1. Primary-regurgitators responded immediately to attack by directing regurgitant at the site of attack. They could control the amount of fluid expelled: a little for a weak pinch; more for a harder pinch. Fluid was re-ingested after the defensive response ended.
2. Secondary-regurgitators first exhibited an alternate defense such as flailing, biting and escape. Regurgitant oozed from the mouth, not in a distinct droplet.
3. Some caterpillars, such as the red spotted purple never regurgitated.
Grant measured the lengths of the crop, midgut and hindgut and found that the relative lengths varied and were correlated with regurgitation defense. Primary-regurgitators had the longest relative crop and shortest relative midgut. Non-regurgitators had the shortest relative crop and longest relative midgut. Secondary-regurgitators were intermediate. The hindgut did not vary with behavior. Caterpillars that use regurgitation for defense may have adaptations to the crop and foregut that provide better control of regurgitation and a larger pool of regurgitant.
*Grant, JB. 2006. Diversification of gut morphology in caterpillars is associated with defensive behavior. The Journal of Experimental Biology 209, 3018-3024.