Arachnids include a tremendous number of carnivorous species, which are good predators of many other taxa. Some tropical tarantula are called Avicularia (e.g. Avicularia avicularia), indicating that they can feed on birds, most often on young birds. Such efficiency can be linked to some of the weapons possessed by some orders of arachnid.
Present in spiders, scorpions but also in small pseudoscorpions, venom is neither produced nor delivered by the same structures. In spiders, the inoculation apparatus is found on the chelicerae, namely on the anterior pair of oral appendages consisting of the coxa and the hook. In scorpions, venom is delivered by the telson and the hook located on the last section of the tail, whereas in pseudoscorpions it is inoculated by a hook on the end of the pedipalps. In the predation process, venom contains a mixture of molecules (up to 1,000 different ones) that allows the predator, sometimes much smaller than its prey,to overpower it and kill it. The biochemical cocktail very quickly paralyses the prey and prevents it from escaping and getting out of the hunter’s reach.
From an evolutionary point of view, it can be seen that in many species, venom is more effective on the usual prey, but is shown to be much less effective on other animal groups. However, while venom is certainly a weapon of attack, it is also a defensive weapon against species that might prey on arachnids. As we shall see later, humans sometimes pay the price. Finally, it should also be noted that venom is considered to contain molecules that initiate extracorporeal digestion of the prey. Indeed, when feeding,arachnids do not have the capacity to ingest “mouthfuls” of their prey but rather to chew their prey by injecting digestive enzymes so as to reduce it to liquefied “mush” that they can swallow.
Pedipalps constitute the second pair of oral appendages in arachnids. In some groups, such as Thelyphonida, Amblypygi, pseudoscorpions and scorpions, they are sometimes inappropriately called “jaw-legs” because these genuine articulated structures are capable of capturing, restraining and even killing prey without the aid of venom. In scorpions, the pedipalps end in powerful pincers used by the animal primarily when hunting,with venom being released only when the prey is too large and moving. The pedipalps of some Amblypygi bear indentations and spines that literally impale the prey when capturing it. In other groups of arachnids, the pedipalps may have completely different roles that are far removed from that of predation. In Solifugae species, for example, they are used for movement to such an extent that to the untrained eye the animal appears to have five pairs of walking legs instead of the traditional four in arachnids. In spiders, particularly in males, the pedipalps house the copulatory bulbs that are used to transfer sperm during mating.
Although spiders’ ability to produce silk and weave webs is extraordinary, it is not unique to this group. The remarkable pseudoscorpions are also able to produce silk in their chelicerae, as are some groups of mites. However, silk is used to build shelters, form cocoons for moulting, hibernate or even “brood” their young in these groups. In spiders, silk plays a major role. Its functions are not limited to capturing prey: silk is also involved in the construction of cocoons, in the production of shelters, in moulting, in communication during reproduction, and in the dispersal of young spiders. These are but a few of its uses. It comprises semi-crystalline biopolymers characterised by the repetition of specific amino acid sequences. It has incomparable resistance and elasticity among biomaterials. Spiders make an incredibly diverse range of fascinating webs; even Aristotle tried to classify them. As a weapon of predation, webs are also extremely diverse. Hence, some species of ground-dwelling spiders dig a burrow surrounded by threads that warn the spider of potential prey. Others, such as the well-known diadem spiders (Aranea diadema), spin a web anchored at various points. It functions like a net where certain viscous threads hold the prey for as long as necessary for the spider to attack. While the diademed spider’s web is flat and relatively vertical, many other species make horizontal or oblique two- or three-dimensional webs with the function of trapping some prey. Their surface area can be particularly substantial, as is the
case with some species of nephila, whose webs reach several square metres and can even trap birds. Another original use of webs is by spiders of the Deinopis genus, which build a miniature web that acts as a net, which the spiders hold with their front legs and throw at any prey that comes within their reach. Finally, mention should be made of spiders of the Mastophorinae family that weave a thread with one or more sticky balls that they leave hanging under their body. When a moth goes past, they swing or retract the thread to engulf it like a fisherman hooking a fish.
Did you know that only spiders, scorpions and pseudoscorpions are poisonous? Some other groups (Amblypygi, Solifugae, Uropygus) may have scary shapes but are completely harmless!