Famiglia Heptageniidae: Epeorus ed Ecdyonurus

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Siamo ad aprile/maggio e finalmente arriva la primavera inoltrata, le giornate si allungano ed il tempo è più clemente. Sul torrente, il primo sole più caldo ci invoglia a guardarci intorno per scorgere nuove forme di vita che “sappiamo” essere presenti, e senza nemmeno accorgercene il nostro viso si dirige verso l’alto come per annusare l’aria.

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Sciame di Ecdyonurus maschi in volo nuziale

Invece cercheremo di scorgere il volo elegante dei primi giganti dell’ordine Ephemeroptera: Epeorus ed Ecdyonurus entrambi della famiglia Heptageniidae.

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Epeorus assimilis subimago female

A dire il vero gli sciami in volo sono costituiti da esemplari maschio allo stadio di imago (insetto maturo), se invece volessimo scorgere le effimere che sono ancora allo stadio di subimago, dobbiamo concentrare la nostra attenzione alla vegetazione riparia ed alle pietre della sponda. Infatti è in questi luoghi che le subimago attendono la muta definitiva in imago che avverrà in circa 24 ore dalla precedente, quella da ninfa subacquea (ninfa piatta) ad insetto alato.

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Ecdyonurus subimago tra la vegetazione

 

A differenza del genere Rhithrogena, quando le ninfe di Epeorus e di Ecdyonurus sono pronte a mutare in insetto alato, si portano nei pressi della riva camminando sul fondo, quindi schiudono e, emergendo, si arrampicano subito sui sassi o sugli arbusti della sponda; infatti ci troveremo magicamente le subimago tra i nostri piedi già belle che mutate. Qualche volta mi è capitato di vedere derivare sull’acqua del sottosponda le subimago di questi due generi di effimere, ma comunque sempre con le ali ben spiegate ed asciutte.

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Ninfa di Epeorus assimilis

 

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Ninfa di Ecdyonurus venosus

I nostri amici pinnuti conoscono bene le dinamiche dischiusa degli insetti di cui si cibano e, pertanto, si disporranno in agguato nel sottoriva per ghermire le ninfe mature e le subimago ascendenti di questi due generi di efemerotteri; si interesseranno alle imago solo quando queste, eventualmente, deriveranno sulla superficie dell’acqua dopo che avranno ultimato il loro ciclo riproduttivo.

Le foto dell’articolo illustrano le situazioni descritte, quindi… “guardatevi sempre attorno”!!!

Roberto Brenda

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