These are new leafmines for me, found during time recently spent checking the leaves of Alder trees, both the more frequently seen Alder (Alnus glutinosa) and the Italian alder (Alnus cordata), a common planting here along street edges and in parks. To me, these mines all look very snake-like!

These are the work of larvae of the fly species Agromyza alnivora. As the British Leafminers website describes, they create ‘An upper surface corridor, gradually widening, which is not associated with leaf margin or veins’, but the most distinctive feature of these mines is the frass, which is always deposited in two rows. The larvae make their mines twice each year – the technical term is bivoltine – in summer and in early autumn.