A number of people have expressed disappointment that Moths of North America and Butterflies of North America do not contain information on the duration of the pupal (cocoon or chrysalis) stages of the various species.

Actually, the information that you need to approximate the emergence ("hatch") date is there for many species if you know how to read it. The secret is in the Flights section of the species account.

A "flight" is a generation of adults. If there is only one flight, say in May-June, it means that adults emerge in May-June (usually earlier in the south, later in the north), mate, and deposit eggs. Depending upon the species, the caterpillars may complete their development in the fall, overwinter as pupae, and emerge in May-June. Or they may overwinter as caterpillars, resume development in early spring, then have a brief pupal stage and emerge as adults in May-June. The Life History section of the species account indicates the overwintering stage for many species.

If there are two well-spaced flights annually, say April-May and July-August, it means that the adults that emerge in spring lay eggs that complete the entire life cycle by July-August, and that the July-August flight produces the overwintering generation. Note that many widely distributed species have only a single flight in the north but two more in the south.

Therefore, by knowing your location within the species' distribution, and when your caterpillar pupated relative to the species' annual flight pattern, you can determine approximately when emergence will occur . However, because temperature (and in some species, day length) affect development rate, timing may be disrupted by indoor rearing (see Rearing Butterflies and Moths).