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The deer tick’s body, which now have fused together through evolution, consists of the capitulum and the idiosoma.  The mouth parts, chelicerae, palps and hypostome, are found in the capitulum. The idiosoma is again broken down into the podosoma and the opisthosoma. The podosoma consists of the walking legs and the genital pore and the opisthosoma is characterized by the spiracles and the anal aperture.  Many of these evolutionary changes have helped the ticks adapt to their environment and feeding ritual.

 The specialized mouthparts have had a huge impact of the tick’s success as a parasite. Like stated above there are three main mouthparts found within the capitulum, the hypostome, cheliceral, and the palps. The two outermost structures of the mouthparts are the two highly mobile palps; between them are the paired chelicerae, which in turn protect the hypostome which is the center rod-shaped structure.

The hypostome is the primary organ for attachment to the host’s skin. It is covered with numerous, recurved teeth that are placed into the host’s skin. The teeth located on the hypostome progressively get smaller and fewer towards the proximal portion. At the distal tip of the hypostome there are small clusters of tiny denticles. The hypostome itself is hollow except for the very distal end. Deep groove on the dorsal side of the hypostome allow the blood from the host to be brought into the mouth and pharynx.

The chelicerae are the cutting organs used by the tick to penetrate the host’s skin and gain access to its blood.  These paired organs compromise three parts, the cheliceral base, an elongated shaft, and the cutting digits.  The cheliceral base is a large club shaped section which contains the muscles that move the digits. The shaft is an elongated tube that contains the tendons for moving the digits as well as the nerves. Each shaft contains two tendons one being larger than the other.  The larger laterally positioned flexor tendon and the smaller medially positioned extensor tendon which function to bend the digits. At the anerior end, the cheliceral is modified with digits. These structures are homologous to the pincer-like chelae of other arachnids. However, in ticks the modified digits are equipped with broad heavily sclerotized cusps or spine-like teeth, different from the scissor-like grasping arrangement of other arachnids. All of the cusps are oriented in the horizontal plane, so that most of the cutting action is lateral. In effect, tick chelicrae can cut but cannot grasp, as they cannot rotate or pronate. The digits are used to rip and tear the hosts skin to expose the dermis. When the digits are not being used they are retracted within the chelicea sheaths that surround the elongated shaft.  When needed, the cheliceral digits are protruded by hydrostatic pressure. These digits are limited by the flexibility and extensibility of the cheliceral sheaths. Another adaptive feature of the digits is mechanosensory, chemosensory, and thermosensory sensilla found on the digit spines.  Together, these sensilla provide information on the biochemical characteristics of the wound site environment. They can use this sensory adaption to detect minute differences in skin tissue temperatures.  These types of sensory information facilitate blood sucking behavior.  In males, the cheliceral digits are used to probe the female’s vulva, and detect the female genital sex pheromone which initiates the mating ritual.

The final part of the capitulum is the leg-like palps. There are four distinct pairs of palps and the most posterior pair is reduced in size and can be retracted or protruded as needed.

The idiosoma is divided into 6 segments which contain 4 pairs of walking legs. The function of the legs is for movement when extended and or for protection when folded against the body.  On the dorsal surface of leg one there is a special organ called the Haller’s organ which is specialized in determining host location, host odors, and also detecting pheromones as well as other sensory functions. 


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