Invertebrates are animals without backbones, and they are the largest group of animals. This group includes insects, arachnids, worms, mollusks, crustaceans, jellyfish, octopus, squid and sea urchins. Invertebrates vary widely in their body composition, and there are several types of life cycles. Four distinct stages complete the invertebrate life cycle, including egg, larvae, pupa and adult, explains Bobbie Kalman in "Animal Life Cycles."
With the possible exception of the seahorse, which gives birth to live young, invertebrates lay eggs. The egg may be hard-shell if the animal is an insect or arachnid and lays eggs on land. It may also be jellylike, if the animal lives in the ocean like a jellyfish or lobster. The female invertebrate lays her eggs in a chosen place, like on a milkweed plant for the monarch butterfly. The eggs have already been fertilised by the male. Some invertebrate females watch over their eggs, while others, like butterflies and lobsters, do not exhibit any parenting.
The second stage of an invertebrate life cycle is the larvae stage. The baby invertebrate hatches from the egg, but is not yet fully formed into an adult. The caterpillar is the larvae stage for butterflies and moths, while the nymph is the larvae stage for the locust.
The caterpillar, or larva, then enters the pupa stage, where it begins to grow. Sometimes pupa do not have to find food, for their parents or other members of a colony bring them the food. For butterflies and moths, caterpillars enter a chrysallis and after a number of weeks, emerge as an adult. Some insects spin cocoons. Other invertebrates like lobsters and locusts shed their skin in a process called moulting.
Once the grub or larvae has reached a certain period of development, and after a set number of days of feeding, they grow into an adult. Caterpillars emerge from the chrysallis to form a butterfly. Often, the outer skin or shell is moulted, and the adult invertebrate emerges. When the invertebrate becomes a full adult, they are ready to mate, lay eggs, and begin the life cycle over again.
Although this is the basic life cycle for insects, it varies from invertebrate species to species. Young octopus and squid merely hatch and grow into adults, without having a pupa or larvae stage. Marine invertebrates like lobsters, crabs and sea urchins also have three stages: egg, young and adult.