PhD Arthur Decae 2010 - Ghent Ecology - Universiteit Gent

forward with great force, and a facility for stabilizing the tunnel walls at its rear end (if it is
tunneling in soft rocks). Now if we look at a mygalomorph spider through the eyes of a civil
engineer we would recognize the perfect TBM (Fig. 2). The only difference between a
mechanical TBM and an orthognate spider is the absence of a conveyor belt to remove the
dug-out material. The problem of soil removal from the tunnel under construction is solved
differently in spiders. The key solution to this problem is that spiders have a particularly
narrow and extremely flexible waist, the pedicel that allows it to pivot in the narrow shaft of
its tunnel and to carry-out particles of dug-out soil.
Observation of a trapdoor spider
excavating its burrow immediately
shows the prime functions of the
orthognate chelicerae (scraping,
digging and carrying soil), the stubby
legs (strong grip on the burrow wall),
abdominal spinnerets (stabilizing the
tunnel behind the spider) and the
narrow pedicel (allowing the spider to
pivot for soil removal).
If we look at spiders in this way it is
clear that they differ very much from
their putative sister group the
Amblypygi. The forward orientation
of the chelicerae may superficially be
the same in orthognate spiders and
the Pedipalpi (Amblypygi + Uropygi
+ Schizomida. Harvey 2003) their
function is fundamentally different.
None of the Pedipalpi use their
chelicerae for digging, in orthognate
spiders however, digging is done solely
with the aid of the fangs and chelicerae.
Comparison of the morphology of the
two hind pairs of legs in orthognate
spiders and the Pedipalpi reveals that in
spiders these legs are short and very
strong, against being quite slender to
very slender in the Pedipalpi. The
pedicel is narrow as well in Pedipalpi
as in orthognate spiders, presumably
because all these animals have to move
in the confines of cracks and crevices,
but although there are no comparative
Fig. 1 Tunnel Boring Machine (TBM). One of many models,
all of basically similar design, offered by construction
companies on the internet for infrastructural tunnel drilling
projects.
Fig. 2 Atypus affinis depicted as a TBM.
data that I know of, I would predict that the pedicel of spiders is much more flexible than that
of the Pedipalpi. Finally, spiders are the only creatures on earth that have developed an
abdominal spinning apparatus, working with extreme precision and ideally suited and placed
for stabilizing loose soil during construction work. Coyle (1981) has studied and described all
the above mentioned aspects of burrow construction behavior for Ummidia in detail. My
personal observations confirm that Nemesia, Iberesia, Cteniza, Cyrtauchenius and
Cyrtocarenum all construct their burrows in an identical manner as is reported for Ummidia.