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Can anyone explain what made these trails on a second-floor skylight (see photo, above)? The skylight is under a tall lime tree.

• The trails were made by grazing molluscs. Slugs and snails have a radula, or “tongue”, rather like a nail file, with which they rasp the surface of their food, in this case a film of algae growing on the skylight. The trail is made as the animal moves forward, swinging its head from side to side to remove the algae from the glass. Close observation of such marks often reveals the imprint of individual spikes from the radula.

Each trail ends when the animal stops feeding and creeps off the glass, which is why there is a lower concentration in the centre of the pane: the slugs start feeding at the edge – and one edge here is obviously more accessible than the others.

I think the trails are likely to have been caused by slugs, as these tend to be much more active than snails (the consequence of not having a shell). I also think they are more likely to reach a second-floor surface.

Tim Bolton-Maggs, Edinburgh, UK

• The marks on the glass are almost certainly the feeding trails of garden molluscs grazing on algae, which is growing on the upper surface of the glass. The glass, sheltered by the lime trees, will provide a nice growing surface for algae. It may be heated from below, and will be fertilised from above by honeydew or excrement from insects feeding on the lime leaves.

Roger Harris, Romsey, Hampshire, UK