A blood-feeding parasitic fly loses visual sensitivity after finding a host and permanently giving up flight, new research shows.

Deer keds—biting flies found across Europe, Asia, Africa and the Americas—use their eyes and flight to locate a host, typically deer, but occasionally humans or other mammals.

Once they land, however, they shed their wings permanently and spend the rest of their lives crawling through fur and feeding on blood.

Researchers at Aberystwyth University and the University of Florence have now shown that this striking lifestyle shift is accompanied by a major change in the insect’s sensory priorities.

After securing a host, deer keds direct resources away from vision, potentially conserving energy for functions better suited to life as a permanent parasite.

Dr Roger Santer from the Department of Life Sciences at Aberystwyth University, who led the research, said: “Vision plays a vital role in animal behaviour, but it is also energetically expensive.

“Evolution favours sensory systems that are efficiently matched to an animal’s way of life.

“Some blood‑feeding flies rely heavily on vision, while others live permanently on hosts and have little need for it. Deer keds are especially interesting because they switch between these two lifestyles.”

The research team analysed deer keds at different stages of their life cycle, including winged adults caught in flight while searching for a host, and wingless adults collected from deer after they had adopted their parasitic lifestyle.

The scientists examined genes underlying visual sensitivity (‘opsins’), comparing their activity before and after the insects shed their wings to understand how the flies’ sensory systems respond to the abrupt lifestyle transition.

Dr Santer said: “We found that a flying deer ked’s visual system is much like that of a tsetse fly, which famously hunt out mammal hosts in Africa. However, after a deer ked loses its wings and becomes an ectoparasite, activity of its opsin genes reduces to around half the previous level.

“This suggests that the flies do not lose vision entirely, but that their visual sensitivity is reduced. We think the fly might be sacrificing sight to conserve energy for functions such as digestion and reproduction.”

Published in the Journal of Experimental Biology, the findings provide new insight into how parasites fine‑tune their sensory systems during major lifestyle changes.

Improved understanding of how deer keds and other biting flies use their senses could help inform more effective monitoring and control strategies in future.