4.1 | Insights into the function of convergent
morphological features
This study is the first to quantify the strong correlation between
behavioral niches and a suite of convergent morphological features
within the Mygalomorphae. In particular, features of the spinnerets, leg
chaetotaxy, and eye group, as well as the rastellum and serrula, exhibit
strong patterns of correlation with behavior, and an examination of
their likely function provides insights into the potential drivers of
convergent evolution within the group.
Spinnerets. Elongate, widely spaced posterior lateral spinnerets
are correlated with web-building (Table 2; Fig. 3). Their length
presumably allows for the efficient application of wide swathes of silk
during the construction and repair of capture webs, as has been observed
in Linothele (Eberhard & Hazzi, 2013; Nicolás Paz, 1988). Their
widely separated position likely also aids in the independent,
unilateral or asymmetrical use of each spinneret during
web-construction, for example during the attachment of individual silk
sheets (as observed in Linothele macrothelifera (Eberhard &
Hazzi, 2013). In contrast, very short apical segments of the PLS (and
short spinnerets in general), are correlated with structural
modification of the retreat entrance (Table 2; Fig. 3) and are probably
better for the precise application of strong, thin bands of silk (as
observed in Ummidia : Coyle, 1981). The precise application of
silk may be important for the integrity of these entrance structures,
for example in the construction of a trapdoor hinge, or in the
substrate/silk-matrix of a trapdoor or turret (Coyle, 1981; Coyle et
al., 1992). During burrow and burrow-entrance construction, these short
spinnerets have been observed to work together synchronously and/or
rhythmically, usually applying silk to the same area, explaining their
position close together on the abdomen in these species (Coyle et al.,
1992; Mayo, 1988).
Rastellum and serrula . The rastellum is strongly correlated with
both burrowing and door construction (Table 2; Fig. 3). Observations of
burrowing taxa indicate that it is used for compaction of the burrow
shaft and entrance structures (Coyle, 1981; Coyle et al., 1992) plus
excavation (Gertsch, 1949; Nascimento et al., 2021). However, both
burrowing and entrance modification occur in taxa that don’t possess a
rastellum (e.g., Theraphosidae and Migidae, respectively), suggesting
that other factors may also influence whether the structure is
necessary, for example the substrate in which the spider burrows. The
function of the serrula in spiders is generally assumed to involve
manipulation of prey items (Jocqué & Dippenaar-Schoeman, 2006). We
found that it was positively correlated with opportunistic retreats, and
negatively correlated with burrowing (Table 2; Fig. 3). The functional
reasons for this are unclear, although a speculative explanation for the
negative correlation of the serrula with burrowing could be a tendency
for it to become clogged with substrate while burrowing, because
substrate is carried using the chelicerae/pedipalps during burrow
construction, and so would likely come into contact with the serrula
(Coyle, 1974, 1981; Mayo, 1988).
Leg chaetotaxy. Surprisingly, the so-called ‘digging spines’ –
strong lateral spines on the anterior legs and pedipalps, did not show a
positive correlation with digging, but only with burrow-entrance
modification (Table 2; Fig. 3). That digging is not the primary role of
these spines is supported by behavioral studies of burrowing taxa that
observed that the chelicerae and fangs are used for substrate
excavation, not the legs (Coyle, 1981; Coyle et al., 1992; Mayo, 1988;
Nascimento et al., 2021). Furthermore, some taxa that do not burrow
(e.g., many Migidae) still possess these spines, although they have lost
other features associated with burrowing (e.g., pro-dorsal spine patches
on patella III). We suggest that these spines function primarily during
prey capture in species with modified burrow entrances, which tend to
have smaller foraging areas (Main, 1982) and hunt by lunging from the
burrow entrance and restraining prey with the anterior legs and
pedipalps (Coyle, 1981, 1986; Hils & Hembree, 2015). Although no
correlation was found between scopulae and behavior, in taxa that modify
the burrow entrance scopulae clearly replace the function of digging
spines because the only entrance-modifying taxa without digging spines
possess scopulae, adding to the well-supported hypothesis that a
function of both structures is to restrain prey (e.g., see Eggs et al.,
2015; Pekár et al., 2011; Wolff & Gorb, 2016).
Enlarged posterior legs, a dorsal bias in spine position on the
posterior legs, and the presence of pro-dorsal thorn patches on patella
III are all correlated with both burrowing and burrow entrance
modification (Table 2; Fig. 3). Behavioral studies on several burrowing
species indicate that the posterior legs are braced against the burrow
wall to anchor the spider (Bond & Coyle, 1995; Coyle, 1981; Decae &
Bosmans, 2014; Hils and Hembree, 2015). This is done during routine
movement, but also serves a defensive function in species that hold
their burrow entrance shut when disturbed. Larger, stronger posterior
legs and dorsal spines likely enhance this bracing function.
Eye group. The eye tubercle was found to be positively correlated
with opportunistic burrowing, and a standard, compact, rectangular eye
group was found to be negatively correlated with burrow entrance
modification (indicating that change from this state generally occurs in
taxa with modified entrances) (Table 2; Fig. 3). It seems most probable
that these changes in the eye group relate to the amount and direction
of light exposure (and therefore visual information) in different
retreat types, for example, almost all opportunist taxa have relatively
open retreat entrances, and when foraging at the retreat entrance, would
be exposed to light from all directions. In contrast, burrowing taxa
with modified entrances would be exposed to light from only one
direction (the entrance), and to far less light in general. This is,
however, in contrast to several previous studies which indicate that
vision is not important for foraging in a range of mygalomorph species
(see Coyle, 1986 for a list of relevant literature).