Caterpillars drop on strand of silk when disturbed is known as

Caterpillars drop on strands of silk when disturbed is known as Reflex dropping.
Reflex bleeding is a defensive behaviour in some insects where a noxious fluid is exuded by the insect when threatened.
The fluid flows out through pores in the exoskeleton. The secretion consists of a bitter fluid, including haemolymph and noxious chemicals.