Gammarus Army

Gammarus flies, colours in "degradé" , ready for deployment

If we observ well, Gammarus species have a very flattened body and most of them have a hump on their back. 
I don't believe in ultrarealistic flies, but may be the shape, the "feeling" and  the colour of a fly is important.
An other important characteristic of the Gammarus flies is their weight. Deeper streams needs heavy flies.
Tungsten beads solve this problem, but what should be done, when a flattened body is needed?
A lead-based, multilayered body may be one solution, although several other techniques may be applied.