Bug mas­quer­ades as din­ner to catch its din­ner

Elab­o­rate dis­guises al­low rove bee­tles to en­ter army ant colonies and feast

Baltimore Sun Sunday - - NEWS - By Sean Greene

Army ant colonies are filled with heaps of plun­dered food and teem­ing with help­less ju­ve­niles that other in­sects would love to feed on.

But one does not sim­ply walk into an army ant colony and start feasting.

Ill-pre­pared in­trud­ers would face swarms of ag­gres­sive in­sects de­ter­mined to de­fend their nest. That’s why many in­sects — in­clud­ing sil­ver­fish, mites and wasps — have de­vel­oped ar­mor or clever dis­guises to raid ant colonies.

Sev­eral species of rove bee­tles, a di­verse fam­ily of tiny, elon­gated in­sects, take this ap­proach to the ex­treme.

These bee­tles are able to in­fil­trate ant nests through an elab­o­rate mimicry scheme, evolv­ing ant-like an­ten­nae and legs, ant-like smells and even ant-like be­hav­iors to in­fil­trate the ranks of their hosts.

Over the past 60 mil­lion years, this bee­tle-to-ant dis­guise has emerged 12 sep­a­rate times in var­i­ous gen­era of rove bee­tle around the world, ac­cord­ing to re­search pub­lished this month in Cur­rent Bi­ol­ogy.

“There’s noth­ing else quite like it in bi­ol­ogy,” said Joseph Parker, the Columbia Univer­sity evo­lu­tion­ary bi­ol­o­gist who led the study. “These bee­tles have in­de­pen­dently evolved to live with army ants. They sort of mor­phed into the shape of their ant hosts — and their be­hav­iors have changed, too.”

Parker and his col­league, Mune­toshi Maruyama of Kyushu Univer­sity in Ja­pan, spent a decade in the field search­ing for the tiny in­sects among thou­sands of march­ing army ants and col­lect­ing them for later DNA anal­y­sis. Many of the species they found were new to science.

By re­con­struct­ing the evo­lu­tion of the ant-mim­ick­ing rove bee­tle sub­fam­ily Aleochari­nae, the re­searchers found that the in­sects de­scend from a com­mon an­ces­tor that lived about 105 mil­lion years ago in the early Cre­ta­ceous Pe­riod, about the same time flow­er­ing plants and mod­ern mam­mals ap­peared.

The ant im­pos­tors’ an­ces­tor would have looked more like to­day’s “free-liv­ing” rove bee­tles, the ones that do not live as par­a­sites in­side ant colonies.

As army ants and ter­mites con­quered the trop­ics dur­ing the Ceno­zoic Era, at least 12 sep­a­rate lin­eages of rove bee­tle took on ant-like char­ac­ter­is­tics.

The re­searchers call this an an­cient ex­am­ple of con­ver­gent evo­lu­tion, in which un­re­lated or sep­a­rate groups of or­gan­isms fol­low seem­ingly par­al­lel evo­lu­tion­ary paths.

Rove bee­tles’ ant mimicry is sur­pris­ing, in part be­cause of the com­plex­ity of the adap­tions. Con­ver­gent evo­lu­tion is gen­er­ally lim­ited to a sin­gle trait, such as the shape of eyes or wings.

It’s also un­usual that the bee­tles de­vel­oped these dis­guises over such a long pe­riod of time.

Usu­ally, con­ver­gence takes place over just a few mil­lion years. That’s be­cause the species, though sep­a­rate, share a sim­i­lar ge­netic blue­print that al­lows their evo­lu­tion to fol­low sim­i­lar cour­ses.

That’s not the case for the rove bee­tles, whose an­cient an­ces­tor split into sep­a­rate groups more than 50 mil­lion years ago. In that time, each bee­tle species be­came more ge­net­i­cally dis­tinct. Even so, more than a dozen rove bee­tles still man­aged to evolve their own ant-like dis­guises.

All of these bee­tles evolved to sur­vive in a very par­tic­u­lar niche, in this case the in­hos­pitable colonies of army ants. In other words, the bee­tles had no free­dom to evolve dif­fer­ent ways of life, he said.

“If the se­lec­tive con­di­tions are right and the start­ing ma­te­rial is right, then evo­lu­tion can be ex­tremely pre­dictable,” Parker said. To live in an ant colony, “you pre­sum­ably have to obey spe­cific rules. That’s smelling like an army ant and be­hav­ing like an army ant.”

The ant im­pos­tors’ last com­mon an­ces­tor likely passed on a set of adap­ta­tions that pre­dis­posed the in­sects to even­tu­ally de­velop traits that ap­prox­i­mated the ap­pear­ance, smells and be­hav­iors of var­i­ous army ant species.

But first, the bee­tles needed a rea­son to ven­ture into a dan­ger­ous army ant den: the nest’s ant brood and the scav­enged arthro­pods the ants hoard are at­trac­tive sources of food. To sur­vive such a hos­tile en­vi­ron­ment, the bee­tles’ de­vel­oped a de­fen­sive gland on their ab­domen that se­creted an ir­ri­tat­ing chem­i­cal, called a quinone, which al­lowed them to de­fend them­selves against ag­gres­sive ants.

“They can walk into an ant colony, eat their re­sources and sort of blast them in the face with quinones,” Parker said. “They can get in the door be­cause they can chem­i­cally de­fend them­selves.”

Fi­nally, rove bee­tles pos­sess a body plan that’s much eas­ier to re­model into the shape of an ant. Free-liv­ing rove bee­tles have a rel­a­tively short wing case and a flex­i­ble ab­domen, so they’re only a few steps away from be­ing ant-like, Parker said. Then all the myrme­coid bee­tles needed was elon­gated an­ten­nae and legs.

Each myrme­coid rove bee­tle is adapted to par­a­sitize a sin­gle species of ant. The bee­tles prob­a­bly live their en­tire lives in­side the ant colony, al­though no one has ever found their lar­vae, Parker said.

It’s also un­clear whether the ants ever re­al­ize that there’s an im­pos­tor among their ranks. But, Parker said, myrme­coid bee­tles have de­fen­sive mod­i­fi­ca­tions, in­clud­ing thick­ened, club-like an­tenna and stur­dier body seg­ments, in case they do get caught.

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