322
horseshoe crabs (Botton et al. 2003) is probably a fair guide to the mode of life in
early euchelicerates. Living horseshoe crabs generally use their prosomal legs to dig
worms and molluscs out of the substrate, masticate them with the spiny gnathobases
at the base of these legs and then transfer the resulting sediment with food particles
forwards to the mouth via a current of water. In other words they are active predators, with no evidence for parasitism, and it seems reasonable to assume that early
fossil euchelicerates from the mid-Palaeozoic—which had a similar general body
plan—were not parasites either. Eurypterids also had gnathobases (Selden 1981),
although some achieved quite large body sizes of more than 2 m (Lamsdell and
Braddy 2010) and had correspondingly huge chelicerae (mouthparts), which suggests they may have been able to capture larger prey items such as fish. Again, a
parasitic mode of life seems highly unlikely and none of the known eurypterids
known show any morphological adaptations that would support this hypothesis. The
feeding ecology of the extinct chasmataspidids is equivocal. Given their general
morphological similarities to both horseshoe crabs and eurypterids a similar mode
of life would be expected.
9.4 Arachnids
Like horseshoe crabs and eurypterids, arachnids are predominantly predatory
arthropods (Beccaloni 2009). Prey capture is normal for groups like spiders, scorpions and pseudoscorpions, and several evolutionary novelties such as modified limbs
(claws, spines) or venom have evolved to help them entrap and subdue their victims.
A predatory mode of life seems likely for the Palaeozoic ancestors of the arachnids
and spider-like biting mouthparts have been documented in, for example, exceptionally preserved Early Devonian fossils belonging to the extinct arachnid order
Trigonotarbida (Haug 2017). Note that some spiders practice kleptoparasitism,
stealing food from the webs of larger spider species (e.g. van Helsdingen 2011), but
this behaviour should not be counted as parasitism sensu stricto. Harvestmen primarily capture live prey, but can have a more varied diet (Acosta and Machado
2007) supplemented by material such as decaying animals, lichen, fungi and even
bird droppings. However, as these authors note, one should not make generalisations about all harvestmen since individual taxa have individual food-preferences. It
may also be worth noting in this context that while most arachnids are liquidfeeders, harvestmen are still able to ingest solid particles.
The other exception to the general predacious rule among the arachnids are the
mites. These arachnids tend to have smaller body sizes and have evolved to present
a wide range of ecologies, which include both predatory and non-predatory lifestyles. They are also the only arachnids to have evolved parasitic behaviour and will
thus be the focus of the remainder of this chapter. Their fossil record was recently
summarised by Sidorchuk (2018) who argued that they have been small throughout
their geological history. It is important to point out that while all mites were traditionally grouped as a single order, Acari, it has become apparent that there are two
J. A. Dunlop
horseshoe crabs (Botton et al. 2003) is probably a fair guide to the mode of life in
early euchelicerates. Living horseshoe crabs generally use their prosomal legs to dig
worms and molluscs out of the substrate, masticate them with the spiny gnathobases
at the base of these legs and then transfer the resulting sediment with food particles
forwards to the mouth via a current of water. In other words they are active predators, with no evidence for parasitism, and it seems reasonable to assume that early
fossil euchelicerates from the mid-Palaeozoic—which had a similar general body
plan—were not parasites either. Eurypterids also had gnathobases (Selden 1981),
although some achieved quite large body sizes of more than 2 m (Lamsdell and
Braddy 2010) and had correspondingly huge chelicerae (mouthparts), which suggests they may have been able to capture larger prey items such as fish. Again, a
parasitic mode of life seems highly unlikely and none of the known eurypterids
known show any morphological adaptations that would support this hypothesis. The
feeding ecology of the extinct chasmataspidids is equivocal. Given their general
morphological similarities to both horseshoe crabs and eurypterids a similar mode
of life would be expected.
9.4 Arachnids
Like horseshoe crabs and eurypterids, arachnids are predominantly predatory
arthropods (Beccaloni 2009). Prey capture is normal for groups like spiders, scorpions and pseudoscorpions, and several evolutionary novelties such as modified limbs
(claws, spines) or venom have evolved to help them entrap and subdue their victims.
A predatory mode of life seems likely for the Palaeozoic ancestors of the arachnids
and spider-like biting mouthparts have been documented in, for example, exceptionally preserved Early Devonian fossils belonging to the extinct arachnid order
Trigonotarbida (Haug 2017). Note that some spiders practice kleptoparasitism,
stealing food from the webs of larger spider species (e.g. van Helsdingen 2011), but
this behaviour should not be counted as parasitism sensu stricto. Harvestmen primarily capture live prey, but can have a more varied diet (Acosta and Machado
2007) supplemented by material such as decaying animals, lichen, fungi and even
bird droppings. However, as these authors note, one should not make generalisations about all harvestmen since individual taxa have individual food-preferences. It
may also be worth noting in this context that while most arachnids are liquidfeeders, harvestmen are still able to ingest solid particles.
The other exception to the general predacious rule among the arachnids are the
mites. These arachnids tend to have smaller body sizes and have evolved to present
a wide range of ecologies, which include both predatory and non-predatory lifestyles. They are also the only arachnids to have evolved parasitic behaviour and will
thus be the focus of the remainder of this chapter. Their fossil record was recently
summarised by Sidorchuk (2018) who argued that they have been small throughout
their geological history. It is important to point out that while all mites were traditionally grouped as a single order, Acari, it has become apparent that there are two
J. A. Dunlop