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Arthropoda: ARACHNIDA

Ticks, Mites Spiders Pseudoscorpions



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       The Class Arachnida includes the spiders, horseshoe crab, scorpions and mites.  It is a very large class of mostly terrestrial arthropods, with the marine horseshoe crab being an exception.


       The general characteristics are the absence of antennae and a body comprised of a cephalothorax and an abdomen, the latter may appear as only a single part without divisions. The cephalothorax bears four pair of walking legs and 6-8 eyes raised on tubercules.


       The head appendages include chelicerae, which are jaw like with claws and poison duct openings at their tips.  The basal portion of pedipalps serves both feeding and sensory functions.


      The Arachnida are air-breathing arthropods, the body of which is divided into two parts:  (1) The cephalothorax, including the fused head and thorax, and (2) The abdomen. The abdomen can be either segmented or unsegmented. The mites and ticks have their entire body fused to form forms a sac. The head appendages are highly modified.  The antennae are lateral and the eyes, when they occur, are simple and sessile, and the eyes, when present, are rather simple and sessile. In the adults there are four pairs of ambulatory legs that are attached to the cephalothorax. The first developmental stage is the larva, which has three pairs of legs.  When respiratory organs occur they are either book lungs or tracheae.  The sexes differ structurally and metamorphosis is incomplete.  The immatures resemble small adults.  The arachnids imbibe fluid from their prey by means of a "sucking stomach."Their mouthparts function either for crushing their prey and sucking up the liquid portions or for piercing and cutting the host tissues to obtain blood (Matheson 1950). 


       The mouthparts consist of a pair of chelicerae located in front of the mouth opening; a pair of pedipalpi that are located on the mouth sides or just posterior to them.  In some parasitic species there is a structure called the "hypostome" that is located directly beneath the mouth opening.  Chelicerae vary structurally in different orders.  In the spiders (Araneida) each chelicera consists of a large basal segment and a terminal one shaped into a claw.  Spiders used these structures to capture and kill their prey.  A poison gland is located near the tip of the claw.  Parasitic species (e.g., ticks) use the chelicera as piercing and cutting tools. The pedipalpi resemble legs in all the groups have 4-6 segments. In the spiders the pedipalpi 4 of the male are greatly modified into very specialized organs insemination of females. Among many of the ticks they are protect the highly developed piercing organs (Matheson 1950).


       The following descriptions include both groups of medical and non-medical importance for distinction purposes:


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       The Order Araneae -- includes the true spiders.  Segmentation is obscure in the abdomen and there are no obvious appendages except 3-4 pairs of spinnerets at the posterior end of the abdomen that are modified abdominal appendages.  Several examples of spiders may be seen in the following diagrams Inv143 - Inv147:



          Food & Digestion -- Insects and other small animals are caught in webs.  The prey is paralyzed and their liquid contents are moved up through the pharynx and esophagus.  A sucking stomach pumps food from the prey through the mouth and into the digestive tract.


          Nine diverticulae from the intestine lead to various body parts.  There is one located forward and four on each side, which function to increase the surface area.  The posterior part of the intestine is surrounded by digestive glands and some food may actually enter the glands.  A rectal caecum occurs at the junction of the rectum and intestine.


          Circulation -- The heart is long and located in the abdomen.  The dorsal aorta in the cephalothorax has subsequent branches to appendages and the brain and eye regions.  Some blood is pumped posteriorly to a short posterior aorta.  The haemocoel is divided into various sinuses.  Blood reaches the book lungs and is aerated after which it returns to the heart.


          Respiration -- Air diffuses directly into the book lungs, as the blood does not carry oxygen.  Some tracheae may occur but they are never well developed.


          Excretion -- Malpighian tubules serve for excretion.  Coxal glands that are modified nephridia may also be involved in excretion.


          Nervous System -- There is a typical pattern where a great concentration of ganglia occurs in the anterior cephalothorax.  Nerves run out to different parts of the body.


          Sensory Organs -- There are the eyes, pedipalps and setae all over the body all of which have sensory functions.


          Reproduction -- The sexes are separate.  Ducts open near the anterior end of the body, but fertilization is internal.


          Males use pedipalps to transfer sperm from their genital pore to that of the female.  Eggs are laid in silken cocoons and maternal care is common.  Development is direct.


          Silk Glands -- There are several varieties of silk glands.  The silk they produce differs in strength, slipperiness, etc.  Different kinds of webbing are produced for particular circumstances.  The tips of the legs are modified for walking on the webs.


          Economic Importance -- Some species of spiders are poisonous to humans and animals.  Spider silk has been used in bombsights during World War II.




           Order:  Scorpiones (Scorpionida) -- scorpions:  These animals have a well marked cephalothorax and segmented abdomen that is equipped with a sting and poison gland at the posterior end.  They can be dangerous in warmer regions.  Chelicerae and pedipalps are both chelate.  They have book lungs.  They feed on other arthropods.  They are also viviparous as they bear living young. See Inv150 & Inv151 for examples:





           Order:  Amblypygi.  (Pedipalpia) -- whip spiders and tailless whip scorpions:  There is a long tail, large palps and small chelicerae.




           Order:  Pseudoscorpionida -- book scorpions:  These are small animals that have the appearance of scorpions because their pedipalps are pincers.  The abdomen is rounded but without a sting.  They feed on small insects. See Inv152 for example:





           Order:  Opiliones (Phalangida) -- harvestmen:  Their extremely long walking legs have earned them the name of "Daddy Long Legs."  The body regions are all compacted into a single division.  They are predators of small insects and other arachnids. See Inv154 for example:





           Order:  Acarina -- mites and ticks:  The chelicerae and pedipalps are modified into projections called a hypostome.  They are parasites and vectors of disease, and serious pests of vegetable and tree crops. See Inv153 for example:




           Class  Pycnogonida -- sea spiders:  These are tiny marine animals.  Included are parasites, commensals and free-living predators.




           Class:  Merostomata: Order:  Xiphosura -- horseshoe crab:  The range is from the East Coast of North America to the coasts of southeastern Asia.  These animals have remained essentially unchanged sinde the Paleozoic.  They and the Pycnogonida are the only marine arachnids.  They are also the only Arachnida with compound eyes.  The chelicerae are chelate and the pedipalps look like walking legs.  But there is four pair of true walking legs.  The abdomen has well developed appendages that have been modified into book gills.


          Horseshoe crabs are of course a misnomer as they are not mollusks.  Their blood, which is blue in color, is high in metallic copper and is harvested regularly for medical research.  See Inv148 & Inv149 for examples:



       Subphylum: Myriapoda, Class: Chilopoda includes the centipedes.  They are dorso-ventrally flattened.  Their body consists of a head and trunk but there is no thorax nor abdomen.  The head bears one pair of antennae, one pair of mandibles, one pair of maxillipedes with poison glands at the bases and ducts leading to pointed tips (Note:  these are absent in the Diplopoda).  There are two pairs of simple eyes called pseudocompound eyes.  They have maxillae on the 1st and 2nd segments.  The trunk bears uniramous appendages and there are 15 to 175 segments.  See examples at Inv141.



          Body Wall -- This consists of a cuticle, muscles and a haemocoel


          Digestive Tract -- A typical mouth to anus arrangement.


          Circulatory System -- The heart is tubular with one pair of ostia per segment.  The blood does not carry oxygen


          Respiration -- The tracheae are lined with ectoderm and cuticle, and heavy rings of cuticle line them.  They branch out and ultimately reach all tissues of the body.  The blood does not have an oxygen carrying function.


          Excretion -- Malpighian tubules are long, thread-like and blind-ending tubules.  They lie in the haemocoel and empty into the digestive tract at the junction of the mid and hindguts.  They extract nitrogenous wastes from the blood.


          Nervous System -- This system is the same as that found in the Crustacea.


          Reproduction -- The sexes are separate.  Genital organs are found at the posterior end of the body and development is direct.


          Locomotion -- These animals are fast movers.  Long posterior legs are sensory and used when moving backwards.


          Food & Digestion -- Chilopoda are carnivorous and their food is paralyzed first by the maxillipedes.





       The Arachnida are divided into about nine orders with six of these being primarily of medical importance (Matheson 1950).  One group, the Acarina, is most encountered (See:  Tick Borne Diseases).   The other five orders do contain species that have poison glands, and their bites or stings can be of such severity as to require medical attention. Some species are vectors of pathogenic agents (Matheson 1950) and Medical Entomology.


Table 1.  Tick Species That Inflict Harmful Bites




Bite Effects




Amblyomma cajennense



Severe pain & wound may not heal readily


Matheson (1950)



Argas brumpti



Severe pain with wound persisting sometimes for years


Brumpt (1927)



Argas mianensis


Severe with fever




Ixodes pacificus (californicus)


Severe pain & other reactions


Matheson (1950)


Ixodes ricinus


Bites may produce paralysis


Nuttall (1908)


Ixodes sp.


Severe pain and ulcerations


Mail & Gregon (1938)


Ornithodoros savignyi


Widespread ecchymosis


Patton & Cragg (1913)


Ornithodoros gurneyi



Causes paralysis, blindness & unconsciouss


Man. Trop. Med. (1945)



Ornithodoros turicata


Severe pain


Nuttall (1908)


Ornithodoros rostratus


Severe pain


Davis (1942)


Ornithodoros talaje


Severe pain


Nuttall (1908)


Ornithodoros moubata


Bites by nymphs painful


Nuttall (1908)


Ornithodoros brasilienses


Severe pain


Man. Trop. Med. (1945)



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  Key References:     <medvet.ref.htm>    [Additional references may be found at:  MELVYL Library]


Brumpt, E.  1927.  Précis de paraaitologie.  4th ed.  Paris, France.

Davis, G. E.  1942.  Tick vectors and life cycles of ticks.  IN:  Symposium on relasping fever in the Americas.  Amer. Assoc. Adv. Sci. Pub.

      18:  67-76

Dunlop, J. A. & M. Webster. 1999. Fossil evidence, terrestrialization and arachnid phylogeny. J. Arachnol. 27: 86-93.

Harvey, M. S.  2002.  The neglected cousins: What do we know about the smaller Arachnid orders? Journal of Arachnology 30(2): 357-372.

Harvey, M. S.  2007.  The smaller arachnid orders: diversity, descriptions and distributions from Linnaeus (1758 to 2007). Pages 363-380 in:

     Zhang, Z. Q.  & W. A.Shear  (eds.) Linnaeus Tercentenary: Progress in Invertebrate Taxonomy. Zootaxa 1668: 1–766.

Harvey, Mark S.  2002.  The neglected cousins: what do we know about the smaller arachnid orders?. J. Arachnol. 30(2): 357-372.

Mail, G. A. & J. D. Gregson.  1938.  Tick paralysis in British Columbia.  J. Canad. Med. Assoc. 39:  532-537.

Matheson, R. 1950.  Medical Entomology.  Comstock Publ. Co, Inc.  610 p.

Nuttall, G. H. F.  1908.  The Ixodoidea or ticks, spirochaetosis in man and animals, piroplasmosis.  Harben Lectures.  J. Roy Inst. Pub.

     Hlth, July, Aug, Sept.

Patton, W. S. & F. W. Cragg.  1913.  A textbook of medical entomology.  Calcutta & London.

Patton, W. S. & A. M. Evans.  1929-1931.  Insects, ticks, mies and venomous animals of medical and veterinary importance. Part I. 

     Medical; Part 2, Public Health.  Croydon, England.

Service, M.  2008.  Medical Entomology For Students.  Cambridge Univ. Press.  289 p

Shultz, J. W.  1989.  Morphology of locomotor appendages in Arachnida - evolutionary trends and phylogenetic implications. J. Linn. Soc. 97: 1-56.

Shultz, J. W.  1990. Evolutionary morphology and phylogeny of Arachnida. Cladistics 6: 1-38.

Shultz, J. W.  1994. The limits of stratigraphic evidence in assessing phylogenetic hypotheses of recent arachnids. J. Arachnol. 22: 169-172.

Shultz, J. W.  2007.  A phylogenetic analysis of the arachnid orders based on morphological characters. Zoo. J. Linn. Soc. Zoological

     150(2): 221–265.

Starobogatov, Y. I.  1990.  System and phylogeny of Arachnida (analysis of morphology of paleozoic groups) [Russian]. Paleontologicheskii

     Zhurnal 24: 4-17.

Weygoldt, P. & H. F. Paulus.  1979.  Untersuchungen zur Morphologie, Taxonomie und Phylogenie der Chelicerata. 1. Morphologische

     Untersuchungen.. Zeit. für Zool. Syst. u. Evolutionsforschung 17: 85-116.


Weygoldt, P.  1998.  Evolution and systematics of the Chelicerata. Exptal. & Appl. Acarol. 22: 63-79.