The terms venomous and poisonous are often used interchangeably, but incorrectly. There is, in fact, a difference between a venomous insect and a poisonous insect.
Both venomous and poisonous animals produce a toxin that is injurious or even lethal to another organism. The real difference between the two terms involves how that toxin is delivered.
Venomous organisms deliver or inject venom into other organisms, using a specialized apparatus of some kind (usually fangs or a stinger). The venom is produced in a gland attached to this apparatus. Venomous insects occur in three groups: the true bugs (order Hemiptera), the butterflies and moths (order Lepidoptera), and the ants, bees, and wasps (order Hymenoptera). Spiders, centipedes, and scorpions may also be venomous. Venomous arthropods may inject their venom into prey to immobilize it, or use it defensively against potential predators - like people!
Poisonous organisms, on the other hand, do not deliver their toxins directly. The entire body, or large parts of it, may contain the poisonous substance. These organisms may be harmful when eaten or touched. Poisonous insects include members of quite a few groups: butterflies and moths (order Lepidoptera), true bugs (order Hemiptera), beetles (order Coleoptera), and grasshoppers (order Orthoptera), and possibly others. The defensive chemicals produced by these poisonous insects may cause vomiting or blistering, or have cardiac impacts on predators that try to eat or handle them.