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TICKS
Ticks are parasites that readily attack mammals and birds for blood meals. Some of the characteristics that have allowed ticks to surve and flourish are, as follows:
- they are persistent when seeking a blood meal
- they attach firmly while feeding and cannot be easily dislodged
- they have a long life span (some, up to five years)
- they can survive harsh environmental conditions
- they are generally free of natural enemies
- they have a wide host range
- they generally lay thousands of eggs
they are remarkably resistant to starvation
Ticks sedrve as vectors (carriers of organisms that cause disease) for a number of diseases including ROCKY MOUNTAIN SPOTTED FEVER, LYME DISEASE, and HUMAN BABESIOSIS.
They parasitize man and other vertebrates both wild and domestic. They are opportunists and may attack any available mammal - if their primary host is unavailable.
There is a lack of precise information on the physiology of most ticks but there is enough general information to provide the following.
There are two distinct types of ticks. They are: l) the isodid (hard shelled ticks) and 2) the argasid (soft shelled ticks).
Externally, the tick body is not divided into segments, and a distinct head is lacking.
At the anterior (front ) end of the tick there is a CAPITULUM (gnathosoma or false head). To it are attached two pairs of appendages: 1) the CHELICERAE (each consisting of two podites, an elongate shaft and a terminal cutting digit), and 2) the sensory PEDIPALPS (each consisting of several podites) which lie lateral to the chelicerae.
At the base of the pedipalps (a region called the basis capituli) is a structure called the HYPOSTOME. The hypostome is a barbed rough-surfaced structure which is ventrally located. The hypostome location forms a "lower lip" or pre-buccal cavity along which nutrients and secretions pass in and out of the tick.
The dorsal surface of the capitulum of the female contains two densely pitted regions called the POROSE AREAS which produce a lubricating fluid to assist the movement of GENE'S ORGAN.
Gene's organ coats each egg produced by the female with a sticky wax-like substance.
The IDIOSOMAL REGION lies posterior to the capitulum. The idiosome bears four pairs of walking legs in the adult and nymphal stages (three pairs of legs in the larval or "seed tick" stage).
The tip of the first pair of walking legs contain a sense organ called HALLER'S ORGAN. Each leg terminates in two curved claws and a suction pad.
The dorsal surface of the idiosome in the male tick is almost completely covered with a hard sclerotized plate called the CARAPACE or SCUTUM. In the female adult, nymphal, and larval stages of both sexes, onlyh the anterior part of the idiosome is covered with a hard carapace. The posterior region of all ixodid ticks is covered with a soft flexible cuticle.
BODY SYSTEMS
There are paired SPIRACLES (openings of the respiratory system) on the fourth pair of walking legs. The spiracles connect to a tracheal system (TRACHEOLES) where gas exchanges occur in the adult and nymphal stages. There are no visible respiratory spiracles in the larval stage and it is assumed that respiratory gas exchange takes place through the body cuticle.
Ticks are TELMOPHAGIC feeders - they insert mouthparts indiscriminately into skin surfaces causing lysis of tissues. They feed on tissue and extracellular fluids until they break walls of capillaries and produce a blood pool from which they then feed.
For sucking blood, both the hypostome and the chelicerae are inserted into the skin of the host. The hypostome plays no part in skin penetration but prevents the animal from being dislodged from the skin. One does not usually feel the tick biting or feeding.
Once attached to the skin, ticks secrete a latex-like material which cements the tick to the skin. Ticks also produce a histolytic secretion which liquefies the tissue and an anticoagulant secretion which reduces blood coagulation.
The digestive tract of the ticks contains a pumping PHARNYX and paired salivary glands which open into the SALIVARIUM. Fluid food passes from the pharynx along the ESOPHAGUS to the storage midgut and digestive CAECA or DIVERTICULA.
Absorbed nutrients are transported by an open circulatory system.
The circulatory system is composed of a dorsally located heart and a single artery leading from the heart. Blood enters the heart from the HAEMOCOEL through a pair of OSTIA and is pumped into the artery and into sinuses which surround the brain, esophagus, and leg nerves. Blood leaves the sinus, enters the haemocoel and returns to the heart.
Digestion results in wastes in the form of a solid nitrogenous produce called guanine. This material is removed by the MALPIGHIAN TUBULES to a RECTAL SAC for temporary storage before being voided through the anus.
Feeding in ixodid ticks is divided into two phases. the first is alow and lasts up to twenty-four hours before full engorgement. During this phase the ENDOCUTICLE grows and thickens. The second phase is rapid and causes the newly formed endocuticle to stretch and distend.
The long feeding time and heat of the host causes loss of water from the ticks. Ticks replace water from blood meals and can concentrate the meal allowing additional food to be taken up.
Each ixodid INSTAR (stage following a molt) feeds only once. So in the life cycle of the tick there are three feeding periods - once each in the larval, nymphal, and adult stages.
The tick brain consists of fused GANGLIA (cell clusters) and is known as the central nerve mass. In ticks, the brain is penetrated by the esophagus. Ticks have various sense organs to locate their hosts. These include: SETAE (hairs) on the pedipalps (chemoreceptors); tactile and thermoreceptor setae on their legs; HALLER'S ORGAN on the tip of the first legs.
Haller's organ contains olfactoreceptors (smell), mechanoreceptors (pressure), hygroreceptors (moisture), and thermoreceptors (heat).
Haller's organ may also be involved in detecting SEX PHEROMONES (chemicals) and AGGREGATION/ATTACHMENT PHEROMONES. Both of these pheromones are involved in mating and/or attracting mating adults and are discussed later in this paper.
