Darkling Beetle/Mealworm Information

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Phylum, Arthropoda; Class, Insecta; Order, Coleoptera

Identifying Features

Appearance (Morphology)
Adult Beetle

  • Black with hardened front wings (elytra)
  • Antennae arise under ridge near eyes
  • Antennae many-segmented, enlarging near tip
  • Shape quite variable, from almost parallel-sided to round
  • Head visible from top, followed by pronotum and elytra about same width
  • Mealworm (larva) averages an inch in length. They have a tough yellowish brown exoskeleton and are cylindrical.

Adult Males and Females
It is difficult to tell the difference between the males and females without a microscope and dissection.

Immatures (different stages)
The larval stage (referred to generally as mealworms) is worm-like and somewhat hardened for burrowing. The egg is white. The pupa is 1/2 to 3/4" long., white initially then darkening just before the beetle emerges. Length of the life cycle is 3-5 months. The larval stage may molt 9-20 times.

Natural History

Food
The beetles and larvae eat decaying leaves, sticks, grasses and occasionally new plant growth. As general decomposers, they also eat dead insects, feces and stored grains.

Habitat
Mealworms live in areas surrounded by what they eat under rocks, and logs, in animal burrows and in stored grains. They clean up after plants and animals, and therefore can be found anywhere where "leftovers" occur.

Predators
Many predators eat mealworms including rodents, lizards, predatory beetles, spiders, and birds.

Interesting Behaviors
When disturbed, some beetles (genus Eleodes)assume a defensive posture in which they stand on their head and release chemicals from a scent gland in the rear that produces noxious odors and turns skin brown. Mealworms prefer darkness and to have their body in contact with an object.

Impact on the Ecosystem

Positive
Clean up organic materials not readily used by others. Mealworms are food for other animals.

Negative
Sometimes mealworms feed on seedlings and clip plants off near soil line. Mealworms can be pests to stored grain.

Collecting Live Insects

Where to Collect
Rather than spend time looking for mealworms in the wild, spend a couple dollars of buy them. Most pet stores and many fish tackle shops sell small and large mealworms. They are sold individually or in amounts of 50, 100 or 200. The large mealworms cost more, but are more lively and easier for students to observe. The large mealworms often are treated with hormones so they will NOT become adult beetles. The small mealworms will change into adult beetles within a month or two. Mealworm larvae and adults can be purchased from: Berkshire Biological Supply Company, The Biology Store (pupae also available), Carolina Biological Supply Company, Connecticut Valley Biological Supply Co., Inc., Nasco (pupae also available), Science Kit and Boreal Laboratories (larvae only), and Ward's Biology (pupae also available).

How to Make a Mealworm Experiment Container

Materials

  1. Scissors
  2. Masking tape
  3. Mealworms
  4. One container for each team or inividual student
  5. Equal numbers of black and clear film cans
  6. Black construction paper

To Make a Container

  1. Remove the lids of the film cans.
  2. Cut squares of black construction paper larger than the film can opening.
  3. Place a mealworm in the clear container.
  4. Place the black paper between the openings of the film cans. Leave enough space for the mealworm to move to the other side.
  5. Join the two cans together and secure the paper with tape. When taping, try not to cover up too much of the clear film container.

Handling Mealworms
To pick up a mealworm, use a plastic spoon or a folded 3x5" card to scoop up. When holding mealworms, keep hands over the container or table to avoid dropping the mealworms on the floor.

Precautions
Do not leave the containers where the sun will hit them. The containers can be stored in the refrigerator for a couple of days if the mealworm is provided with a little bran. Keep in mind, adding the bran also adds a new variable to the experiment. After the experiment is completed, dismantle the containers and give the mealworms food.

Container Diagram

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