DOES WA­TER MAKE A MAN­TIS SUI­CI­DAL?

iD magazine - - Nature -

The man­tis is al­ways ready for bat­tle. When it’s time to strike, the in­sect lunges for its prey within one-tenth of a sec­ond, trap­ping it firmly in front legs that are re­in­forced with spines and mak­ing short work of it. When it’s not at­tack­ing prey, a man­tis will cam­ou­flage it­self as a leaf or flower. It stays mo­tion­less like this for hours on end, and then out of nowhere it can spring forth, even pluck­ing a fly from the air with its “death grip” arms.

The man­tis is not too both­ered by rain, but dur­ing a heavy shower it seeks shel­ter un­der the same leaf it was just im­i­tat­ing. The in­sect would also rather avoid deep stand­ing wa­ter, and it will only pad­dle its way across shal­low pud­dles when there is no other op­tion. And yet, float­ing man­tis corpses are a fairly com­mon sight in lakes. Is this be­cause of some mys­te­ri­ous form of mass man­tis sui­cide? No, the cul­prit is the horse­hair worm, a tiny par­a­sitic or­gan­ism. It nes­tles its own lar­vae in the lar­vae of a mos­quito, for ex­am­ple, which in turn are eaten by the man­tis. What hap­pens next is not en­tirely clear to sci­en­tists: Some­how the par­a­site man­ages to tap into cer­tain synapses in the man­tis’s brain so that it A) loses its free will, and B) stag­gers to­ward wa­ter as if con­trolled by an ex­ter­nal force. “Per­haps the par­a­site trig­gers a sen­sa­tion of in­tense thirst in the man­tis,” sug­gests en­to­mol­o­gist Damir Kova.

Such a feel­ing of thirst is so ex­treme that the dew­drops on the leaves are no longer suf­fi­cient for the man­tis. Once it ar­rives at a lake, the worm at­tacks the in­nards of the man­tis, which fi­nally falls into the wa­ter as a hol­low shell. Hav­ing achieved its goal, the worm ex­its its host: The con­di­tions for its re­pro­duc­tion are op­ti­mal in the wa­ter. Some­times a man­tis will not be en­tirely gut­ted by the worm, and af­ter de­posit­ing its pay­load it may leap back onto dry land. Hav­ing es­caped death so nar­rowly, it is likely to avoid large pud­dles in the fu­ture.

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