Green Fly­pa­per

by Merry Youle

Trou­bled by pesky flies in your home or of­fice? Tempted to reach for the toxic fly­pa­per? Here's a green al­ter­na­tive long known by the In­di­ans of the Cen­tral Mex­ico high­lands. Step out­side and look around for a tree with en­tire branches cov­ered by a large spi­der­web. Clip out a sec­tion of the web and hang it in­doors, com­plete with res­i­dent spi­ders. A nat­ural fly trap. Com­mon house flies will be at­tracted to the web, lured to their death by a sweet smelling bait with a mi­cro­bial source.

Source

Make cer­tain, though, that you've got the right kind of web. Only those made by the so­cial spi­der Mal­los gre­galis, will work. So­cial spi­ders? Granted, most spi­ders are can­ni­bal­is­tic lon­ers, but ap­prox­i­mately 23 of the 39,000 spi­der species have opted for vary­ing de­grees of per­ma­nent, non-ter­ri­to­r­ial so­cial­ity (qua­si­so­cial­ity). The spi­ders in these caste­less com­mu­ni­ties co­op­er­ate in hunt­ing, feed­ing, nest con­struc­tion, and, in some cases, car­ing for the spi­der­lings, as well.

Soli­tary M. gre­galis spi­ders kept in the lab can con­struct webs and hunt pretty suc­cess­fully, but the webs are sim­ple struc­tures. In na­ture, as many as 20,000 live to­gether in a sin­gle, in­tri­cate, com­mu­nal web. The outer sur­face is the pri­mary site for prey cap­ture. The in­te­rior labyrinth of silk-lined tun­nels and cham­bers is where the spi­ders hang out ex­cept when vi­bra­tions from strug­gling prey send them to the sur­face to feed. By co­op­er­at­ing, this spi­ders can cap­ture more and larger prey, with less silk pro­duc­tion re­quired per in­di­vid­ual.

Mus­coid (house­fly-like) flies like to swarm around M. gre­galis webs in par­tic­u­lar, drawn by  their no­tice­able "sweet, yeast-like" odor. It is the not silk that draws flies, but the odor from "web dec­o­ra­tions." These spi­ders, un­like most oth­ers, leave the car­casses of par­tially con­sumed flies in their webs. Re­move the car­casses from the web and the scent dis­ap­pears.

The web of a dif­fer­ent so­cial spi­der from the Ama­zon River re­gion of Brazil. Source

What's go­ing on here? Re­searchers in the 1980s us­ing sim­ple cul­tur­ing as­says de­ter­mined that fed-upon fly car­casses are host to abun­dant yeasts of sev­eral types, whereas un­fed-upon car­casses or those fed-upon by other spi­der species host pre­dom­i­nantly bac­te­ria. Cul­tures of yeasts de­rived from the car­casses fed-upon by M. gre­galis, em­anated the same sweet scent and at­tracted flies. They con­cluded that the feed­ing by the spi­ders al­ters the con­di­tions in the car­casses so as to ei­ther give the yeasts a growth ad­van­tage or in­hibit the growth of bac­te­ria or both. To my knowl­edge, this is the first ex­am­ple of a preda­tor us­ing mi­croflora to at­tract prey by means of odor. The tac­tic, though good, has a flaw. If more prey are at­tracted than can be  con­sumed by the spi­ders, their bod­ies will be de­com­posed by the bac­te­ria, thus re­leas­ing a dom­i­nant am­mo­nia odor that re­pels the flies. The spi­der pop­u­la­tion will now crash or em­i­grate en masse.

herid­ion ni­groan­nu­la­tum, an­other so­cial spi­der, with egg sacs. Credit: Ingi Ag­nars­son. Source

The re­searchers re­ferred to this as a spi­der-yeast sym­bio­sis, but I sus­pect there is an­other spi­der-as­so­ci­ated mi­cro­bial player that may be the source of an an­timi­cro­bial that keeps the bac­te­ria in check. Just my guess. But as the story stands now, it pro­vides post­scripts to two of our ear­lier blog posts by pro­vid­ing one more rea­son why spi­ders dec­o­rate their webs and an­other ex­am­ple of an arthro­pod ring­ing a mi­cro­bial din­ner bell. The time seems ripe for some­one to pick up the thread and pur­sue this story fur­ther us­ing cur­rent 'omics tech­niques.

Ref­er­ence

Tietjen,W, Ayya­gari L, Uetz G (1987). Sym­bio­sis Be­tween So­cial Spi­ders and Yeast: The Role in Prey At­trac­tion. Psy­che: A Jour­nal of En­to­mol­ogy, 94 (1–2), 151–158. DOI 10.1155/1987/67258