Tick

Ticks are ectoparasitic arachnids belonging to the order Ixodida under the superorder Parasitiformes. They are extremely resilient arthropods that feed solely on vertebrate blood, often acting as a vector for transmitting pathogens, such as bacteria, viruses, and protozoans, to the host’s body and causing infectious diseases. 

About 900 known tick species are divided into 23 genera and three families: Ixodidae (Hard Ticks), Argasidae (Soft Ticks), and Nuttalliellidae.

Description

General Body Plan

They lack proper segmentation of their abdomen (or opisthosoma), which, like mites, are fused to their cephalothorax (or prosoma). Like other chelicerates, the tagmata has developed into two main parts: 

1. Gnathosoma (Head)

It contains the mouthparts comprising the feeding apparatus with the two palps, two chelicerae, and a hypostome.

The chelicerae are specialized appendages used for cutting and puncturing the skin, while palps are leg-like appendages that are sensory in function. On the other hand, the hypostome, adapted for piercing and sucking, helps anchor the tick’s mouthparts firmly onto the host’s skin.

2. Idiosoma (Body)

This part contains the walking legs, digestive tract, and reproductive organs. The ventral side of the idiosoma has sclerites (hardened plates) and a gonopore between the fourth pair of appendages. While nymphs and adults bear eight legs, the larval forms hatch with six legs, acquiring the other two only after molting into the nymph stage.

All three families have some distinct anatomical features that set them apart from each other.

Hard Ticks

• In males, a characteristic hard protective ‘scutellum’ covers almost the entire dorsal surface, whereas it is restricted to a small shield-like structure in nymphs and females.
• The nymphs and adults contain a prominent capitulum (mouthparts) used for feeding that projects outward from their body.

Soft Ticks

• Soft ticks have a pear-shaped or oval body with a characteristic soft, leathery cuticle.
• They possess a centrally positioned dorsal plate with ridges projecting slightly above the body surface and have circular depressions, indicating the position where the muscles are attached to the integument.

Nuttalliellidae

• They have a combination of projecting gnathosoma and soft, leathery skin.
• Other distinguishing characteristics include a strongly corrugated integument, the distinct position of the stigmata (pores on the integument), and the lack of setae.

Taxonomy

These arachnids are placed under the Superorder Parasitiformes, within which they are most closely related to Holthyrida, a small order of mites. Among almost 900 species, the majority (over 700) belong to the family Ixodidae. The family Nuttalliellidae, named after the bacteriologist George Nuttall, is monotypic, with the species Nuttalliella namaqua.

• Family: Ixodidae (Hard Ticks)
  • Genus: Africaniella (2 species, Africaniella transversale)
  • Genus: Amblyomma (130 species, including Amblyomma variegatum, Amblyomma cajennense, Amblyomma gervaisi)
  • Genus: Anomalohimalaya (3 species, including Anomalohimalaya lamai)
  • Genus: Archaeocroton (1 species, Archaeocroton sphenodonti (Tuatara Tick))
  • Genus: Bothriocroton (7 species, including Bothriocroton hydrosauri)
  • Genus: Cosmiomma (1 species, Cosmiomma hippopotamensis)
  • Genus: Dermacentor (34 species, including Dermacentor albipictus, Dermacentor andersoni, Dermacentor occidentalis (Pacific Coast Tick))
  • Genus: Haemaphysalis (166 species, including Haemaphysalis longicornis (Asian Longhorned Tick), Haemaphysalis leporispalustris)
  • Genus: Hyalomma (27 species, including Hyalomma marginatum, Hyalomma truncatum, Hyalomma dromedarii (Camel Tick))
  • Genus: Ixodes (246 species, including Ixodes scapularis (Deer Tick), Ixodes hexagonus, Ixodes pacificus (Western Blacklegged Tick))
  • Genus: Margaropus (3 species, including Margaropus winthemi)
  • Genus: Nosomma (2 species, including Nosomma monstrosum)
  • Genus: Rhipicentor (2 species, including Rhipicentor bicornis)
  • Genus: Rhipicephalus (82 species, including Rhipicephalus sanguineus, Rhipicephalus appendiculatus)
  • Genus: Robertsicus (1 species, Robertsicus elaphensis)
  • Genus: Cornupalpatum (Extinct)
  • Genus: Compluriscutula (Extinct)
• Family: Argasidae (Soft Ticks)
  • Genus: Antricola (5 species, including Antricola guglielmonei)
  • Genus: Argas (57 species, including Argas africolumbae, Argas brumpti, Argas persicus)
  • Genus: Nothoaspis (3 species, including Nothoaspis amazoniensis)
  • Genus: Ornithodoros (51 species, including Ornithodoros savignyi, Ornithodoros turicata, Ornithodoros moubata)
  • Genus: Otobius (3 species, including Otobius megnini)
• Family: Nuttalliellidae
  • Genus: Nuttalliella (1 species, Nuttalliella namaqua)
• Family: Deinocrotonidae (Extinct)
• Family: Khimairidae (Extinct)

Distribution

Although ticks are cosmopolitan in distribution, they thrive better in warm, humid climates, given the optimum temperature and moisture required for their metamorphosis.

Habitat

These arachnids inhabit shadowy environments with leaf litter, especially those with an overstory of trees or shrubs. They also prefer ‘ecotones,’ the transitional habitat between woodlands and open areas.

While hard ticks typically reside on the host, soft ticks do not have a fixed dwelling and keep switching between sand, crevices, burrows, or buildings.

Diet

They are obligate hematophages that exclusively feed on vertebrate blood for survival.

Behavior

• Some ticks quickly attach to their hosts’ bodies, while others wander, searching for thinner skin, as found in a mammal’s ears. Once it locates a suitable feeding spot, the tick grasps the host’s skin and punctures its surface. It then inserts its hypostome into the wound to extract blood and releases an anticoagulant or ‘platelet aggregation inhibitor’ to prevent the host’s blood from clotting.
• Hard ticks employ a unique behavior called ‘questing’ to find their hosts. They cling to vegetation with their third and fourth pairs of legs and stretch out their first pair, ready to grasp onto any passing host. While nymphs and smaller species quest close to the ground, adults climb higher into the vegetation, where they have a greater chance of encountering larger hosts.
• Certain ticks, particularly soft ticks, exhibit nidicolous behavior by searching for hosts in their own nests, burrows, or caves. Like questing ticks that actively seek out hosts in vegetation, nidicolous ticks rely on the same sensory cues, such as body heat and odors, to locate hosts within their habitat.

Lifespan

They may survive up to three years, depending on the species.

Reproduction and Life Cycle

All three families of ticks go through the same four stages in their life cycle: egg, larva, nymph, and adult. 

Hard Ticks

Depending on the species, they can possess a one-host, two, or three-host life cycle.

• One-host ticks, like the winter tick (Dermacentor albipictus) and the cattle tick (Boophilus microplus), remain on a single host throughout the larval, nymphal, and adult stages, leaving only to lay eggs. The larvae molt into nymphs that, after engorging on the host’s blood, molt into mature adults that remain on the host to feed and mate. 
• The life cycle of two-host ticks, like Hyalomma anatolicum excavatum, spans two years. The pregnant female drops off her second host in the fall to lay eggs that hatch during winter and attach to their first host during spring.
• Most Ixodids are three-host ticks, typically spanning three years. The female drops off the host to lay thousands of eggs, the larvae of which hatch in winter. These larvae feed and molt into nymphs that seek another host, feed again, drop off, and finally molt into adults. The adult ticks often search for a larger third host, like cattle or humans, for mating.

Soft Ticks

Unlike Ixodids, Argasid ticks may go through up to as many as seven nymphal stages. Mating and egg-laying typically occur away from the host in a secure environment. After hatching, larvae feed on a nearby host for several hours to days, dropping off the host and molting into their first nymphal instars. These nymphs seek out and feed on their second host (often the same as the first host) within an hour. This feeding is repeated until the last nymphal instar is reached, allowing the tick to molt into the adult form.

Nuttalliellidae

Nuttalliella namaqua is thought to have multiple hosts, though very little is known about its life cycle.

Predators

Natural predators of ticks include birds, small mammals like opossums, insects, nematodes, and predatory mites.

Adaptations

1. Depending on the species, tick saliva contains around 1,500 to 3,000 anti-inflammatory proteins, called evasins, that allow them to feed on blood for about 8 to 10 days without the host noticing their presence.
2. The tarsus of a tick’s first pair of legs contains Haller’s organ, a sensory structure crucial for detecting host odors, chemicals, temperature changes, air currents, and even infrared light emitted by their warm-blooded hosts.
3. The cuticle of Ixodids can enlarge as their weight increases during their feeding stage by 200 to 600 times compared to their pre-feeding weight.

Interesting Facts

1. Due to their slow metabolism during dormancy, an adult tick can go without food for a year or up to 430 days. During drought conditions, they can survive up to eighteen weeks without feeding.
2. Surprisingly, ticks can survive in vacuum-like conditions for as long as half an hour.
3. The oldest known tick fossils are those of a bird tick from the Late Cretaceous Period (94 to 90 million years ago). Several ticks from the genera Khimaira and Deinocroton were also unearthed from Burmese amber in Myanmar.