The woolly Pyrrharctia isabella caterpillar (left) has many setae, while the saddleback caterpillar Acharia stimulea (right), has very defined scoli. Incidentally, A. stimulea has stinging hairs while P. isabella does not (Woolly caterpillarphoto by Tod Robbins on iNaturalist, public domain. Saddleback caterpillarphoto by gillydilly via iNaturalist, public domain.)