Assassin Spider (Archaeidae)

 

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).