As the sun sets, the high pitched buzz of
cicadas winds down and a chorus of frogs greet the night. An array of
insects and animals with adaptations and appearance so bizarre as to be
almost incredulous emerge. Amongst the oddest are the Archaeids.
These strange spiders go by a variety of
names which reflect their form and behaviour, though they are most
commonly referred to as spider assassins and pelican spiders. The former
epithet refers to their araneophagic diet, while the latter pays homage
to their morphologically unique ‘necks’ (actually an extension of the
cephalothorax). Despite the natural interest which these spiders garner
by their looks alone, little is known about their natural history which
can be explained by a variety of factors; their diminutive size (2-8
mm), nocturnal habits, location under foliage suspended head down, or by
their short-range endemism. Not to
mention that their cryptic colouration, browns and greys, which
facilitate their camouflage as they drop evasively to the leaf litter.
As a result, these spiders are poorly represented in the scientific
literature, leaving it up to the imagination of the reader or better
yet, the observer to fill in the details of their lives.
Even scarcer
are images, most of which show specimens preserved in amber from up to
50 million years ago. These relictual specimens from the Mesozoic era
bear an unmistakable resemblance to present day Archaeidae,
demonstrating a remarkable conservation of phylogenetic traits. These
archaeological findings elucidate a former range which extended up into
the sclerophyll (woody stemmed plants with evergreen leaves) forests and mesic (moisture-rich environment) heathland of the European Baltic, mirrored in the present day ecology of the Austrachaeidae of Australia, to Burma, Kazakstan and the Xunan province, China.
Though no
longer present in Europe, recent findings have shown an extended range
in Australia from montane to tropical rainforest.
As one recedes further and further in
geological time, one can observe a progressive shortening of the ‘neck’
and chelicerae until they resemble more and more the infraorder of
Mygalomorphae (Orthognathae). The chelicerae now appear to move from
side to side rather than up and down as in the Araneomorphae. Therefore,
it may be inferred that the length of the neck has evolved hand in hand with the length of the chelicerae.
Otherwise as the former lengthens, the distance between the mouth and
the chelicerae becomes too great to efficiently transfer the food from
one to the other.
Not only can we see a change in this morphometric ratio through time,
but also across the various species and genera. Even within
the monophyletic graciliosis group (Eriauchenius), the evolution of appropriate ‘neck’ and chelicerae ratios appears to have evolved as two separate events, an interesting example of convergent evolution.
Phylogenetic analyses have placed the
Arachaeidae in 3 genera (Eriauchenius, and Afrarchaea [Madagascar,
Africa], and Austrarchaea [Australia]) with a combined total of 37
species. Most species can be separated based on gross morphological
features present in the pedipalps, the differences in ‘head’ and ‘neck’
(based on various morphometric ratios), eye positions and surface
details of the haplogyne (female) genitalia. However, more discerning
methods include the use of genetic markers such as mitochondrial c
cytochrome oxidase subunit I (COI) and the adjacent COII genes in
combination with 12S and 16S mitochondrial DNA (4).
Despite the
genetic approach being extremely helpful in clarifying genealogy (found
at the base of the Araneomorph lineage, close to where they diverged
from the their sister clade, the Mygalomorphs), the
deconstructionism of phylogenetic and morphological analyses can only
yield helpful insight into the function accompanying the form of
preserved specimens. Therefore, the natural history can often only be
determined by field observation. Up to this point there is only a small representation of live Archaeid photos which best represent our knowledge of in situ behaviour,
and those that exist are divided between the Archaeidae and the sister
family Mecysmaucheniidae. The latter being better represented in the
literature and having an extended range from Southern South America to
New Zealand. Additionally, these spiders lack the araneophagic diet of
the Archaeidae and behave more like other generalist spiders. The
Archaeidae on the other hand are found in Australia, South
Africa and Madagascar with the latter two comprising the lion’s share
of the research (this is perhaps due to the relative abundance of
specimens in these countries compared to Australia which may have more
Archaeid predators).
