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MEALWORM – BACKGROUND INFORMATION

The mealworm is not a worm; it is a larva. Any similarity to a true worm is incidental. Mealworm larvae are golden yellow and have 13 segments—ahead, three thoracic segments, and nine abdominal segments. Mealworm larvae are the counterpart of the familiar caterpillar in the butterfly story. They pull themselves around on six stubby legs, one pair on each thoracic segment.

Mealworms are actually darkling beetles. Beetles, along with all their other insect kin (true bugs, flies, bees, wasps, ants, on and on), are members of the phylum Arthropoda, a word meaning jointed legs. Like all members of their phylum, insects wear their skeleton on the outside like a suit of armor. This is practical when they are under attack, but very inconvenient when they are trying to grow.

Arthropods have solved this problem by molting (shedding) this outer shell-like cuticle periodically. Immediately following the molt, the soft white larva expands before the new larger cuticle hardens. For mealworms this process repeats five times over a 2-month period, after which the larva is about 3 cm long. The final larval molt reveals the next stage, the pupa.

Pupa don’t eat, and they don’t move except for a twitch or two when disturbed. Inside, however, the mealworm is undergoing a dramatic transformation into a beetle, much the same as a caterpillar turns into a butterfly while inside a chrysalis. After 2-3 weeks the cuticle covering the pupa splits near the head, and the mealworm molts for the final time. Out walks an adult darkling beetle, white at first, but soon turning to reddish brown, and finally black. Adults can live up to a month if they are supplied with brand and apple, potato or lettuce (which has been used at school) for moisture. The beetles will climb on a piece of egg carton if you put one in the container. Late in the day you might notice a lot of activity and wing flapping as the beetles prepare to mate. A few days after mating, the females lay eggs in the bran. In time the eggs will hatch, producing tiny mealworm larvae. The life story of the mealworm has come full circle.

Darkling beetles in the larval, pupal, and adult stages are excellent pets for children to observe. They exhibit interesting behaviors, they are small but not tiny, they don’t bite, smell, fly, or jump, and they are extremely easy to care for. Larval mealworms live right in a container of their food: bran, cornmeal, or rolled oats. Wheat bran is recommended.

Mealworm waste, called frass (“poop” to students), is produced in tiny granules that are clean and safe to handle. When students notice them for the first time, they often think the tiny grains are eggs. Frass will accumulate until the culture container holds nothing else. The frass can be separated from larvae and food with a sieve if you like, but if you do this, you will also toss out the eggs (if adults have been present) because they are indistinguishable from the granules of frass.

Mealworm culture must be kept dry. Mealworms can go through their complete life cycle without any added water (they are very efficient at extracting water from their food), but it is recommended that moisture continually be provided in the form of small bits of apple, sweet potato, or carrot. Otherwise the larvae and adults may attack and eat each other in search of additional moisture. If carrot or sweet potato is used as the moisture source, the frass will be orange, adding evidence that the granules are waste rather than eggs.

Large cultures of mealworms (200 or more) should be kept in large, relatively flat containers. They seem to thrive best when the colony has a large surface area. Keep the bran 2-4 cm (1-2”) deep. If you want to expand your mealworm activities, any basin, bus tray or old aquarium will do. If the container sides are steep and smooth, it is not necessary to cover the container.

The mealworm’s preferred environment is very dry, moderately warm, and dark. As the temperature increases, so does the rate at which mealworms advance through their life cycle. Under ideal conditions the complete life cycle can take place in as little as 3 months, but more likely will take 4. However, students should be able o see their mealworms advance through the three important stages of larva, pupa, and adult in 4-6 weeks if the larvae are large and well advanced at the time they are introduced. Mealworms and darkling beetles are rarely seen in the wild, but when they are, it is likely to be in a field where wild grasses flourish and seeds are plentiful. They are most often found in barns, grain storage facilities, and food-preparation areas. This organism has benefited by living close to humans, because we unwittingly provide a much better environment for them than can be found in the natural world.