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).