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Foedus perianale is an invasive arachnid species whose reproduction process makes it an obligate parasite of large mammals (including hominids).

Sexual dismorphism is present to a very marked degree : while the males rarely exceed four milimeters in length, weighing between one and two grams, the females generally exceed 20 grams. The largest known specimen weighed in at no less than 42 grams (about an ounce and a half). Their abdomen, very vividly colored and protected by six rings of hairs, go from approximately peanut sized before impregnation to almost the size of a walnut right before partum.

The female does not lay eggs, to be fecundated outside the body as is common with arthropods ; instead, the eggs form in a special amniotic sac inside its lower abdomen. Once fecundated, the eggs require a relatively high temperature for their maturation, which is probably the proximate cause of this spider's very peculiar behaviour : the female will seek out large mammals during the night, and will burrow itself abdomen-first into their anal cavity.

The installed female assumes a very specific position, with its legs spread out in a tell-tale pattern (the "brown star" or "kiss of dark" as it is commonly known). It bites down into the perianal tissue of the host, with its chelicerae constantly feeding a complex venom into the perianal lymphatic circuit of the host. The as-yet not fully described cocktail of alkaloids and other substances has a direct effect on peristalsis, rendering the host completely constipated for the six to ten day duration of the nesting. There are also less well understood effects, with the human host apparently incapable to conceive of the infestation, apparently unable to see it even if confronted with a mirror, and in any case entirely unaware of its own constipation.

Once the eggs mature, the spiders burst forth out of the mother's abdomen, in the process killing her. They voraciously feed on feces for the 8 to 36 hour interval necessary for the host constipation to resolve, after which are ejected into the environment and assume adult life directly (the males will never significantly grow throughout their shortⁱ lives).

Some studies seem to suggest infestation leaves a propensity of further future infestation in the host, with human subjects observed to engage in specified risk behaviours (such sitting bare-bottom in the dewy grass or on wet tree stumps by the edge of broadleaf forests, often deliberately removing garments and undergarments without apparent awareness of the matter) which are either denied by the subject or explained away unconvincingly.

There are no adverse health effects known ; the rate of infestation is anecdotically placed at "between one and ten in a hundred" by a straw poll conducted among active proctologists. Further studies are necessary to clarify various aspects of this mysterious parasitism.

1. An interesting element of Foedus perianale's life is that males rarely survive two weeks, but sexually mature within about a week and go on to fecundate already adult females, born either one or two generations prior. There's always cross-generational genetic interaction because of this pattern, and the species could not survive the extinction of all but one generation -- it requires at least two females of different ages to maintain itself. [↩]