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MEALWORMS AND DARKLING BEETLES (Tenebrio
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—a head, 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 the larval stage of darkling (aka Tenebrio) 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 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.
Life cycle. Darkling beetles follow a life history
known as complete metamorphosis. Like butterflies and moths, they
go through four distinct stages during their life cycle. A female
beetle lays eggs, as many as 500 in her brief lifetime of a month
or two. The eggs are about the size of the period at the end of
this sentence. After a couple of weeks the equally tiny larvae
emerge from the eggs. The larvae are known as mealworms, but of
course they are not true worms. The larvae are golden yellow and
have 12 body segments. They are the counterpart of the familiar
caterpillar in the butterfly story. Mealworms pull themselves
around on six stubby legs that are all crowded at the front.
The larvae seem to have two purposes in life: eat and grow. Beetles
are arthropods, and like all members of their phylum they wear
their skeleton on the outside like a suit of armor. This is very
practical when they are being attacked, but very inconvenient
when they are trying to grow. The arthropods have solved this
problem by shedding (molting) their shell periodically. Immediately
following the molt the soft, white larvae expand before the new
larger shell hardens. This process may repeat half a dozen or
more times over a 3-month period, after which time the larvae
are about 2 cm (3/4") long. The final larval molt reveals
the next stage, the pupa.
The pupae don't eat and they don't move except for a twitch or
two when disturbed. Inside, however, the mealworm is turning into
a beetle, much the same as a caterpillar turns into a butterfly
while sequestered inside the chrysalis. In 2 or 3 weeks the pupa
splits open and out walks a beetle, white at first, but soon turning
to brown and finally black after a day. The beetles mate and lay
eggs, and the cycle repeats.
Habitat and food. 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 human enterprises, because we unwittingly provide a much
better environment for the success of mealworms than could be
found in the natural world. For this reason mealworms have become
a minor pest in grain storage areas.
Mealworms and darkling beetles are excellent classroom animalsthey
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. Mealworms live right in a container of their food source:
bran, cornmeal, rolled oats, breakfast flakes, or chick starter
mash. All are excellent foods, but bran and chick starter are
recommended. The food must be kept dry. Mealworms can go through
their complete life cycle without any added water (they are very
efficient at extracting water from the food), but it is recommended
that small bits of apple, potato, or carrot be added from time
Mealworms should be kept in large, relatively flat containers.
They seem to thrive best when the colony has a large surface area.
Keep the bran about 2 or 3 cm (±1") deep in a basin,
bus tray, aquarium, or plastic shoe box. If the container sides
are steep and smooth, it is not necessary to keep the container
covered. Adults and larvae seem to prefer hiding under bits of
paper or light cardboard; the pupae give no indication that they
The mealworm's preferred environment is very dry, moderately
warm, and dark. A bit of apple provides extra moisture for the
mealworms and seems to stimulate rapid growth. As the temperature
increases, so does the rate at which mealworms advance through
their life cycle. Under ideal conditions, in a classroom, the
complete life cycle can take place in as little as 3 months, but
more likely it will take 4 months. Cold slows the process almost
to the point of suspended animation. Mealworms can be put into
the refrigerator (not the freezer) for periods of time to stop
In addition to providing reliable opportunities for observing
a complete life cycle in the classroom as in the Insects
Module, mealworms can also be used for other activities.
In the Environments Module their response to
various environmental factors is investigated. Mealworms can be
used for structure/function observations and behavior investigations.
And they are just nice to have around to remind us that life on
earth takes a seemingly endless variety of forms, and that part
of being human is to have compassion and respect for all life.
Food and Water. The 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 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
Mealworm Homes. 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 5–10 cm (2–4") deep in the clear
plastic basin provided in the kit. 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 mealworms'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 it will take 4. However, students
should be able to see their mealworms advance through the three
important stages of larva, pupa, and adult in 4 to 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.
What to do when they arrive. Mealworm beetles
are shipped in a container with a "breathing" cap to
provide air. They need no special care but should be used as soon
as possible, as they have a rather short life span. Keep beetles
at normal room temperatures in low light. Store in a cool place
at 45 to 65º F out of direct sunlight. At warmer room temperatures,
larvae will soon pupate. Cover loosely with a paper towel to provide
crawling space. Add slices of potato or carrot for moisture and
add a substrate of bran for food. Replace as necessary or if it
Mealworm Life Cycle
||5 molts occur
||Death: 30 days
||The cycle continues.
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