3.2 | Variation in somatic morphology and its
relationship with behavioral niche
The NMDS ordination shows the heavy influence of behavioral niche on
mygalomorph somatic morphology, although evolutionary history also plays
a role (Figure 2). A clear behavioral gradient can be seen, with
opportunistic, web-building taxa representing one extreme of the
morphological/behavioral spectrum in the bottom-left of the ordination,
and burrowers and nest-builders with a trapdoor entrance representing
the other, on the right. Between these two extremes lies opportunists
and burrowers with open entrances (generally clustering slightly left of
center), and burrowers with other entrance modifications besides a
trapdoor (slightly right of center).
Clearly, many aspects of somatic morphology are strongly influenced by
evolutionary history, as many major phylogenetic clades do not overlap,
and the proximity of these clades to one-another is generally reflective
of their phylogenetic relationships (Figure 2). For example, the
Atypoidea are at the bottom of the ordination, separate from the
Avicularioidea (all other Mygalomorphae), and the Crassitarsi
(Nemesioidina + Theraphosoidina) and Venom Clade+ and Domiothelina form
clusters. However, many of these clades are spread widely across
morpho-space from left to right, reflecting the diversity of behavioral
niches that their species inhabit.
Members of different clades with similar burrowing behaviors are often
closer together in morpho-space than members of the same clade that
behave differently, presumably reflecting the convergent evolution of
morphological characters that are adapted to particular behavioral
niches (e.g., see Table 2). For example, those members of the
Antrodiaetidae, Actinopodidae, Stasimopidae, and Bemmeridae that are
burrowers with structurally-modified burrow entrances all cluster closer
to the Domiothelina than to other more closely related taxa that behave
differently. Indeed, the position of taxa in morphospace often mirrors
previous phylogenetic hypotheses based on morphology, for exampleAtrax (Venom clade), which has independently evolved
opportunistic habits and a web-entrance, is recovered close to the
Hexathelidae, the Actinopodidae (Venom Clade) cluster within the
Domiothelina, and the bemmerid genera Spiroctenus andHomostola cluster closest to nemesioid and euctenizid genera
respectively, mirroring their previous taxonomic positions.