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Spider bites and necrosis!

By JW Dowey - 13 Mar 2015 9:37:0 GMT
Spider bites and necrosis!

The close US relative of the Chilean venomous spider, Loxosceles, is figured here. The danger of severe medical problems is possibly worse where high temperatures and rapid breeding of the spiders produces large numbers in houses and similar buildings.Loxosceles reclusa ; Credit: © ShutterstockLoxosceles image

Two spiders are featured in this paper. One is a spitting spider, Scytodes globula and the other a house spider, bites humans when it encounters them in warm places at night. Beware the bath towel ! We have encountered spitting spiders in the story on ants and a jumping spider here, but I digress. The danger of the biting recluse spider would be unfamiliar north of Tennessee in the US or in Europe. But the Loxosceles species of South America and the one species in southern North America has a special effect known as loxoscelism. The result is a large area of browning skin, leading to skin dying and a deep open sore. The medical description is necrotic arachnidism, and it is fortunately unique to this genus of spider.

The research, here in Valparaiso and Santiago, in Chile, is important because of the lack of any treatment for this spider bite. The spitting spider is imagined to kill the recluse spider, which would help a lot in controlling the population. Both spiders come out at night and live in warm places, leading to frequent encounters. The Chilean situation is reported here with the spitting spider often said to predate on Loxosceles laeta. Other spiders throughout South America prey on the recluse spider, as seen by their bodies left in webs. The great advantage seen in employing the spitting spider is that it is an araneophagic. It is known to prey on Loxosceles species, jumping spiders and Drassodes spp.

With these two nocturnal species living together (80% of their thermal niche is shared.) The spitting Scytodes mostly wins the contests between the two, but this very much depends on its size. Many end with the recluse species winning though. In the extensive laboratory experiments, spiderlings of the recluse had a high mortality rate because of this predation. As Loxosceles is both larger and quicker, the aggression of, for example, a large female defending her young was liable to result in the death of the spitting spider.

Encounters were organised in an experimental situation lasting 60 minutes, and then continued for a further hour before a draw between the 2 contestants was decided. Instead of simply death of a spider, many behavioural events and/or loss of legs were also studied. Out of 32 encounters, only 19 were counted as being aggressive, with 13 won by Scytodes. The bare figures point to the evidence that these spiders often co-exist, even within an experimental chamber. The possibility of biological control is low, given that the spitting spiders do control the recluse spiders to some extent. The ecology of the various spiders in houses involves many species in warm countries. Food supply shortages may ensure that Scytodes eats more Loxosceles (or perhaps the reverse!) But animals adapt to their particular niches, dependent on all other species and physical factors around them. The severe effect on humans needs to be addressed, while it is obvious from experiments that killing all spiders, as in hotels and guest houses may make us more susceptible to population explosions of the reclusive Loxosceles

Mauricio Canals, Nicolás Arriagada and Rigoberto Solís of Universidad de Concepción, Universidad de Chile, Santiago and Santiago’s Instituto de Salud Poblacional, Escuela de Salud Pública Salvador Allende G present the arachnophobe’s favourite paper in the Journal of Medical Entomology.