4.3 | Constraints on the mygalomorph evolutionary
landscape
Despite differences in the niche dimensions mentioned above, overall,
mygalomorph life histories are remarkably homogeneous: all are
long-lived, sedentary spiders that live in permanent retreats on or
within the substrate or foliage (Raven, 1985). Because extant members of
the suborder Mesothelae also live this way, it is often assumed to
represent the ancestral life history of extant spiders. In contrast, the
Araneomorphae occupy an incredibly diverse array of niches, and include
aerial-web builders, burrowers, cursorial hunters, and
ambush-specialists living in all types of microhabitats both on and off
the ground (Foelix, 2011). We can therefore gain insight into the
constraints on the mygalomorph adaptive landscape by understanding how
the Araneomorphae have broken free from it.
Key morphological innovations allowing the Araneomorphae to inhabit new
niche space were probably the piriform + ampullate gland-spigot system
(P+A system), and tracheal posterior respiratory systems (Levi, 1967;
Ramírez et al., 2021). The P+A system allows the attachment of
individual silk strands to the substrate or to each other and is crucial
for the use of drag-lines and the construction of complex silk
structures away from the substrate, such as aerial webs (Coddington &
Levi, 1991; Ramírez et al., 2021; Wolff et al., 2019). It is present in
almost all araneomorph spiders, and ancestral state reconstructions have
now confirmed its origins in the ancestor of the group (Ramírez et al.,
2021). Silk glands and spigots of the Mygalomorphae deserve more
attention, but presently, no mygalomorph is known to possess an
equivalent silk-attachment system (Palmer, 1991). This probably means
that, despite their extensive use of silk, they cannot create complex,
load-bearing silk structures away from the substrate.
Tracheal respiratory systems, which have only evolved in the
Araneomorphae, allow oxygen to be directed to muscles where it is needed
most, facilitating localized, energy demanding activities (Levi, 1967;
Ramírez et al., 2021). In their recent study of respiratory system
evolution in spiders, Ramírez et al. (2021) showed that tracheal systems
evolved several times independently and proposed that their original
benefit was directing oxygen to the spinneret muscles to facilitate the
new, energy-expensive spinning procedures associated with the P+A
system. Tracheal systems have, however, been co-opted to direct oxygen
into the prosoma in highly active, hunting groups such as the Dionycha
(Ramírez et al., 2021). Because of their small spiracle openings,
tracheal systems probably also reduce susceptibility to desiccation and
are therefore likely to be adaptive in active, cursorial niches,
especially in small spiders (Levi, 1967). Mygalomorphae possess the
symplesiomorphic posterior respiratory system consisting of a pair of
booklungs. These allow only localized oxygen exchange and have larger
more exposed openings, and this is probably a major constraint limiting
the evolution of active, cursorial niches in the Mygalomorphae.
A final consideration is the ecological constraint of niche
availability. Both the aerial web-building niche, and active, cursorial
niches were inhabited early in araneomorph evolution (Kallal et al.,
2020), and therefore opportunity for mygalomorph ancestors to exploit
these niches would have been limited by direct competition with their
araneomorph relatives. The mygalomorph adaptive landscape is narrow, but
they are well-adapted to their sedentary lifestyle. The substrate-bound,
retreat-building niche has revolved in many araneomorph families (e.g.,
members of the Segestriidae, Filistatidae, Eresidae, Zodoriidae,
Udubidae, Lycosidae, Sparassidae), yet the Mygalomorphae must be thought
of as the masters of this niche space, having remained a major faunal
component within it for over 350 million years (Opatova et al., 2020).