The Spider Assassin (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 (1), 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 rainforests (2).
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).
Bearing this in mind, an excellent dichotomous key with some distinguishing morphological characteristics (the colour of the bars represents the mean morphometric ‘neck’/chelicera ratios) has been created for 15 species of the Eriauchenius genus.
Perhaps the most striking features of this family are the elongated ‘neck’ and slender chelicerae (jaws) with recursive fangs, which together with the head comprise the seemingly teetering cephalothorax. Of note are also the long delicate legs with modified tarsi that enable the legs to extend beyond even the chelicerae. Each of these features; however seemingly bizarre has a specific purpose essential to its predatory, araneophagic diet. The aptly named spider assassin, nomadic in nature and lacking a web of its own can often be found in the leaf litter foraging for prey. As it navigates this landscape, the spider assassin is particularly attuned to finding the draglines of other spiders (a silken line which serves as a safety line) by mechano- and perhaps chemosensory means. With its long forelegs, it traces the path like a skilled hunter until it happens upon its hapless prey and spears it with its long forceps-like chelicerae.
However, Archaeids may also go so far as to invade the webs of other Araneomorphs. With its anterior legs barely touching the outermost draglines of another spider’s web, it may strum the silken lines like a siren, its deadly tune irresistible to its prey. Archaeid pattern recognition of spider-prey courtship web strumming has not yet been been thoroughly investigated. As such there is no evidence that it approaches the complexity of the Portia spp. jumping spiders’ remarkable interspecies differentiation. However, like the Portia they appear to be quite adaptable and opportunistic. This is demonstrated by their boldness in plucking prey right out of their own webs. They achieve this dexterous act of negotiating their prey’s web thanks to the length of their legs, which essentially act as shock absorbers. Simply put, this aspect of their morphology minimizes the amplitude of disturbance such that it is no greater than that normally attributed to more benign natural causes (wind, rain, etc.).
Due to the relatively poor eyesight of most Araneomorphs, the Archaeid is able to approach within striking distance without alerting its prey. Thanks to its disproportionately long chelicerae, it impales its prey without exposing itself to collateral damage. Able to move both horizontally and vertically, the jaws close upon the prey like a vice. The fangs pierce the exoskeleton, and venom is pumped into the other spider. As the poison circulates in the hemolymph, the prey struggles even more violently but the hooked fangs remain embedded. Next, in a still unexplained behaviour, one chelicera lowers 90 degrees in a stereotyped action. Meanwhile the other chelicera holds the prey at a safe distance as it continues to thrash about in its death throes. Minutes pass and the struggling becomes weaker and weaker until it stops entirely. There may be a sporadic jerk, autonomic neurons releasing their final action potentials. But it is now safe to consume. The Archaeid lowers its meal to its mouth and feeds. The proteolytic enzymes in the venom have had time to work, and have broken down the internal organs, rendering them sufficiently liquid to imbibe.
The concomitant lowering of a single chelicera with feeding as aforementioned has yet to be explained. However, a couple plausible theories might account for this behaviour: 1) lowering one chelicera minizes the risk of injury from the potentially dangerous flailing movements of the dying prey or 2) conservation of energy (imagine holding 1 arm out in front of you instead of two).
[Please note that these conclusions are drawn from my own field experience and observations, though I did find a corresponding lecture video seen below (as presented by researcher Hannah Wood) which corroborates my own reasoning. Unfortunately there seems to be a dearth of readily available information with regards to the natural history of Archaeidae.]
At rest, Archaeids dangle head down under leaves or between branches with a dragline or two to support them. This position seems to be the most comfortable to accomodate their unique form (Nb. most online photos have been rotated for easier viewing, often without mention that this is not the in situ behaviour).
Even more poorly understood is the courtship and mating behaviour of Archaeids. The extent of my personal experience is the observation of a single female holding an egg sac well above her head with one of its mid-legs. She was able to maintain this posture while galloping along the bottom of a leaf.
Their cryptic nature and seemingly low abundance have kept Archaeids as one of the best kept secrets of the arachnid world. Fortunately, recent photos (J. Miller, 2008) have spotlighted this fantastic creature, which has since appeared in diverse fora easily accessible to the public. Hopefully this newfangled celebrity will encourage further study and illuminate the many mysteries surrounding one of the strangest spiders on the planet.
As more information becomes available, I will update this Article.
1) Penney D. Afrarchaea grimaldii, a new species of Archaeidae (Araneae) in Cretaceous Burmese amber. Journal of Arachnology 2003: unit 31: pp.122-130. doi:10.1636/0161-8202(2003)031[0122:AGANSO]2.0.CO;2
2) Michael G. Rix, and Mark S. Harvey.Australian Assassins, Part I: A review of the Assassin Spiders (Araneae, Archaeidae) of mid-eastern Australia. ZooKeys 123: 1–100, doi: 10.3897/zookeys.123.1448. http://www.pensoft.net/journals/zookeys/article/1448/australian-assassins-part-i-a-review-of-the-assassin-spiders-araneae-archaeidae-of-mid-eastern-australia.
3) A revision of the assassin spiders of the Eriauchenius gracilicollis group, a clade of spiders endemic to Madagascar (Araneae: Archaeidae). Hannah, Wood. 2008. The Linnean Society of London, Zoological Journal of the Linnean Society, 2008, 152, 255–296. http://onlinelibrary.wiley.com/doi/10.1111/j.1096-3642.2007.00359.x/abstract
4) Wood HM, Griswold CE, Spicer GS. Phylogenetic relationships withing an endemic group of Malagasy ‘assassin spiders’ (Araneae, Archaeidae): ancestral character reconstruction, convergent evolution and biogeography. Mol Phylogenet Evol. 2007 Nov;45(2):612-9. Epub 2007 Jul 19. http://www.ncbi.nlm.nih.gov/pubmed/17869131.