Får i ulveklær

Humlebiller Trichius fasciatus på hvitbladtistel. Munkvoll, Oppdal i Sør-Trøndelag. Foto: F. Ødegaard.

Humler har giftbrodd og kan være giftige for insektetere.  Og rovdyrene lærer seg fort hvilke dyr man bør unngå. Det er derfor en stor fordel dersom man klarer å etterligne giftige dyr. Dette er bakgrunnen for at en rekke andre insekter ligner sterkt på humler selv om de er fjernt beslektet.  Fenomenet kalles «batesisk mimikry» og er svært vanlig i naturen. Det er med andre ord bedre å være et får i ulveklær, enn ulv i fåreklær i insektverdenen.

Fluer i forkledning

Hos enkelte insektgrupper er mimikry mer utpreget enn hos andre. Det er særlig hos insekter som lever i de samme miljøer og som har en genetisk disposisjon til det, at mimikry er spesielt vanlig. I Norge står blomsterfluene (Syrphidae) i en særstilling og inkluderer en rekke arter som etterligner både humler, andre bier og stikkeveps. Den vanligste og kanskje mest spesielle av disse er humleblomsterflua Volucella bombylans. Denne kan opptre i flere ulike fargemorfer som minner om hver sine grupper av humler. Det finnes både jordhumle-, steinhumle-, og åkerhumlelignende varianter. Ikke nok med at den etterligner humlenes farger og atferd gjennom summing og bevegelser, men når flua sitter på en blomst kan den strekke frambeina framover langs hodet slik at det ser ut som den har kraftige humleantenner. Arten utvikler seg i humlebol der den lever av rester og avfall, men den kan også ta humlelarver. Den har derfor ikke bare behov for å   beskytte seg mot fiender, men også for å kamuflere seg mot humlene selv.

Totalt sju av våre blomsterfluearter er typiske humleetterlignere. Bjørneblomsterflua Arctophila bombiformis har jordhumlemønster og er kanskje den største av dem alle. Den er ikke uvanlig i skogkanter på Østlandet og i Trøndelag om sommeren. Loddenblomsterflue Pocota personata er en mer sjelden og litt mindre art med jordhumlemønster som er knyttet til løvskogsområder på Østlandet. Svart pelsblomsterflue Criorhina ranunculi er en bemerkelsesverdig art som ofte er å finne sittende på trestammer, gjerne osp, tidlig om våren. Denne vakre blomsterflua etterligner de svarte humlene med rød bakstuss og finnes spredt nord til Trøndelag. En annen art i sammen slekt, Criorhina berberina, er sjelden og kun påvist et par ganger i Norge. Denne opptrer i to fargeformer; en som ligner jordhumler og en som ligner trehumle. Ellers finnes ytterligere et par humleetterlignere med lyse tegninger på brystet og rød bakstuss: praktdroneflue Eristalis oestracea og humlebarskogflue Erizona syrphoides som begge er relativt vanlige hos oss.

Det er også en del andre tovinger (Diptera) som kan minne om humler. For eksempel den store  rovflua Laphria flava og møkkflua Mesembrina mystacea. Sistnevnte er en utpreget humleetterligner, og minner mest om trehumle eller barskoghumle. Det passer fint for en art som er mest vanlig i skogen. Blant snyltefluer (Tachinidae) finnes også noen store og kraftige arter som kan minne om humler, for eksempel den helsvarte Tachina grossa og den mer oransjetegnete Tachina fera, selv om disse ikke har typiske fargemønster som humler. Innen den parasittiske fluefamilien brems (Oestridae) finnes også flere arter ligner humler. I fjellet kan både reinbrems Hypoderma tarandi og reinens nesebrems Cephenemyia trompe minne om (særlig hanner av) berghumle og lapphumle.

reinsbrems

Reinens nesebrems Cephenemyia trompe samles ofte på fjelltoppene og kan minne om ei lapphumle. Finnshøa i Oppdal, Sør-Trøndelag. Foto: F. Ødegaard.

Pelsbier og gjøkhumler

Når det gjelder andre arter blant årevinger (Hymemoptera), er det i grunnen ikke så mange humleetterlignere, selv om de store klubbevepsene (Cimbicidae) i slektene Trichiosoma og Cimbex, både i størrelse og farger, kan ha likhetstrekk med humler.  Også andre villbier kan ligne humler. Blant disse er humlepelsbie Anthophora furcata en åpenbar kandidat til å bli feilbestemt som en enghumlearbeider. Noen gjøkhumler ligner sine verter slik som steingjøkhumle og jordgjøkhumle, men dette er ikke typisk for andre gjøkhumler som markgjøkhumle og tregjøkhumle. Kanskje er ikke pelsfarge så viktig for å unngå å bli avslørt som gjøk i humleverdenen? Man har likevel spekulert i om årsaken til at lundgjøkhumle har rød bakstuss, i motsetning til verten lundhumle, skyldes at den opprinnelig er tilpasset underarten proteus av lundhumle som har rød bakstuss og finnes lenger sør.

Mange humler ligner hverandre sterkt i fargetegningene. Dette er en av årsakene til at det ofte er vanskelig og usikkert å artsbestemme humlene på basis av slike mønstre, men det er faktisk en fordel for humlene å ligne på hverandre. Når flere giftige arter (eller arter med giftbrodd som humler) ligner på hverandre, kan hvert enkelt individ dra fordel av det ved at man lettere unngår å bli spist av insektetere. Dette fenomenet kalles «Müllersk mimikry» eller «mimikrykomplekser». Det er også vanlig at humler kan ha et spekter av ulike fargemorfer (en variant av en art med et bestemt fargemønster) med større variasjon i fargemønster innen en art enn mellom arter. Av humler i Norge er enghumle det beste eksempelet på dette. Den arten opptrer i en mørk form og i en lys form hos oss. Og for å gjøre det ekstra vanskelig for oss som prøver å artsbestemme humlene, er det slik at ulike arter kan ha et identisk variasjonsspekter i fargemorfer. Hagehumle kan i en melanistisk form ligne helt på våre slåttehumler. Samtidig opptrer ofte individer av ulike arter med identisk fargemorf på samme sted. Dette ser vi blant annet hos berghumle og lapphumle.

Summende sommerfugler

Også blant sommerfuglene finner vi humleetterlignere i flere familier. Glassvingene (Sesiidae) er trolig de mest utpregete der alle arter ligner mer på andre insekter enn sommerfugler. For eksempel ospeglassvinge Sesia apiformis, som med sin tykke bakkropp og gule og svarte tegninger noen ganger blir forvekslet med ei humle. Det er imidlertid en stor stikkeveps, muligens geithams Vespa crabro, som har vært modell for denne. Gulkrageglassvinge Sesia bembiciformis, er en annen meget sjelden art på samme størrelse og med lignende fargetegninger.

Ellers er humlesvermerne kanskje de aller beste etterlignerne blant sommerfuglene. De tilhører familien tussmørkesvermerne (Sphecidae), men er dagflygende. Vi har to arter i Norge: smalkantet humlesvermer Hemaris tityrus, som finnes nord til Trøndelag, og bredkantet humlesvermer Hemaris fuciformis, som er mer begrenset utbredt på Sørøstlandet.

Man kommer ikke utenom å nevne humlebille Trichius fasciatus (bilde over). Selv om det er lett å se at dette er ei bille, har den humlelik behåring og fargetegninger som gjør at det er lett å ta feil ved første øyekast. Særlig når den flyr og nærmer seg en blomst med sin summende og humlelike atferd, skal det mer til å avsløre forkledningen.

117 thoughts on “Får i ulveklær

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