I know you said they were spiders and not mites.but it sounds like red spider mites.
These (if they are really tiny, and leave a bit of red dye when smashed) could be common red spider mites.
There are two types of common red spider mite; glasshouse red spider mite Tetranychus urticae and Fruit tree red spider mite Panonychus ulmi. Damage caused by these mites can be significant when they occur in high numbers. As plant sap feeders, the first sign of damage is a speckling on leaves eventually turning them yellow. Young plants growth is distorted and older plants loose their leaves. Fine webbing is formed, which is then used to transport very high numbers of red spider mites about.
http://res2.agr.ca/parc-crapac/pubs/bene/lp29f .jpg
http://www.evergreengardenworks.com/mites.htm