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General Morphology

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Hard Ticks

The family Ixodidae is by far the largest and economically most important family with 13 genera and approximately 650 species (Sonenshine, 1991).

Hard ticks feed for extended periods of time on their hosts, varying from several days to weeks, depending on such factors as life stage, host type, and species of tick.

Figure 1: Hedgehog with multiple attached, engorged ticks.

The main attribute of this family is a plain dorsal sclerotised scutum or shield, which is often ornate with patterns in white or gold against a brown or grey background and which distinguishes these ticks from other families. This sclerotised plate covers the entire dorsal surface of the male, but only one third of the female's dorsal surface. The size of the scutum remains constant during the feeding of the female. During its engorgement it covers a smaller part of the body. The outside surface, or cuticle, of hard ticks actually grows to accommodate the large volume of blood ingested, which, in adult ticks, may be anywhere from 200-600 times their unfed body weight.

Characteristically, the capitulum of hard ticks just as the mouthparts is visible from a dorsal view. Hard ticks can be easily differentiated by the shape of the basis capitulum and by the form of anal grooves.

In all ixodid ticks, the palps contain 4 segments; the tiny terminal (4th) segment (with sensory sensillae) is retractable and found in a cavity on the ventral surface of segment III. The mouthparts include the paired chelicerae, the segmented palps, and the ventrally situated, toothed hypostome, all mounted on the basis capituli (Sonenshine, 1991; Mehlhorn, 2001).

Figure 2: SEM-micrograph of mouth parts of ixodid ticks.
Top: View from the dorsal side on a larva of Ixodes ricinus. x 150.
Bottom: View into the mouth (M) of Amblyomma variegatum. Note the teeth at the hypo- and epistome. x 400.
C: chelicera; M: mouth; P: pedipalps

Eyes, if present, occur on the postero-lateral margins of the scutum.

Figure 3: Diagrammatic representation of an ixodid tick (e.g., Dermacentor sp.) from its ventral side.
AN: anus; CH: chelicera; CL: claw; CS: sheath of chelicera; CX: coxa; E: esophagus; EM: pulvillus; FE: festoon; GN: gnathosoma (capitulum); GO: genital opening; H: hypostome; PP: pedipalpus; SA: salivary duct; SC: scutum; STI: stigma; TA: tarsus

The peritreme or groove is big and clearly visibly around the stigmal plate. Grooves are deep, linear depressions in the body cuticle, usually on the ventral surface.

Also typical is a strong build hypostome with robust teeth. They work as barbs, so it is impossible for the tick to detach spontaneously. This led to the name of the family (greek: Ixos = glue) (Kimmig et al., 2000).

   

Further information

  • Bowman DD: Georgis’ Parasitology for Veterinarians. 9th edn., 2009 , Saunders/Elsevier, St. Louis
  • Kimmig P, Hassler D, Braun R: [Ticks – small bite with nasty consequences.] 2000, Ehrenwirth, Munich [in German]
  • Mehlhorn H: Encyclopedic Reference of Parasitology. 2nd edn., 2001, Springer Verlag, Berlin
  • Sonenshine DE: Biology of Ticks. Part 1, 1991, Oxford University Press, New York

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