bladesofgrass
Offline
• • •
Upload & Sell: Off
|
 p.1 #1 · Satanic Deinopsis portrait (Madagascar) | |
Net casting spiders generally hang suspended close to branches and other low points ready to ambush prey. Therefore this kind of shot can be difficult and it is one that I have been after for a long time. This particular one was suspended in a resting state after having hunted for the night. Marojejy national park, Madagascar.
In a curious quirk of evolution, the Deinopidae or net casting spiders which branched early on from the araneids combine web building with a more active hunting strategy. They build a small rectangular web, stretch it between their front two pairs of legs and dangling motionlessly above their prey, ambush them. Unlike in araneids, the capture silk is replaced with cribellate silk (which gives it a wooly appearance), however it remains just as efficient at trapping prey. Deinopid webs remain similar to the aforementioned orbweaver template with several variations important for prey capture. Dangling as it does, how does a deinopid drop fast enough to ensnare its prey? The answer lies in two modifications to web design. First, a vertical safety thread tethers the web and gives it a slight conical shape while construction is in progress. When the web is completed, the spider holds this high tension safety thread with its second pair of hind legs. When prey passes by, it releases the safety line catapulting the spider forward at great speed. Another difference is the use of a bridge line, built slightly above the capture web. This line is held with with the first pair of hind legs and allows the spider a surface to pull down on to generate a downward force.
Hanging pendulously from her perch, she remains still, her camouflaged form allowing her to blend in seamlessly with the branches overhead. She waits until nightfall when her huge anterior median eyes provide an unrivalled night vision, their lenses with an f/0.58 (f=aperture size, smaller number being large diameter) mean they are able to concentrate light more efficiently than a cat (f/0.9) or an owl (f/1.1). She owes this sensitivity to the light activated molecule rhodopsin, tightly packed into a microvillar membrane (which dramatically increases the surface area). Amazingly, 1500 times as many photons reach the light receptors in her eyes than the rods in our own eyes. She even accomplished this without the presence of a tapetum, a reflective membrane used to concentrate available light in many other nocturnal animals. Her preparation is impressive, she has staked out an ambush location, first having inspected it for loose debris and anything that might entangle her web, next she builds her web tentatively prodding the ground with her foot, ensuring it is set at the proper height. She may have even gone so far as to drop several faecal spots to guide her aim. In this manner she hangs, and patiently waits … An insect passes below, oblivious to the danger above. In a fraction of a second, the safety line has been cut, the web has been stretched 4 times its former size and before the insect even recognizes the danger, it finds itself helplessly trapped, venom coursing through its body. She feeds. However her work is not done. The rhodopsin which enables her unparalleled night vision is so sensitive to light that daytime exposure would actually destroy it. Thus, at dawn, the spider spends the first 2 hours dismantling the light sensitive microvillar membrane and rhodopsin molecules. The latter subsequently migrate behind a protective pigment layer, effectively rendering them less sensitive to light. When dusk falls, the light sensitive membrane is once again renewed, web building is resumed and the hunt can begin anew.
|
