The mark of Cain spider was first described by the second expedition to Madagascar. It was noted that these spiders seemed to drink blood from around the eyes and ears of the cain - these being the most vulnerable areas of the animal. Some individuals were seen carrying as many as three marks, but it seemed to be more usual to see a cain with a single mark. Though still unproven, the theory goes that a mark will find and eat others of its species when it encounters them outside the mating period (P. rodloxi females may also eat the male after mating).

Mark of Cain spider, Parasitarachne rodloxi (Madagascar)

The discovery of this apparently parasite-spider caused something of an uproar in the field of invertebrate specbiology. As a result a new expedition was sent to study (among other things) this unusual arachnid. This expedition discovered something very unusual: the cains carrying one or two marks seemed to be healthier than the cains with no marks. This apparent paradox was solved when it was found that besides P. rodloxi, cains serve as hosts for another arachnid, Sauroacarus madagascariensis, a diminutive species of dinomite. The mark of Cain spider was actually mainly feeding on these mites, as well as a host of other ectoparasites, but occasionally stopped to drink the blood from a wound from which it had removed a dinomite. Thus it was found that the supposed parasite was in fact a symbiote that protected the dinosaur from bloodsucking pests.

Though a mark of Cain spider will spend most of its life perched on a cain of its own, they still retain the ability to spin webs. These are only used twice during the life of P. rodloxi: to spin a long string of web for gliding with the wind, traveling from the cain of the mother to another place where the young can find their own cain (or other dinosaur carrying S. madagascariensis) and for the second time after mating for spinning a cocoon for the eggs. A cain mother will attach the cocoon into one of the back spines of a cain and abandon it there. The hatching young will find their own way off the cain and onto a new one. It has been speculated that the ancestor of P. rodloxi was blown to Madagascar from the mainland by strong winds, and evolved its unique lifestyle there, as no similar symbiotic spiders have been found anywhere else on Spec.

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