Leatherjackets are the larvae of crane fly - they usually inhabit lawns and can cause significant damage because they eat through the roots. One clue as to their presence is birds pecking at the turf to get at the larvae. Around September/October, the larvae have matured and they then (having laid eggs) exit and fly away - (often into my sitting room where they bumble around crashing into lights and people, flying daddy long legs, ugh...)

Thanks, Bamboo.