Insects cannot grow gradually; their exoskeleton simply prevents it. Therefore, they must grow in stages, changing skin sometimes in the course of development and growing a size each time. The insects’ development can proceed in two radically different ways. Some insects undergo a complete metamorphosis. A good example is butterflies, where out of the egg comes a caterpillar that in no way resembles the adult. It must undergo a pupal stage where the complete metamorphosis to adult butterfly takes place.
In others, and among them the bed bugs, the little bugs that come out of the eggs broadly resemble the adults. They undergo an incomplete metamorphosis. In case of the bed bug, it goes through five nymphal stages (fig 18); they grow bigger for each molt and resemble the adults more and more.
The bed bug nymphs must have a blood meal between each molt.