8 11. Team Project - D takes notes, C speaks a. Compare and contrast web builders and lie-n-wait spiders with respect to costs and benefits of their mode of prey capture. Recall that 'compare and contrast' means 'how are they similar and how are they different?' 12. Courtship, mating, and reproduction a. Considering spiders are predators, courtship and mating can be a dangerous proposition. If a potential mate is misidentified as either being prey or predator, death may be all the hopeful suitor finds. b. Spiders have developed complex mechanisms to ensure mating occurs without injury. c. All male spiders use their pedipalps to transfer a spermatophore to the female. The spermatophore is packaged in silk. Most males show specific modifications to pedipalps associated with mating. d. Courtship and species-specific behavior and mating positions also help insure mating is with conspecifics. Spiders - Page 7 of 8 Spiders - Page 8 of 8