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Background Information on the
Biology of Ticks
Larisa Vredevoe, Ph.D, Deparment
of Entomology, University of California, Davis
Ticks are blood feeding external parasites of mammals, birds,
and reptiles throughout the world. Approximately 850 species have been described
worldwide (Furman and Loomis 1984). There are two well established families of
ticks, the Ixodidae (hard ticks), and Argasidae (soft ticks). Both are important
vectors of disease causing agents to humans and animals throughout the world.
Ticks transmit the widest variety of pathogens of any blood sucking arthropod,
including bacteria, rickettsiae, protozoa, and viruses. Some human diseases of
current interest in the United States caused by tick-borne pathogens include
Lyme disease, ehrlichiosis, babesiosis, rocky mountain spotted fever, tularemia,
and tick-borne relapsing fever.
Soft tick (left) and Hard
tick (right)
Hard Ticks: Family Ixodidae
Hard ticks have three distinct
life stages. Larvae which emerge from the egg have six legs. After obtaining a
blood meal from a vertebrate host, they molt to the nymphal stage and acquire
eight legs. Nymphs feed and molt to the next and final stage - the adult, which
also has eight legs. After feeding once more, the adult female hard ticks lay
one batch of thousands of eggs and then die. Only one blood meal is taken during
each of the three life stages. The time to completion of the entire life cycle
may vary from less than a year in tropical regions to over three years in cold
climates, where certain stages may enter diapause until hosts are again
available. Many hard ticks can go for several months without feeding if not
unduly duressed by environmental conditions.
The mouthparts of hard ticks
are readily visible from above. There are three visible components: the two
outside jointed parts are the highly mobile palps; between these are
paired chelicerae, which protect the center rod-shaped structure, the
hypostome. The palps move laterally while the tick is feeding and do not
enter the skin of the host. The rough hypostome has many beak-like projections
on it. This is the structure which plunges into the host's skin while feeding.
The backward directed projections prevent easy removal of the attached tick. In
addition, most hard ticks secrete a cement-like substance produced by the
salivary glands which literally glues the feeding tick in place; the substance
dissolves after feeding is complete.
Scanning
Electron Micrograph
of Tick Mouthparts
Hard ticks seek hosts by an interesting behavior called
"questing." Questing ticks crawl up the stems of grass or perch on the edges of
leaves on the ground in a typical posture with the front legs extended,
especially in response to a host passing by. Certain biochemicals such as carbon
dioxide as well as heat and movement serve as stimuli for questing behavior.
Subsequently, these ticks climb on to a potential host which brushes against
their extended front legs. Hard ticks are most commonly collected for research
by the use of "flags" or "drags" which are made from 1 m square pieces of
roughly textured fabric such as fleece or flannel attached to a rod handle. The
flags are slowly dragged across the surface of vegetation to collect questing
ticks. 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. 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 (Sonenshine 1991).
Questing Hard Tick
(Ixodes scapularis)
Lifecycles of Hard Ticks
Hard ticks have a variety of life histories with respect to
optimizing their chance of contact with an appropriate host to ensure survival.
Some ticks feed on only one host throughout all three life stages. These ticks
are called one host ticks. This type of tick remains on one host during
the larval and nymphal stages, until they become adults, and females drop off
the host after feeding to lay their batch of eggs. Other ticks feed on two hosts
during their lives and are called two host ticks. This type of tick feeds
and remains on the first host during the larval and nymphal life stages, and
then drops off and attaches to a different host as an adult for its final blood
meal. The adult female then drops off after feeding to lay eggs. Finally, many
ticks feed on three hosts, one during each life stage, and are appropriately
named three host ticks. These ticks drop off and reattach to a new host
during each life stage, until finally the adult females lay their batch of eggs.
In each case, the fed adult stage is terminal, that is, after laying one batch
of eggs the female dies, and after the male has reproduced, he dies as well.
 
One Host Tick
Lifecycle Two Host Tick Lifecycle
Three Host Tick Lifecycle
Soft Ticks: Family Argasidae
The life stages of soft ticks are not readily
distinguishable. The first life stage to come out of the egg, a six legged
larva, takes a blood meal from a host, and molts to the first nymphal stage.
Unlike hard ticks, many soft ticks go through multiple nymphal stages, gradually
increasing in size until the final molt to the adult stage. Some soft ticks pass
through up to seven nymphal molts before they become adults. Soft ticks feed
several times during each life stage, and females lay multiple small batches of
eggs between blood meals during their lives. The time to completion of the
entire life cycle is generally much longer than that of hard ticks, lasting over
several years. Additionally, many soft ticks have an uncanny resistance to
starvation, and can survive for many years without a blood meal (Furman and
Loomis 1984).
The mouthparts of soft ticks are are not readily visible from
above. There are three visible components: the two outside jointed parts are the
highly mobile palps; between these are paired chelicerae, which
protect the center rod-shaped structure, the hypostome. The palps move
laterally while the tick is feeding and do not enter the skin of the host. The
rough hypostome has many beak-like projections on it. This is the structure
which plunges into the host's skin while feeding. The backward directed
projections prevent easy removal of the attached tick.
Some soft ticks seek hosts by questing on low-lying
vegetation, but the vast majority are nest parasites, residing in sheltered
environments such as burrows, caves, or nests. Certain biochemicals such as
carbon dioxide as well as heat and movement serve as stimuli for host seeking
behavior. Soft ticks feed for short periods of time on their hosts, varying from
several minutes to days, depending on such factors as life stage, host type, and
species of tick. The feeding behavior of many soft ticks can be compared to that
of fleas or bedbugs, as once established, they reside in the nest of the host,
feeding rapidly when the host returns and disturbs the contents. The outside
surface, or cuticle, of soft ticks expands, but does not grow to accommodate the
large volume of blood ingested, which may be anywhere from 5-10 times their
unfed body weight (Sonenshine 1991).
Soft ticks can be readily collected via dry ice traps. Blocks
of dry ice emit large amounts of carbon dioxide, a host seeking stimulant. Traps
are set in and around nesting areas of animal hosts. Soft ticks can be observed
running along the surface of the ground towards the trap and are collected by
hand, or inside a collection chamber in the trap. Many types of soft ticks can
be artificially fed in the laboratory, thus reducing or eliminating the use of
animals for certain studies. Some (ie: Ornithodorus coriaceus) will feed
through sausage casings filled with almost any type of mammal blood heated up to
37° C. Others can be fed blood through various types of membranes in the
presence of biochemical and environmental feeding stimulants such as ATP, carbon
dioxide, heat, and animal fur (Sonenshine 1993).
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