This is the third post in a six part blog series about rare insects in the Cairngorms. A new project launched this year to save six endangered invertebrates in the north of Scotland and project officer Gabrielle Flinn will take a closer look at one of these species each month. This time, it's the turn of the northern silver stiletto fly. The Rare Invertebrates in the Cairngorms project is a partnership involving RSPB Scotland, the Cairngorms National Park Authority (CNPA), Buglife Scotland, Butterfly Conservation Scotland and Scottish Natural Heritage (SNH).

Soaring into the air about two to three metres from the ground and flashing its reflective body, the northern silver stiletto fly (Spiriverpa lunulata) suddenly drops to the floor again. Males will do this, flying up and dropping again, several times before ascending into a full dance to try and attract the attention of a female.

If a female appears, the males will form 'leks' (much like some bird species, for example black grouse) to compete for her attention. Despite the allusion to high heeled shoes in its name it is not called the northern silver stiletto because of this performance. The abdomen of the fly is shaped like a stiletto knife and covered in silvery hairs to give it a shimmering appearance. 

The fly survives on shingle - the sandy, rocky banks that can be found alongside and on rivers. It prefers slightly vegetated river shingle where it can shelter and feed as an adult. River shingle habitat can be impacted by annual flooding and so long-life banks that manage to withstand this flooding are of great importance as reliable sites to the fly.

River Shingle. Photo credit: Stephen Hewitt. 

Amongst the sandy deposits, the eel-like larvae travel just below the surface searching for invertebrate prey to bite and destabilise with their venom before consumption. The venom is not dangerous to large animals such as humans, however, so there is nothing to fear whilst having your picnic by the Spey!

After enough feeding the larvae will pupate into an adult with June and July being the best time to see them. Females can be distinguished from males by their smaller eyes (as with all flies) but also by the black stripes on the abdomen between the silvery hairs.

During the training day that we ran this month, volunteers were lucky enough to find several larvae and observe the males starting to dance. Over the course of the Rare Invertebrates in the Cairngorms project we hope to better understand the distribution of this species so we can help to conserve it and the integral habitat on which it and many other invertebrates rely.

The first two blogs in this series focused on the shining guest ant and the Kentish glory moth. Give them a read by clicking the links.