Friday, 5 May 2006

Overlooked Dipteran on the wing

Patches of early-flowering plants in sheltered locations provide an ideal opportunity to watch many of our often-overlooked insects. Such spots are typically a few degrees warmer than surrounding areas, something that suits those insects that may be limited by air temperature. In early spring, a couple of degrees difference in air temperature can mean that an insect is on the wing, rather than sitting immobile in a state of semi-torpor. With a bank of young nettles and a range of spring-flowering plants, one warm corner of the Nunnery Garden in Thetford holds a great deal of interest for the entomologist. Just the other afternoon, there were several species of hoverfly on the wing, along with queen wasps, five species of butterfly and our commonest species of bee-fly, Bombylius major.

Bee-fly - Bombylius major

Bee-flies are the dipteran equivalent of hummingbirds, able to manoeuvre with ease and adjust their positioning while hovering to insert their long proboscis into tubular flowers. Bombylius major is about 11mm in length, furry in appearance and ginger in colour. The long proboscis is a prominent feature and one that may give cause of alarm. However, this fly is harmless and does not sting or bite. The high pitched whine made by its flight does remind me of a mosquito though and has, on occasion, given me cause to flinch as one has drifted past my ear. Although this species reaches as far north as Scotland, like other bee-flies it is on the northern limit of its range and so is most commonly encountered in southern Britain. Research has revealed that it cannot take to the wing if the temperature is below 17ÂșC and will vibrate its wings rapidly to increase its body temperature if necessary.

Bee-flies can be difficult to approach and I find that the best thing to do is to sit still, just in front of flowers that are likely to be visited. The territorial nature of Bombylius major also helps (as it does when watching many hoverfly species), since an insect that has been flushed will often return to the same patch after a few minutes. Male bee-flies will make darting forays to intercept other insects entering their airspace, presumably in the hope that one of the intruders will turn out to be a female bee-fly. Mated females produce a large number of eggs which, for most species, are distributed in a rather speculative manner within areas likely to be used by nesting solitary bees and wasps. It is the nests of these species upon which young bee-flies parasitize. The strategy must work, since bee-flies seem to do well around here and I can usually count on seeing several within sheltered spots each spring.

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