Cheliceriformes: Arachnida Module 4
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CHELICERIFORMES: ARACHNIDA MODULE 4 MODULE 4 CHELICERIFOMES: ARACHNIDA Unit 1 Subclass Arachnida (G. arachne, spider) Members of the Arachnida are essentially terrestrial except for mites, which have become secondarily adapted to aquatic life. Adaptation to terrestrial life include waterproof cuticle, Malpighian tubules, internal fertilization and internal gaseous exchange system. Arachnids are able to feed on highly mobile preys because of the possession of poison glands with secretions that paralyzes prey. Subclass Arachnida is divided into 11 orders as follows: 1. Order Scorpiones (scorpions) e.g. Centurus, Buthus 2. Order Pseudoscorpiones (pseudoscorpions) e.g. Chelifer, Garypus 3. Order Solpugida or Solifugae (solifuges; wind spiders) e.g. Galeodes 4. Order Palpigradi (micro-whip scorpions) e.g. Koenenia 5. Order Uropygi (whip scorpions or vinegaroons) e.g. Mastigoproctus 6. Order Amblypygi (whip spiders, tailess whip scorpions) e.g. Charinus, Acanthophrynus 7. Order Schizomida (schizomids) e.g. Agastoschizomus 8. Order Araneae (spiders) e.g. Araneus, Aranea, Argiope 9. Order Ricinulei (ricinuleids) e.g. Ricinoides, Phalangium 10. Order Opiliones (daddy long legs or harvestmen) e.g. Trogulus 11. Order Acari e.g. Ixodes (ticks), Chorioptes (mites) Unit 2 The major orders to be discussed in this module are the Scorpiones, Araneae and Acari. A brief mention of order Pseudoscorpiones is given. Order Scorpiones These are the scorpions. Scorpions are predaceous carnivores, generally cryptic by day while feeding at night on insects, spiders and other small sized invertebrates. The body is divided into the prosoma and opisthosoma. The short prosoma has fused segments covered by AOE 1 CHELICERIFORMES: ARACHNIDA MODULE 4 a single carapace; it bears a pair of large median eyes and two to five pairs of small lateral eyes. The chelicerae is short, three segmented with gnathobases for grinding food; pedipalps are large, powerful, chelate and six-segmented for grasping. The four pairs of legs are eight segmented and end in a pair of claws. The segmented opisthosoma is further subdivided into a broad anterior seven-segmented mesosoma and a narrow posterior five-segmented metasoma terminating in a stinging apparatus called aculeus. A narrow stalk called pedicel occurs between the prosoma and mesosoma. The first mesosomal segment bears a gonopore covered by genital operculum; the second bear sensory appendages called pectines (mechanoreceptors to detect ground vibrations and chemoreceptors for sex recognition) while the third to sixth segments bear a pair of book lungs each. The segments of the metasoma (tail or postabdomen) are elongate and bear no appendages. Scorpions seize their prey with the pedipalps, flex the abdomen over the back of the body and stabs the prey with the sharp, curved venomous barb, the aculeus, to introduce venom and immobilize the prey. The juices are then extracted by the action of the sucking pharynx. Scorpions do not sting except provoked. Respiration is by lung books. Sense organs Figure 1: Pandinus imperator include the eyes, sensory hairs, lyriform organs and pectines. The reproductive system of scorpions lie in the mesosoma. Fertilization is internal and is preceded by a courtship dance, ‘promenade a deux’, which involves drawing the female over the extruded male spermatophore. Scorpions may be ovoviviparous or viviparous. Examples include the big, black scorpion, Pandinus Figure 2: Hottentotta hottentotta imperator, of southern Nigerian rainforests; the small light brown scorpion, Hottentotta hottentotta, of the northern Nigerian savanna; and the North African desert scorpion, Androctonus australis. Figure 3: Androctonus australis. AOE 2 CHELICERIFORMES: ARACHNIDA MODULE 4 Order Pseudoscorpiones These are the pseudoscorpions. They are extremely small arthropods. The body consists of a prosoma, covered dorsally by a carapace but segmented ventrally, and broadly joined to the opisthosoma (not divided as in true scorpions). The chelicerae are chelate and bear spinnerets; the pedipalps are large, chelate and bear poison glands while the opisthosoma lacks the caudal stinging apparatus. Respiration is by means of tracheae. Eyes may be present or absent. They exhibit phoresy by using other animals as transport. Example include the house or book scorpion, Chelifer cancroids, frequently found as phoretic “guest” on houseflies. Figure 4: A pseudoscorpion Order Araneae These are the spiders and are the most diverse of the arachnids. They are generally terrestrial predaceous carnivores ranging in size from less than a millimeter to about 9 cm. The body is divided into a prosoma and an opisthosoma, usually joined together by a narrow waist called Figure 5: Black widow spider pedicel. The prosoma bears carapace with two rows of simple ocelli. The chelicerae is two segmented and chelate, followed by a six- segmented pair if pedipalps and four pairs of walking legs terminating in two to three claws. The unsegmented opisthosoma bears one to four pairs of spinnerets which bear openings of the abdominal silk glands. These special glands secrete fine threads of silk that are used in building nests, cocoons and special copulatory chambers; shelters; encasing egg and sperm at copulation; and as a dragline (adhesive silk that fastens them at intervals to the Figure 6: A spider’s web substratum) for safety. The silk is a small albuminoid protein produced in a liquid, water insoluble form that hardens into an AOE 3 CHELICERIFORMES: ARACHNIDA MODULE 4 extremely strong elastic thread when it leaves the body. The cribellum, a platelike structure located anterior to the spinnerets, is another spinning organ. Spiders feed largely on insect and ingest their food in a liquid or semiliquid form. Whenever a spider catches a prey, it injects poison into it through the chelicerae; the prey is immobilized, bitten and its tissue macerated during digestion. Majority of spiders are harmless to humans although some are moderately harmful. Respiration is by book lungs. The heart bears eight pairs of ostia (two in the prosoma and six in opisthosoma. Excretion is through Malpighian tubules while the excretory wastes are mainly in form of guanine. Sense organs include the eyes, sensory hairs and chemoreceptors. The pedipalps in male is used as external copulatory organs (with sperm reservoirs). Prior to mating, the male spins a tiny web and deposits a drop of sperm and sucks it into cavities in the pedipalps. It searches for a female and engages in courtship display and copulation. The male prevents being eaten up by the female by grasping her chelicerae in his own and quickly escapes after mating. In one particular species, Xysticus cristatus, the male bids the female’s legs to the ground with silk threads; and escapes after mating before the female could free herself. The female spins sheets of silk called cocoon over the fertilized eggs. Spiderlings eventually emerge from the cocoon. Examples include Gasteracantha sp (occurring in Figure 7: Gasterocantha sp. Nigeria); European garden spider (an orb-web spider), Araneus disdematus; American black widow, Latrodectus; Torania variata and Scodra griseipes. Order Acari These are ticks and mites. They are regarded as the most heterogenous group of the Arachnida and are ubiquitous. There is no boundary between the prosoma and the unsegmented opisthosoma. The prosoma is covered by a carapace-like shield. There is Figure 8: (a) Tick (b) Mite a capitulum (or false head) bearing mouthparts basically of two or three segmented chelicerae and pedipalps coxae; the mouthparts are modified for biting, piercing, tearing or sucking. The four pairs of legs are each six-segmented. Acari is divided into three suborders: Opilioacariformes, Acariformes and Parasitiformes. Members of the Opilioacariformes are AOE 4 CHELICERIFORMES: ARACHNIDA MODULE 4 primitive mites referred to as opilionids or ‘daddy long legs’; they have extremely long and slendeer legs. An example is Leiobunum. Most members of the suborder Acariformes are free-living, while some are ectoparasites of both vertebrates and invertebrates. An example is the human itch mite, Sarcoptes scabiei (family Sarcoptidae). The female mites of S. scabei burrow into the superficial layers of the skin, lays Figure 9: Sarcoptes scabiei about four to six large eggs which hatch within few days into a six-legged larvae. These larvae crawl out of their burrow on to the surface of the skin and may either burrow into the skin again or enter a hair follicle to produce ‘moulting pockets’, feeds for about two days and moults into an eight-legged nymph, moults twice again before becoming adults. Other examples include the harvest mite, red bugs or chiggers notably Trombicula species; the tropical fowl mite, Ornithonyssus bursa and the transient bat mite Meristaspis Figure 10: Trombicula sp kenyaensis found infesting the straw-coloured fruit bats, Eidolon helvum in south western Nigeria. Suborder Parasitiformes includes free- living and symbiotic forms such as the ectoparasitic ticks (family Ixodidae and Argasidae). Examples are the hard Figure 11: Ixodes ricinus ticks such as Boophilus decoloratus and Ixodes with sclerotized shield covering the dorsum; and the soft ticks such as Argas that lack the heavily sclerotized dorsal shield. Developmental stages to adult may involve one, two or three hosts. One-host tick include the ‘blue tick’ of livestock in West Africa, Boophilus decoloratus; two-host tick include the brown ear tick, Rhipicephalus appendiculatus, with immature stages