4. GROWTH AND DEVELOPMENT OF THE MEALWORM
Mealworms have indirect development, which means that the larvae do not resemble the adults. This type of development is termed complete metamorphosis. There are always four stages of development: egg, larva, pupa and adult. Only the adults have wings and can reproduce.
In this species, all stages are active both during the day and at night.
A. Egg. The eggs are oval in shape, and milky white in color with a slight shine. They are foundsingly or in clumps mixed in the rearing medium. The eggs are small, about 1.8 x 0.7 mm.
The incubation time at 80°F is about 7 days.
B. Larval stage. The egg hatches into the first instar larva, which after feeding molts into the second instar larva. First and second instar larvae are very small and difficult to see without a magnifying glass. At each molt, the old skin (cuticle) is shed and a new and larger cuticle is formed. The first instar larva molts into the second instar. The larvae eat and grow in each larval instar to the limit their armored cuticle permits, then they must molt again in order to grow. Immediately after a molt the larva is soft and white, but within a few hours the cuticle becomes hard and assumes its normal yellow and black color. The number of larval instars varies from about 8-12, depending on temperature and available food. Higher temperature means fewer molts and faster development. The duration of the larval stage is from a few weeks (high temperature) to several months (room temperature). A full-grown last instar larva is about 30 mm long and about 4 mm in diameter.
B. Pupal stage: The full grown, last larval instar molts into a pupa, which looks somewhat like the adult. Note the wing pads and the adult-like legs and antennae. The pupa is quite vulnerable, it cannot move about, and it does not feed. If rearing conditions are poor, the larvae or the adults will eat the immobile pupae. If cannibalism is noted, add more moist material (quencher, potatoes, etc.). During the pupal stage, the internal organs of the larvae are digested and the adult internal organs develop. The pupa is dark yellow grading into dark brown or black, and about 25 mm long by 7 mm in width. The duration of the pupal stage is about 6 days at 80°F.
C. Adult stage: Mealworms in nature emerge as pupae and the adults emerge in the spring. Indoors at room temperatures the adults emerge from the pupae throughout the year. The adult males and females can only be distinguished by examination of the genitalia (see below under External Anatomy). The adult males live an average of about two months, the females about 3 months. Mating of the beetles starts within a few days after emergence and is repeated at intervals throughout the life of the beetles, probably 2 or 3 times in their lifetime. Semen is transferred directly through copulation. The females generally start laying eggs 8-10 days after pupal emergence at 80°F and females will lay their eggs singly or in small batches at intervals of 2 to 8 days. As mealworms consume feed, fresh feed can be added to the terrarium.
