An "invertebrate" is any animal lacking a "back bone" or vertebral column. The group includes 95% of all animal species! Ladybugs belong to the group "Arthropod." Arthropods are the largest group of animals, and includes all insects, arachnids, and crustaceans. Ladybugs are insects. Covered by a hard exoskeleton made of chitin. It is impossible to say what the average length of a ladybug is without knowing exactly what species of ladybug you are asking about. There are more than 5000 different species of ladybugs worldwide; 400 kinds live in North America alone! Generally speaking, they are small, brightly colored beetles, best recognized by their spotted carapaces. They vary widely in size, color, and number of spots, but you could hold tens of thousands of them in a single gallon sized jar. Ladybug males tend to be smaller than females. Ladybugs and their larvae (offspring) are voracious eaters of harmful, plant-destroying bugs, but they are quite harmless to humans, and are beloved by farmers and gardeners everywhere.