Overview & Lesson Sequence

Focus on Standards & Assessment

Materials Needed, Preparation & Planning, Management Strategies

Background Info

Intro Activity
Why Study Predator - Prey interactions?

Activity 1
Observation of a predator-prey interaction

Activity 2
What makes a predator successful?

Research Project
Designing a predator - prey experiment

Optional Activity
Biological Control vs. Pesticides

Case Study
Rabbits in Australia




Resource Sheets
-Plant Propogation
-Rearing Aphids
-Eyelash Brush
-Petri Dish Habitat
-Sampling Methods
-Observation Check Sheet
-Sample Rubric for Group Presentation
-One Gallon Cage
-Predators in the Environment Data Sheet
-Predation Inquiry
-Green lacewing Larval Mouthparts
-Chewing Mouthparts

-Hemipteran Mouthparts

-Green lacewing
-Ladybird Beetle
-Big eyed Bug
-Praying Mantid
-Fruit Fly

-Green lacewing
-Ladybird Beetle
-Praying Mantid
-Big eyed Bug

Enforcers Home



Fruit Fly Rearing

Natural History
Drosophila melanogaster, or the red-eyed pomace fly, is classified in the family Drosophilidae, and order Diptera (which also includes flies, mosquitoes and midges.)

In the wild, they are found near ripe fruit where the adults and larvae feed on yeast and bacteria growing on rotting fruit. In the lab, they can be fed on yeast cells growing on a high carbohydrate prepared diet.

The average life span of Drosophila  depends on the environmental conditions. There are records of 80-153 days, but the average life span of a Lab fly is 26 days for a female, and 33 days for a male. (Under crowded conditions this may be reduced to 12 days. Also mutant flies generally have a shorter life span).  Temperature greatly effects the rate of development. At room temperature (25˚C): 10 days from egg to adult; at 20 ˚C: 13 days; at 15˚C: 90 days. There are four phases to the life cycle: egg - 3 larval (instar)stages - pupa - adult.

Courtship begins by the male tapping the abdomen of the female with his foreleg. This is his means of identifying his own species. He approaches the female fly from the front, and circles around her making half turns. He sticks a wing out, vibrating it for several seconds. If the female is receptive, copulation results. Sperm from the male are held in the seminal receptacle or spermatheca.  There may be more than one mating.  Eggs are .5 mm long, and are laid generally after the third day of the female’s adult life.

Larvae hatch in 22 hours, and grow and feed for four days, (longer at lower temperatures). The larvae are transparent, and one can see the inside organs such as the coiled intestines, whitish fat bodies, and gonads (visible in the male but very small in the female). During the third instar stage of larval development, observers of Drosophila will notice the larvae crawling up the sides of the culture container in preparation for pupation. At this time one can see dark projections called pupal horns, off the anterior end. These are the spiracles (outside opening of the respiratory tubes) turned inside out.

The pupal case forms, darkens and hardens. Sexing of the pupae can be done by looking for tarsal sex combs of the male, visible from the ventral side. The sex combs in the adult males, are tufts of dark hair found on the most proximal tarsal joint of the foreleg, used for holding the female during mating. Even in the pupal phase, these dark patches are visible in males.

The pupal stage lasts for 4 - 6 days, during which time metamorphosis occurs. Larval tissues are broken down (except for the brain and a few other tissues), and imaginal discs (pockets of cells stored in the larvae) develop into adult organs. There is a disc for each leg, wing, eye, antennae etc.  Finally the pupa is ready to eclose (emerge) into the adult stage.  Adult Drosophila males and females can be easily distinguished. Males are smaller, with a rounded, blackened tip to their abdomen (posterior segment). Females have a pointed abdomen, with a pattern of even dark bands.

Raising Drosophila in the classroom is easy and inexpensive.  It is wise to keep the cultures at room temperature, and to keep the culture vials or tubes humidified. (A small, moistened piece of filter paper, or water at the bottom of the rack in which the culture tubes rest will do this job.)  Subcultures can be made at any time, but adults should at least be transferred to fresh medium every 20-30 days to avoid problems with mites and mold. Overcrowding slows developmental time because of competition for food and a place along the side of the tube to pupate.

When raising mantids, a large and continual supply of fruit flies is needed.  To do this set up several culture vials (subcultures) every two weeks.  Getting your culture started is the hardest part. Preferably before your mantids start to hatch, order or obtain 2-3 vials of feeder Drosophila  from one of the biological supply houses.  (Note that the mantids can go for a couple of days without food as long as you keep their cage humid).  Once the flies arrive, set up about 4 fresh vials placing approximately 10 flies in each, and set them aside.  Keep the remaining flies in the vials they were shipped in and use them for mantid food.  You may want to set up 4 additional vials the following week to jump-start your colony, but after that you need only set up 4 vials every 2 weeks.  This should give you plenty of flies for each egg case hatched. Keep all your vials (labeled with the date they were set up) in a plastic shoe box in rows with new vials at one end and older vials at the opposite end. Freeze vials and any remaining flies when the media gets dark, moldy and/or dries out.  You can cut this whole operation back to 1 or 2 vials once your mantid nymphs are ready to take on larger prey.  You may want to keep 1 or 2 vials going the rest of the year so that you have a starter culture the next spring.  If you take care of them well, you may not need to purchase any new flies the following year.  Just plan to step up your fly culturing operation in the early spring before you purchase your mantid egg case.

Note: the biological supply houses all carry flightless Drosophila melanogaster  for feeding small animals.  Some suppliers also carry a larger species such as D. hydei (flightless) and D. virilis  (a hardy species, but they do fly).  These are usually too big for some mantid species when they first hatch, but work great when the nymphs get a little larger.  You might want to keep a culture of both D. melanogaster and D. hydei.

Since fruit flies are so small, it’s best to handle them with a small paint brush or not at all.  When feeding mantids, you can just gently pour and tap flies into the mantid cage.  This works well with flightless fruit flies.  If you capture your own wild flies (which can easily fly out of your rearing vials) you may need to chill them in the refrigerator for a few minutes before tapping them into the mantid cage.

excerpted from Yale-New Haven Teachers Institute: “Using Drosophila to Teach Genetics” by Christina E. Arnini   http://www.yale.edu/ynhti/curriculum/units/1996/5/96.05.01.x.html


    Center for Insect Science Education Outreach
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