Rather struggling with 13d, 14ac by an egg-case and adult insects (8). I have -e-a-o-a. 14ac answer is powder. Thinking Hexapoda, though not seeing wordplay. Best I can get from it is 'a pod' for "an egg-case", and 'a' for adult, to give '-apoda'. Unclear about the 'hex'. Metabola fits, and pertains, bit not even seeing a tenuous wordplay!