Three hundred yards north of the Ferry Kitchen, in the damp, underground silence of an abandoned Roman sewer gallery that ran beneath the academy cliff, Imran Bell and young Tavi moved quietly by the soft blue soft glow of their wicker insect cages. The air down here was cold, dead. Then it heavily scented with ancient limestone damp, decaying river weeds, and powdery pipe lime flaking from the ancient Roman brickwork.
Imran knelt on the muddy flat stone slabs. He held his brass aspirator tube near a dark fissure in the living rock where the mountain met the foundation stonework. Beside him, Tavi carried a wooden rack holding four woven willow cages, each containing eight blue lantern beetles crawling calmly across their cedar shavings.
‘Notice how their wing-cases are resting, Tavi,’ Imran whispered, his breath misting in the cold underground draft. ‘When the air is sweet, their wings stay closed and their blue light pulses with a slow, four-second breath. But if sulfur dioxide or stone vapor leaks from Sorell’s upper pipes, the beetles will open their wing-covers and crawl toward the highest corner of the wicker weave. They can taste a single part per million of sulfur long before a man’s nose can detect the poison.’
