Vretenci i sličnokrilci

Ova izvrsna stvorenja nježnog izgleda igraju važnu ulogu u prirodnom lancu. Osobito su korisna ljudima zbog svog proždrljivog teka za komarce i druge molestante. Dok se pohlepno goste svojim željenim plijenom, za ljude su potpuno bezopasni.

Veliki car (Anax imperator) Veliki car (Anax imperator) Foto: Steve Jones
Vretenci su relativno veliki insekti, čak i danas, iako ni blizi toliko veliki kao oni koji su živjeli prije nekoliko milijuna godina, što se dade zaključiti po drevnim fosilima njihovih predaka nađenih, primjerice, u Francuskoj, Ujedinjenom Kraljevstvu i SAD-u. Oni nastanjuju Zemlju od davnina i to, prema procjenama, već 250-300 milijuna godina. Razvrstani su u red Odonata, što znači "nazubljeni" te spadaju u podskupinu insekata koji su svrstani u skupinu uniramskih člankonožaca. Red Odonata broji oko 5600 vrsta, od kojih 130 u Europi, a oko 165 u mediteranskom slivu, i 64 u Hrvatskoj - (oko 70 2009.godine). Ovaj red podijeljen je u tri skupine: Anisoptera (vretenci), Zygoptera (sličnokrilci) i Anisozygoptera, reliktna skupina koja se sastoji od samo dvije preživjele vrste. Vretenci i sličnokrilci pripadaju potkoljenu heksapodu (šesteroglav) koljena artropoda, zajedno sa svim ostalim letećim i ne-letećim insektima (koljeno je grupa koja ne rangira visoko kao kraljevstvo, ali rangira više od klase).
Vatreni jurišnik (Crocothemis erythraea) Foto: Steve Jones
Vatreni jurišnik (Crocothemis erythraea), ženka. Foto: Steve Jones

Steve Jones, naš dolski promatrač prirode, u srpnju 2019. godine izvjestio je: "Promatranje ptica bilo je mirno, iako se, uzevši to u obzir, po razinama ribnjaka vidi da je više aktivnosti nego lani. Prikupio sam nekoliko novih vrsta za ovu godinu ali niti jedna koja je za mene novost na Hvaru. Vidio sam više vretenaca i sličnokrilaca nego ikad prije, osobito u ovom razdoblju. Prethodne četiri godine prirodni bazen prikazan dolje u ovom je periodu inače potpuno prazan (26.07.19.). Očito su obilne kiše u svibnju izuzetno pridonijele aktivnostima vretenaca ove godine. Uzevši to u obzir, nema mnogo različitih vrsta, ali ih je znatno više nego prijašnjih godina. Primjerice, tri vrste sličnokrilaca, u usporedbi s dosadašnjom jednom. Ovo je dobra vijest, jer je prisutnost vretenaca i sličnokrilaca najčešće znak da je ekosustav dobre kvalitete. Sličnokrilci su mnogo manje osjetljivi na onečišćenje od vretenaca, a neke vrste Odonata toleriraju različite uvjete u okolišu pa se mogu razvijati u različitim staništima. Oni koji se ne nalaze na određenim mjestima, bilo da se radi o  tekućim potocima ili rijekama, mirnim jezerskim vodama ili močvarnom zemljištu.

Lokva puna nakon 4 godine. Foto: Steve Jones 

Obilne proljetne kiše na Hvaru ponekad stvaraju tečne potoke u inače suhim vodotocima, pa je moguće da je to uzrokovalo porast vrsta koje je Steve vidio 2019. godine.

Žućkasti strijelac (Sympetrum fonscolombei). Foto: Steve Jones

Vretenci i sličnokrilci od svojih su najranijih trenutaka osobito dobro opremljena za proždiranje drugih insekata. Njihove ličinke (nimfe) razvijaju se iz jaja koja su položena u vodene biljke ili mahovinu, ili na površinu vode poput ribnjaka ili jezera. Imaju dojmljive čeljusti za gutanje svog plijena, no njihovo specijalno oružje je proširiva, povećana donja usna (stručni naziv labium) koja je, dok se ne koristi, sklopljena i skrivena pod glavom ali se može ispucati velikom brzinom kad je pogodna žrtva nadohvat ruke. Labium ima dvije kuke za hvatanje plijena obuhvatnim pokretom te ga na taj način povlači u čeljust kako bi pojeo. Ova metoda lova koristeći izvrdavanje poznato je kao krinka i taj mehanizam je, kažu, jedinstven za vretence i sličnokrilce. Insekti, punoglavci i male ribe koje formiraju svoju prehranu, vjerojatno na ličinke gledaju na isti način kao što ljudi gledaju na veliku bijelu morsku psinu.

Velike more - parenje (ischnura elegans). Foto: Steve Jones

Postoje razlike između vretenaca i sličnokrilaca, iako izgledaju slično, a mnogi o njima govore i pišu pod općim naslovom "vretenci", ne praveći razliku među njima. Za početak, jajašca vretenaca su okrugla i veličine oko 0,5 mm, dok su jajašca sličnokrilaca valjkasta i veličine su oko 1 mm. Neki sličnokrilci i vretenci odlažu svoja jaja u stabljike vodenih biljaka, neki klize nad vodom u koju uzastopno umaču vrh svog tijela dok izguruju jaja koja se potom spuštaju u vodu, a neki vretenci guraju jajašca u pijesak ili šljunak uz rubove potoka.

Velika carica - polaganje jaja. Foto: Steve Jones

Ličinke (nimfe) se, ovisno o staništu, razlikuju po obliku te njihova duljina, ovisno o vrsti, može iznositi tek nešto više od 6 mm do nekih 6 cm. Generalno, ličinka vretenca je kraća i masnija (DEBLJA??) od ličinke sličnokrilca. Škrge se nalaze u abdomenu, a on diše uvlačeći vodu u rektum i potom ga izbacujući. Abdomen se, u svrhu inercije, može stisnuti i snažno raširiti potiskujući vodu i stvarajući tako veću snagu za kratke nalete brzih pokreta. U ličinci sličnokrilca tri škrge u obliku peraja pričvršćena su na vrh abdomena koji je uži i duži od vretenčeva. Ako ih ne pojedu grabežljive ribe, ličinke većine europskih vrsta zadržavaju se u svom staništu duži vremenski period od oko dvije godine, pa do oko šest godina. Za to vrijeme, presvlače se između šest i petnaest puta.

Grof skitnica (Hemianax ephippiger) izlazi. Foto: Steve Jones

Kako bi posljednji put odbacila vanjsko kućište, ličinka iz vode izlazi na kamen ili stabljiku biljke kako bi se uspješno preobrazila u odrasli oblik s funkcionalnim krilima. Preskočit će stadij između nezrelosti i odrasle dobi koji drugi kukci prolaze, kao na primjer leptiri ili tvrdokrilci. Vretenci i sličnokrilci ne razlikuju se toliko u odrasloj dobi kao njihove ličinke. Za razliku od njihovih ličinki, oni nemaju puno prirodnih neprijatelja, jedna od kojih je sokolovka, mala ptica grabljivica, iako su u svojih prvih nekoliko odraslih dana ranjivi za druge ptice grabljivice i insekte dok im se tijelo ne stvrdne i ne počnu letjeti punom brzinom. Opremljene su izrazito velikim očima s oko 28.000 jedinica (omatidija) i navodno koriste oko 80% snage uma za obradu vizualnih informacija iz svih smjerova: mogu otkriti kretanje s udaljenosti od 12 metara (40 stopa). Za usporedbu, njihove relativno velike glave imaju male antene, ali one pridržavaju veliku proširivu labiju koja služi za hvatanje plijena.

Vatreni jurišnik i (Orthretrum caledonicum) ). Foto: Steve Jones

Njihovih 6 nogu uglavnom služe za hvatanje plijena ili grabljenje, manje za hodanje. Ovisno o vrsti i staništu, imaju različite načine kontroliranja tjelesne temperature, između ostalog sunčanjem i korištenjem krila. Imaju dva para krila, i to specijaliziranu zglobnu strukturu na prednjem rubu, zvana nodus, koja omogučuje krilu da se tijekom leta deformira a da pritom ne pukne. Ovo je samo jedna od mnogih specijaliziranih detalja pomoću kojih krila funkcioniraju na izrazito kompleksne načine.

Vilin konjić (Libellula depressa). Foto: Steve Jones

Postoji nekoliko razlika među odraslim jedinkama vretenaca i sličnokrilaca. Potonji imaju manje oči odvojene izraženim razmakom te tijela tanjim od vretenčevih. Glavna razlika jest u krilima. Stražnja krila vretenca veća su od prednjih te šira pri dnu dok su krila sličnokrilca jednake veličine i oblika, sužena gdje se spajaju s tijelom. To znači da sličnokrilci ne mogu letjeti tako brzo kao vretenci. U stanju mirovanja, sličnokrilci preklope krila preko leđa dok vretencima krila uvijek stoje uspravno. Vretenci mogu dostići velike brzine i izvoditi vrlo komplicirane manevre u zraku. Pojedine vrste migriraju te je zabilježeno da u povratnom putovanju između Indije i južne Afrike mogu prijeći i do 18000 km.

Sedozemna zelendjevica (Lestes barbarus). Foto: Steve Jones

Strašni grabežljivci kakvi jesu, vretenci žestoko brane svoja mjesta parenja. Neki sličnokrilci također su teritorijalni dok drugi parnjake jednostavno potražuju leteći uokolo i istražujući. Za vrijeme parenja, vretenac obično ne gubi vrijeme na nježno udvaranje. Jednostavno se prilijepi na ženku zgrabivši je za potiljak nakon čega se namjeste u položaj koji sliči na srcoliko kolo kako bi mužjak mogao prodrijeti u ženku i prenijeti svoju spermu u nju. U nekim slučajevima, mužjak uklanja spermu suparnika prije polaganja svoje.

Veliki vilenjaci - parenje (Orthetrum cancellatum. Foto: Steve Jones

Sličnokrilci također formiraju kolo kada se pare, kao na gornjoj fotografiji plavorepih sličnokrilaca, ali imaju više raznih metoda traženja parnjaka od vretenaca. Kopulacija može biti kratka ili produžena, a u nekim slučajevima mužjak ostaje priljepljen na ženku i tako lete uokolo sve dok ženka ne položi jajašca.

Žuti ban (Aeshna isoceles). Foto: Steve Jones

Neke vrste vretenaca i sličnokrilaca su ugrožena (linkovi na engleskom). Pad u njihovom broju veliki je gubitak za našu okolinu. Možemo se samo nadati da će vrtlari, poljoprivrednici i lokalne vlasti stati na kraj masovnom korištenju insekticida, larvicida i adulticida i svih ostalih štetnih pesticida koje trenutno uništavaju “civilizirani” svijet. Otrovi su kontraproduktivni. Uzrokuju ogromnu kolateralnu štetu bez ikakvih pozitvnih učinaka. Na Hvaru, šteta prouzročena populaciji insekata i divljih životinja sve je vidljivija, a ljudsko zdravlje je narušeno. Potreba za potpunim odbacivanjem kemijskih pesticida je kritična.

Tekst © Vivian Grisogono MA(Oxon) 2019.
Fotografije © Steve Jones 2019.
Prijevod: Dinka Barbić

Velika hvala Dave Smallshireu koji je pomogao Steveu da identificira insekte za koje nije bio siguran. Dave je, uz Andy Swasha, suautor vrlo hvaljenog vodiča “Britain’s Dragonflies. A Field Guide to the Damselflies and Dragonflies of Britain and Ireland“ 2018.

Modri kralj (Aeshna cyanea). Foto. Steve Jones

Izvori (na engleskom):

Gullan, P.J., Cranston, P.S. 2005 (3rd edition) The Insects: An Outline of Entomology. Blackwell Publishing. ISBN: 978-1-4051-1113-3

Burton M., Burton R. 2002 (5th edition) The Encyclopaedia of Insects and Invertebrates. Silverdale Books, Leicester. ISBN 1-85605-708-9

Sverdrup-Thygeson. 2018. Extraordinary Insects. Weird. Wonderful. Indispensable. The ones who run our world. English edition 2019, pub. Mudlark - HarperCollins. ISBN: 978-0-00-831635-8.

Tennesen, K.J. 2009. Odonata: Dragonflies and Damselflies. In: Resh, V.H., Cardé, R.T. Eds. 2009 (2nd edition). Encyclopedia of Insects, Chapter 185 pp 721-729. Academic Press. ISBN: 978-0-12-374144-8.

Jongerius, S.R., Lentinck, D. 2010. Structural Analysis of a Dragonfly Wing. Experimental Mechanics 50 (9) 1323-1334.

Rajabi, H. Ghoroubi, N., Stamm, K., Appel, E., Gorb, S.N. 2017. Dragonfly wing nodus: A one-way hinge contributing to the asymmetric wing deformation. Acta Biomateralia 60 - 330-338

UCMP Berkeley Edu. Introduction to the Odonata, Dragonflies and Damselflies. (Internet resource)

McKay, T., Herman, T. 2008. Thermoregulation in Three Species of Damselflies, with notes on Temporal Distribution and Habitat Use (Zygoptera: Lestidae). Odonatologica 37 (1) 29-39.

Sheikh, E.M., Douglas, M. 2012. Biodiversity, Phenology, and Thermoregulatory Strategies of Odonates at Pierce Cedar Creek Institute. Undergraduate Research Grants for the Environment Report, Grand Rapids Community College.

Bumblebee. org: Hexapoda 5 (Insects) https://www.bumblebee.org/invertebrates/Hexapoda5.htm. (Internet resource)

Castro, J., 2014. Animal Sex: How Dragonflies Do It. LiveScience. (Internet resource)

Cordero-Rivera, A. 2017. Sexual conflict and the evolution of genitalia: male damselflies remove more sperm when mating with a heterospecific female. Nature: Scientific Reports, 7. Article number 7844.

Riservato, E., Boudot, J-P., Ferreira, S., Jović, M., Kalkman, V.J., Schneider, W., Samraoul, B., Cuttelod, A. 2009. The Status and Distribution of Dragonflies in the Mediterranean Basin. IUCN, Gland, Switzerland and Malaga, Spain. ISBN: 978-2-8317-1161-4

Mellal, M.K., Zebsa, R., Bensouilah, M., Houhamdi, M. 2018. Aspects of the emergence ecology of the regionally endangered Coenagrion mercuriale (Odonata: Coenagrionidae) in Northeast Algeria. Zoology and Ecology 28 (3) 224 - 230.

 

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Novosti: Cybermed.hr

Novosti: Biologija.com

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