The apterae of the aphid Chaitophorus tremulae are elongate oval with the dorsum solidly blackish sclerotic, rather densely sculptured with denticular spinules (=small toothlike spines), and very often with a paler line along the mid-dorsum.  More information can be found using the link below.

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Well wooded areas, where Aspen is present. 

Oviparae and alate males occur in October.

Aspen leaf aphids live in small colonies on undersides of leaves of Aspen (Populus tremula) and a few related species of Populus. They have also been found in leaves spun together by other insects, or in leaf-nest galls made by another group of aphids (Pemphiginae).

Can be locally common in areas of Britain where the host species occur.

Status in Leicestershire and Rutland not known.

https://influentialpoints.com/Gallery/Chaitophorus_tremulae_Aspen_leaf_aphid.htm