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scorpions for example, or shaped like a pocket-knife as in spiders, in which a movable fang articulates against a basal element. The chelicerae are typically used to
bite and/or masticate food items, and at least in spiders they bear the openings of the
venom glands.
Compared to predation, parasitism among chelicerates is fairly rare. In terms of
definition (e.g., Esch and Fernández 1993) a parasite is taken here to mean an animal that lives in intimate association with another organism, and benefits from this
relationship while harming the host. There are some problems with this definition in
that the amount of ‘harm’ caused is relative, and it is important to differentiate parasitism from a one-off predatory attack which kills (and consumes) the host. Spiders
can autotomise a leg grabbed by a predator and escape their attacker alive, but this
non-lethal attack does not make the predator a parasite. Parasitism may alternatively
be envisaged as a relationship characterised by the parasite generally being smaller
than the host and to a certain degree host-specific and reliant on the host for nourishment. In general, parasites only feed on part of the host’s tissues. This is usually
non-lethal, although large-scale infestation may reduce the host’s fitness or even
become fatal. In this context, we should also reiterate the distinction between a
parasite and a parasitoid. A parasitoid spends much of its life cycle on (or in) the
body of the host and infection is invariably fatal, such as a wasp larvae consuming
a caterpillar and eventually erupting from its body. A handful of mites can be considered parasitoids (see Sects. 9.4.1 and 9.4.2).
Parasitism in chelicerates is restricted to the larvae of several sea spiders (Sect.
9.2), as well as a number of subgroups of mites (Sects. 9.4.1 and 9.4.2) within the
arachnids. Ticks are perhaps the most obvious and familiar examples of arachnid
parasites, and are of considerable economic importance as disease vectors. Other
mite groups are also parasitic, at least at some stage of their life cycle. Chiggers,
feather mites, scabies mites and the Varroa mite affecting honeybees are further
examples of arachnid parasites which are significant for humans (Diaz 2010:
Table 1) or domestic animal husbandry.
9.1.1 The Chelicerate Fossil Record
The significance of the fossil record for understanding parasitism was reviewed by
De Baets and Littlewood (2015); De Baets et al. (2021a) and Leung (2017), who
discussed, among other subjects, the value and limitations of fossil data for calibrating evolutionary frameworks and investigating co-evolutionary relationships
between hosts and their parasites: see also Warnock and Engelstädter (2021).
Although focussing in insects, chelicerates were mentioned as parasitic terrestrial
arthropods by Nagler and Haug (2015). The fossil record of Chelicerata in general
was reviewed by Dunlop (2010), Dunlop and Penney (2012), and more recently by
Wolfe et al. (2016) as part of a wider survey of the arthropods. For a review of recent
developments in chelicerate phylogeny see Giribet (2018).
J. A. Dunlop
scorpions for example, or shaped like a pocket-knife as in spiders, in which a movable fang articulates against a basal element. The chelicerae are typically used to
bite and/or masticate food items, and at least in spiders they bear the openings of the
venom glands.
Compared to predation, parasitism among chelicerates is fairly rare. In terms of
definition (e.g., Esch and Fernández 1993) a parasite is taken here to mean an animal that lives in intimate association with another organism, and benefits from this
relationship while harming the host. There are some problems with this definition in
that the amount of ‘harm’ caused is relative, and it is important to differentiate parasitism from a one-off predatory attack which kills (and consumes) the host. Spiders
can autotomise a leg grabbed by a predator and escape their attacker alive, but this
non-lethal attack does not make the predator a parasite. Parasitism may alternatively
be envisaged as a relationship characterised by the parasite generally being smaller
than the host and to a certain degree host-specific and reliant on the host for nourishment. In general, parasites only feed on part of the host’s tissues. This is usually
non-lethal, although large-scale infestation may reduce the host’s fitness or even
become fatal. In this context, we should also reiterate the distinction between a
parasite and a parasitoid. A parasitoid spends much of its life cycle on (or in) the
body of the host and infection is invariably fatal, such as a wasp larvae consuming
a caterpillar and eventually erupting from its body. A handful of mites can be considered parasitoids (see Sects. 9.4.1 and 9.4.2).
Parasitism in chelicerates is restricted to the larvae of several sea spiders (Sect.
9.2), as well as a number of subgroups of mites (Sects. 9.4.1 and 9.4.2) within the
arachnids. Ticks are perhaps the most obvious and familiar examples of arachnid
parasites, and are of considerable economic importance as disease vectors. Other
mite groups are also parasitic, at least at some stage of their life cycle. Chiggers,
feather mites, scabies mites and the Varroa mite affecting honeybees are further
examples of arachnid parasites which are significant for humans (Diaz 2010:
Table 1) or domestic animal husbandry.
9.1.1 The Chelicerate Fossil Record
The significance of the fossil record for understanding parasitism was reviewed by
De Baets and Littlewood (2015); De Baets et al. (2021a) and Leung (2017), who
discussed, among other subjects, the value and limitations of fossil data for calibrating evolutionary frameworks and investigating co-evolutionary relationships
between hosts and their parasites: see also Warnock and Engelstädter (2021).
Although focussing in insects, chelicerates were mentioned as parasitic terrestrial
arthropods by Nagler and Haug (2015). The fossil record of Chelicerata in general
was reviewed by Dunlop (2010), Dunlop and Penney (2012), and more recently by
Wolfe et al. (2016) as part of a wider survey of the arthropods. For a review of recent
developments in chelicerate phylogeny see Giribet (2018).
J. A. Dunlop