By-the-by, I was in a party supplies shop today and among the items on sale was a pack of 50 ants, 25 spiders and 25 flys, in black plastic for £1.50.
With a little bit of mutilation (eg, removal of head and attenae) and some tidying up of flash, the ants looked about the right size for 28mm face huggers. The pack was sealed so I couldn't tell if they were soft or hard plastic. They didn't have anything in the way of individual detail (just the body carapace plus arched legs) but they certainly had the volume!