Arrowhead High School
Notes to the Teacher: to top
Few insects are as well known as the common mosquito. Everyone is familiar with these annoying little insects that take blood and leave an itching red welt. They are extensively studied by the scientific community because some species are capable of carrying parasites and/or disease. The earliest reference to mosquitoes as disease-causing organisms is found from an Indian Brahmin priest named Susruta who, in about 500 BC, declared that mosquitoes were responsible for malaria. Mosquitoes fit into the stories and folklore of countless cultures and go by many names. The word "mosquito" is of Spanish origin meaning "little fly." The approximately 3500 species of mosquitoes occur world-wide in habitats from the poles to the equator. Even though 75% of mosquito species occur in the subtropics and the tropics, the arctic tundra, where there are fewer than 12 species, has the greatest concentrations (huge swarms!!!) of mosquitoes. North America hosts about 130 species with roughly 60 species found in my home-state of Wisconsin (and on some summer evenings it feels like it!). Mosquitoes are classified as follows:
Order Diptera—the two-winged flies
Subfamilies (3) Toxorhynchitinae
As indicated by their classification within Diptera, mosquitoes go through the process of complete metamorphosis. This means that there are four distinct phases of growth: egg, larva, pupa, and adult (figure 1). The entire life cycle can take anywhere from around 10 to 40 days depending upon the species and temperature. The adult is terrestrial, though it requires water for reproduction. An advantage of a life cycle where the larvae are aquatic and the adults terrestrial is that the two forms utilize different food sources, thus eliminating competition between them. Eggs are deposited by the female in any place with standing (or moving) water such as cavities in trees, plant leaves, old tires, ponds, streams, marshes… Aristotle, in the 4th century BC, recognized that mosquitoes were aquatic creatures during the early part of their lives and only later changed into the flying form. He believed, though, that the aquatic stage was a result of spontaneous generation in putrefying waters. This opinion was still around in 1562 when it was written that mosquitoes arose spontaneously from the waters found in elm tree cavities.
||Female mosquitoes will lay eggs singly or in rafts of up to 300 eggs on the surface of the water. These rafts can be up to 5 mm in length and differ in size and shape based on the species. The eggs will develop for a couple days to over a week before hatching into the small, worm-like larval form. This length of time depends upon the species and the temperature. Eggs that are left out of the water may go into a sort of dormancy and may hatch upon rehydration.|
The larvae are generally filter feeders, though some are predaceous
and feed on the larvae of other, and their own, species. The filter-feeding
larva creates currents which help to bring bacteria, protists, and detritus,
the larva's food, to its mouth. Because of their small size, mosquito larvae
experience water as a much more viscous medium than we do as humans.
To them it seems much more syrup-like. As aquatic creatures, mosquito
larvae need to obtain oxygen and get rid of carbon dioxide. Aquatic
critters can do this in two ways: (1) live in the water and extract
dissolved oxygen from the water, or (2) live in the water and surface often
enough to get atmospheric oxygen. Mosquito larvae make use of the
second option. Larvae have tubes, called spiracles, on the posterior
end that act like straws that allow them to breathe atmospheric air.
The pupa is the next stage in the mosquito life cycle. Unlike the pupal stage of many insects, the mosquito pupa is mobile. During this stage, the body plan of the organism is completely reorganized from the aquatically-adapted larva to the terrestrial, flying adult. The fact that this change can occur in the 2 or 3 days of the pupa’s existence is amazing!
The adult emerges from the pupal casing very soft and pliant and not very ready for flight. The new adult generally has just enough energy left over from the larval stage to make a short, quick flight for some food. The first meal as an adult is usually nectar from flowers. Both male and female mosquitoes feed on the nectar of flowers for a carbohydrate/energy source. Nectar is ingested and proceeds into the crop for storage. This nectar is then transferred to the stomach for digestion as needed. As the mosquito goes for the nectar he/she brushes against pollen-filled anthers and plays a part in distributing pollen and fertilizing flowers. The importance of mosquitoes as pollinators is something that is unknown or overlooked by most people.
While the adult mosquito is mostly known for its attacks on humans and other animals to obtain blood, it is only the female that carries out these forays; the male does not need blood. There is debate as to how, exactly, mosquitoes find and choose their victims. It is thought that body heat, moisture, carbon dioxide, and smells all play a part. Most females will take 90 seconds or more to take a full blood meal. When the female takes a blood meal, the blood goes directly to the stomach where it is immediately acted upon to remove water and salts.
Male and female mosquitoes both take a day or two after emergence from the pupa to become receptive to mating. Mating usually takes place before the female’s first blood meal. Males locate females by responding to the frequency of the wing beats. Males use feathery antennae and an organ at the base of the antennae called Johnston’s organ to locate females of the same species. The flight tone of the female differs between species and is slightly lower than that of the male of the same species. The flight tone is between 250 and 500 beats per second, depending upon the species and gender. When mating occurs sperm is stored in a structure called the spermathecae and is then used as needed for fertilization by the female. One mating early in the female’s life is enough for a lifetime of egg-laying.
The following student activity is designed to encourage students to practice and use their skills of observation and data collecting. It can also be used to introduce or reinforce the concepts of metamorphosis and life cycle.
This activity does require some planning and preparation as well as familiarity with your area. Most people will not have a difficult time finding mosquito habitat. It is not a bad idea to check your collection area ahead of time to make sure mosquitoes are present and that they will lay eggs in your bucket of water. Many species of mosquitoes are rather particular about their habitats and may not deposit eggs in your bucket. Fill a bucket with water and some leaf litter and place it in a shady spot. This needs to be conducted during the normal mosquito season (unless you purchase your insects from a biological supply company). Periodically check the surface of the water for the small eggs (floating individually or in small rafts of eggs) or for the dark, wiggling larvae. Transfer small groups of eggs or larvae from the bucket of water to individual glass culture dishes.
Student groups receive the culture dishes of eggs with instructions to observe them and give as much information as possible. Do not tell the students that these are eggs or even that they are living things. They should not handle the eggs nor remove them from the water. After making observations, discuss and distinguish between observations and inferences. Pose the question…is this object living or non-living? Why? What could they do to find out? You may want to allow older students to try to dissect one or two of the eggs and view them under a dissecting or compound scope. Depending upon the age of the eggs there may not be much to see; the students may not see much that is helpful or they may find some clues to the identity of the object…either outcome will enhance the mystery and will likely intensify the debate. Students should keep a journal of observations and be given an appropriate amount of time each day or every other day to make these observations. Some periods of the cycle will require very little class time (not much observable change taking place) while other days will bring about significant changes. Probably all of the stages will require the use of a dissecting or compound microscope to adequately see the organism. Allow the students in each group to determine what equipment and procedures they need for the day's observations. Encourage students to make drawings, estimate sizes and lengths, and measure water and air temperature. These should be entered into the journal. Are all the eggs/larvae the same? It is possible for more than one species to be present in one bucket.
During the larval stage of the life cycle, students (or the teacher) will need to feed the little critters! They will do just fine on brewer’s yeast or flake fish food. Do not give them too much…a small pinch will go a long way. View the larvae using a dissecting or compound scope and include a drawing in the journal. Can students see the food being ingested. A drop of food coloring may indicate the water current being produced by the larvae's mouthparts.
After larvae go into the pupa stage, it is important to move the pupae into a container with clean water. Pupae do not eat and food will only foul the water and possibly kill the pupae. Have students bring in empty, rinsed two-liter bottles…one per group. Cut the bottles off as shown
(figure 2). Fill each one part way with conditioned (aged) water and transfer pupae to the new container. Have students cover the container with nylon mesh, cheesecloth, or similar material and secure with a rubber band.
||Once the adults begin to emerge from the pupae cases, you may want
to transfer them to another container without water. Students will
need to feed them some manufactured "nectar." Soak a cotton ball
in 10% to 20% sucrose solution and place it on top of the mesh. The
mosquitoes will feed on this sugar solution. To provide a blood meal,
you will need to find a volunteer—either yourself, the students, a rodent,
etc.—or place a couple drops of blood onto the mesh covering of the container.
Clements, A.N. 1992. The biology of mosquitoes, vol. 1.
Chapman & Hall, New York.
Gillett, J.D. 1972. The mosquito. Doubleday & Company, Inc. Garden City, NY. 358 pp.
Lawrence, D.H. 1968. The mosquito, from Poems Selected for Young People. Macmillan, London.
Bergeron, L. 1995. The mosquito & me. ScienceNotes. http://natsci.ucsc.edu/scicom/SciNotes/9502/Mosquito.html