Hvordan jagtrøver flyver og snupper ofre fra luften

Røverfluer snupper ofre fra luften ved hjælp af samme aflytningstrategi som guidede missiler og falke, men hvordan opsnapper de luftbårne stenbrud, når deres udsyn er blokeret af løv og rod? Forskere fra University of Cambridge, UK, og University of Minnesota, U.S. rapporterer i Journal of Experimental Biology at Holcocephala fusca røverfluer kombinerer to strategier, en der giver dem mulighed for at svinge rundt om forhindringer, med deres sædvanlige strategi til at opfange bytte.

Mens mange fluer nøjes med at lande på frugtstykker eller ådsler, engagerer røverfluer (Asilidae) sig i dødelig kamp. Røverfluer opsnapper mindre insekter på vingen og spiser alt, hvad de kan underkue. Når udsigten er fri, bevarer de kunstflyvningsinsekter den samme sigtelinje – justerer den, mens deres mål drejer og drejer – for at opsnappe deres stenbrud; og det gør de med ekstraordinær fart med en hjerne på størrelse med et sandkorn. Alligevel må de stædige insekter ofte gribe bytte i komplekse rodede omgivelser. "At navigere til mål og undgå at støde ind i ting undervejs er opgaver, som vi håber at opnå i vores hverdag," siger Samuel Fabian fra Imperial College, London. Så hvordan klarer røverfluer rod? Fabian, Trevor Wardill og Paloma Gonzalez-Bellido fra University of Minnesota, U.S., besluttede at finde ud af, hvordan de rovdyre røverinsekter justerer deres aflytningstrategi for at håndtere distraktioner. De offentliggør deres opdagelse af, at insekterne kombinerer to strategier – den ene, der giver dem mulighed for at svinge uden om forhindringer, med deres konventionelle aflytningstrategi, som de bruger, når udsigten er klar – i Journal of Experimental Biology .

"Vi brugte Holcocephala fusca på grund af dens forudsigelige aflytningssti," siger Fabian, der sluttede sig til Mary Sumner fra University of Minnesota i York County, PA, USA, i fire intensive uger for at filme de beslutsomme fluer i 3D, mens de jagtede en lillebitte perle trækkes langs en gennemsigtig fiskeline. "Felteksperimenter er en glæde, fordi man får den mest naturalistiske adfærd fra frie dyr," siger Fabian. Selvom det betød, at insekterne frit kunne bevæge sig ud af skud, lige når kameraerne var klar til at rulle. Heldigvis var de også ivrige efter at opsnappe den bevægelige perle; "hvis noget er lille nok, ser de generelt ud til at antage, at det er mad," siger Fabian. Og da han og Sumner analyserede insekterne, der opsnappede perlen, bevarede dyrene den samme sigtelinje til målet under hele deres tilgang for at fange det med succes. "The flies really didn't know it's not real prey, even when very close," chuckles Fabian.

However, when Fabian and Sumner partially obscured the flies' views, with either a wide (5 cm) or narrow (2.5 cm) bar of black acrylic, as they approached the moving bead, the flies took evasive action and even abandoned the interception when the broader band obscured their view for more than 0.1 s. Yet, when the bar blocked their line of sight for briefer periods, the flies veered away dramatically until they had passed it, before swerving back and resuming their interception course. On other occasions, when the visual obstacle remained in sight but did not obscure the fly's view, the insect still veered away from the bar as they approached the bead, even though there was no need to swerve as the bead remained in sight the entire time.

So how was the fly controlling its approach? Fabian, Gonzalez-Bellido and Wardill compared the robber flies' flight paths and the paths they would have taken if there was no obstacle, and realized that the flies were using a very simple obstacle avoidance strategy; 'the faster the obstacle is getting larger in their field of view the more they turn away from it," Fabian explains. However, once the fly has passed the obstacle and it begins to grow smaller, the fly is then drawn back toward it, resulting in the veering flight paths that Fabian and Sumner recorded, even when the fly's view wasn't obstructed.

The flies are simultaneously using a combination of the obstacle avoidance strategy and the conventional interception paths used when their view is unobstructed, resulting in a simple hybrid approach that allows them to intercept their prey while avoiding distractions and obstacles in their paths. "They are paying attention to their surroundings even when focused on the target," Fabian concludes, who hopes to inspire robotic designs that use simpler and more computationally lightweight solutions for complex navigation problems.