SAMPLE LAB REPORT
Perception of Different
Sugars by Blowflies
by Alexander Hamilton
October 24, 2009
To feed on materials that are healthy for them, flies (order Diptera) use taste
receptors on their tarsi to find sugars to ingest. We examined the ability of blowflies to
taste monosaccharide and disaccharide sugars as well as saccharin. To do this, we attached
flies to the ends of sticks and lowered their feet into solutions with different concentrations
of these sugars. We counted a positive response when ...view middle of the document...
In this experiment we tested the ability of the blowfly Sarcophaga bullata to taste
different sugars and a sugar substitute, saccharin. Because sucrose is so sweet to people, I
expected the flies to taste lower concentrations of sucrose than they would of maltose and
glucose, sugars that are less sweet to people. Because saccharin is also sweet tasting to
people, I expected the flies to respond positively and feed on it as well.
We stuck flies to popsickle sticks by pushing their wings into a sticky wax we
rubbed on the sticks. Then we made a dilution series of glucose, maltose, and sucrose in
one-half log molar steps (0.003M, 0.01M, 0.03M, 0.1M, 0.3M, and 1M) from the 1M
concentrations of the sugars we were given. We tested the flies’ sensory perception by
giving each fly the chance to feed from each sugar, starting with the lowest concentration
and working up. We rinsed the flies between tests by swishing their feet in distilled water.
We counted a positive response whenever a fly lowered its proboscis. To ensure that
positive responses were to sugars and not to water, we let them drink distilled water before
each test. See the lab handout Taste Reception in Flies (Biology Department, 2000) for
Flies responded to high concentrations (1M) of sugar by lowering their probosces
and feeding. The threshold concentration required to elicit a positive response from at
least 50% of the flies was lowest for sucrose, while the threshold concentration was highest
for glucose (Fig. 1). Hardly any flies responded to saccharin. Based on the results from all
the lab groups together, there was a major difference in the response of flies to the sugars
and to saccharin (Table 1). When all the sugars were considered together, this difference
was significant (t = 10.46, df = 8, p < .05). Also, the response of two flies to saccharin was
not statistically different from zero (t = 1.12, df = 8, n.s.).
The results supported my first hypothesis that sucrose would be the most easily
detectable sugar by the flies. Flies show a selectivity of response to sugars based on
molecular size and structure. Glucose, the smallest of the three sugars, is a
monosaccharide. The threshold value of glucose was the highest in this experiment
because a higher concentration of this small sugar was needed to elicit a positive response.
Maltose and sucrose are both disaccharides but not with the same molecular weight or
composition. It has been shown that flies respond better to alpha-glucosidase derivatives
than to beta-glucosidase derivatives (Dethier 1975). Because sucrose is an alphaglucosidase derivative, it makes sense that the threshold value for sucrose occurs at a lower
concentration than that for maltose. This might also be the reason why sucrose tastes so
sweet to people.