Do your best to identify this plant. Some butterflies lay eggs singly, and others lay them in huge masses. Many caterpillars are solitary, while others feed in large groups. To tell someone it's a large, black caterpillar doesn't help. Give an approximate length and diameter.
Many caterpillars have 'eyespots' on their heads.
Some caterpillars have one horn on their rear-end, some have spines all over their bodies, and some have 'knobs' right behind the head. Describe how your caterpillar looks as accurately as you can. Observe it for a few moments and note any thing you see. All these clues will help in identification of your mystery caterpillar. It makes the job easier in identification.