329
on blood, may mate on the host, and the engorged female usually produces thousands of eggs. Different instars often use different hosts, such that three-host lifecycles are commonplace while two- or one-host lifecycles are rarer (Estrada-Peña
and de la Fuente 2014). Ticks usually live dormant in the undergrowth, but specific
environmental conditions (e.g. weather or photoperiod) induce ‘questing’ behaviour
in which the parasites typically climb up grass stems, or similar vegetation, and wait
for a host to brush by. Once on the host they seek a suitable feeding site, ‘cement’
their serrated mouthparts into the host’s skin and then subsequently pass compounds
from the salivary glands into the host which help maintain a constant flow of blood
during feeding (Kazimírova and Štibrániová 2013). Any pathogens in the host’s
blood enter the gut of the tick. In order to be transferred to new hosts these pathogens must migrate into the salivary glands, where they will enter a new host during
the next round of feeding behaviour. This is referred to as trans-stadial (or horizontal) transmission. In some cases the pathogen migrates into the ovaries and eggs
where it is passed onto a new generation of (already infected) ticks. This is referred
to as trans-ovarial (or vertical) transmission. Pathogens transmitted by ticks include
viruses, bacteria and protozoans, and they are significant vectors of conditions such
as typhus or Rocky Mountain spotted fever (both caused by Rickettsia bacteria) and
Lyme disease (borreliosis caused by spirochetes) among others; see e.g. de la Fuente
et al. 2008; Poinar 2021 for a review.
Extant ticks can be divided into three families. Nuttalliellidae is restricted to
Africa and has a single species that is probably sister to the remaining extant ticks.
Argasidae are often referred to as soft ticks and occasionally also as bird ticks,
although this is somewhat misleading as several use mammals as hosts. Ixodidae are
the hard ticks and have a shield-like dorsal surface, or scutum. Nuttalliellidae lacks
a fossil record, but both Argasidae and Ixodidae can be traced back to the Cretaceous.
The fossil record of the ticks can be summarised (form oldest to youngest) as
follows:
1. One of the oldest ticks is Deinocroton draculi (Fig. 9.5a) from Cretaceous (ca.
100 Ma) Burmese amber (Peñalver et al. 2017). These authors placed the inclusions in a new, extinct family Deinocrotonidae which appears to be more closely
related to Nuttalliellidae. Deinocroton is associated with specialised setae (hastisetae) belonging to the larvae of dermestid beetles, modern examples of which
are typically inhabitants of bird nests. This may imply that these extinct ticks
also inhabited the nests of feathered dinosaurs and used them as hosts.
2. The oldest hard ticks (Ixodidae) also come from Burmese amber, from which
four genera have now been recognised. Cornupalpatum (Fig. 9.5d) and
Compluriscutula are extinct taxa, described by Poinar Jr and Brown (2003) and
Poinar Jr and Buckley (2008), respectively. Poinar Jr (2015a) subsequently
claimed to have observed rickettsial-like cells in Cornupalpatum Poinar (2021).
A nymph of the same genus was recorded grasping a pennaceous feather in
Burmese amber (Peñalver et al. 2017). This strongly suggests it used either nonavian feathered dinosaurs and/or early birds as hosts.
9 Chelicerates as Parasites
on blood, may mate on the host, and the engorged female usually produces thousands of eggs. Different instars often use different hosts, such that three-host lifecycles are commonplace while two- or one-host lifecycles are rarer (Estrada-Peña
and de la Fuente 2014). Ticks usually live dormant in the undergrowth, but specific
environmental conditions (e.g. weather or photoperiod) induce ‘questing’ behaviour
in which the parasites typically climb up grass stems, or similar vegetation, and wait
for a host to brush by. Once on the host they seek a suitable feeding site, ‘cement’
their serrated mouthparts into the host’s skin and then subsequently pass compounds
from the salivary glands into the host which help maintain a constant flow of blood
during feeding (Kazimírova and Štibrániová 2013). Any pathogens in the host’s
blood enter the gut of the tick. In order to be transferred to new hosts these pathogens must migrate into the salivary glands, where they will enter a new host during
the next round of feeding behaviour. This is referred to as trans-stadial (or horizontal) transmission. In some cases the pathogen migrates into the ovaries and eggs
where it is passed onto a new generation of (already infected) ticks. This is referred
to as trans-ovarial (or vertical) transmission. Pathogens transmitted by ticks include
viruses, bacteria and protozoans, and they are significant vectors of conditions such
as typhus or Rocky Mountain spotted fever (both caused by Rickettsia bacteria) and
Lyme disease (borreliosis caused by spirochetes) among others; see e.g. de la Fuente
et al. 2008; Poinar 2021 for a review.
Extant ticks can be divided into three families. Nuttalliellidae is restricted to
Africa and has a single species that is probably sister to the remaining extant ticks.
Argasidae are often referred to as soft ticks and occasionally also as bird ticks,
although this is somewhat misleading as several use mammals as hosts. Ixodidae are
the hard ticks and have a shield-like dorsal surface, or scutum. Nuttalliellidae lacks
a fossil record, but both Argasidae and Ixodidae can be traced back to the Cretaceous.
The fossil record of the ticks can be summarised (form oldest to youngest) as
follows:
1. One of the oldest ticks is Deinocroton draculi (Fig. 9.5a) from Cretaceous (ca.
100 Ma) Burmese amber (Peñalver et al. 2017). These authors placed the inclusions in a new, extinct family Deinocrotonidae which appears to be more closely
related to Nuttalliellidae. Deinocroton is associated with specialised setae (hastisetae) belonging to the larvae of dermestid beetles, modern examples of which
are typically inhabitants of bird nests. This may imply that these extinct ticks
also inhabited the nests of feathered dinosaurs and used them as hosts.
2. The oldest hard ticks (Ixodidae) also come from Burmese amber, from which
four genera have now been recognised. Cornupalpatum (Fig. 9.5d) and
Compluriscutula are extinct taxa, described by Poinar Jr and Brown (2003) and
Poinar Jr and Buckley (2008), respectively. Poinar Jr (2015a) subsequently
claimed to have observed rickettsial-like cells in Cornupalpatum Poinar (2021).
A nymph of the same genus was recorded grasping a pennaceous feather in
Burmese amber (Peñalver et al. 2017). This strongly suggests it used either nonavian feathered dinosaurs and/or early birds as hosts.
9 Chelicerates as Parasites