"bubbling" behavior a fly con- 

 centrates liquid foods, through 

 evaporation. 



In our third test, we looked 

 closer at fly regurgitation or 

 "bubbling" behavior. We pre- 

 dicted that flies would increase 

 this behavior with decreasing 

 concentration of food encoun- 

 tered. We therefore presented 

 flies with a constant quantity of 

 food diluted in droplet size of 

 increasing volume (decreasing 

 concentration). Results from us- 

 ing both yeast (Table 2) or su- 

 crose solutions confirmed our 

 prediction. Indeed, through 

 "bubbling", hungry flies appear 

 to eliminate excess water from 

 the body to make space available 

 for more food. Longer total rest- 

 ing times between the various meals corresponded to 

 increasing volume sizes of food droplets ingested. Flies 

 apparently evaporated through "bubbling" sufficient 

 water to enable them to ingest progressively, in various 

 small meals separated by "bubbling" periods, up to 8 

 ftp of diluted food. We found that this behavior oc- 

 curred not only in females, but also in males. Although 

 "bubbling" was observed over a broad range of tem- 

 peratures, it occurred more readily under higher tem- 

 peratures and could possibly, therefore, also fulfill a 

 role as a cooling mechanism for flies under warm 

 weather conditions. (What an "inventive" way to deal 

 with a hot July day!) On occasion, flies also regurgi- 

 tated "bubbles" onto the leaf surfaces and reingested 

 the remaining dry solids once the drops had dried out. 



Food Size and Finding Time 



In a final experiment, we tested whether flies can 

 find larger droplets of food easier than smaller 

 droplets. For this test, food droplets (yeast) 

 were not placed on the release leaf, but one on 

 each of the second and third uppermost leaves 

 of the twig. A fly was observed until it found a 

 droplet, left the twig, or 30 minutes had 

 elapsed. 



Results in Table 3 indicate that while more 

 flies did find larger food droplets, the average 

 time these flies took to discover larger droplets 

 was not different from flies discovering smaller 

 droplets. This outcome was to be expected 

 based on the stereotyped search behavior of 

 apple maggot flies we have observed on foliage: 



for both sizes of droplets flies took a similar time 

 hopping upward from leaf to leaf until reaching the 

 leaves with the droplets. Once there, and walking 

 across the upper leaf surface, flies were more prone to 

 miss smaller droplets, and came more readily into 

 contact with larger droplets. As we would have pre- 

 dicted for such small food droplets of such a low food 

 quality (low yeast concentration), odor apparently did 

 not play a role in droplet discovery. 



Conclusions 



We found that the quantity and quality of food en- 

 countered on a leaf surface determines subsequent 

 foraging time of apple maggot flies on the immediate 

 surrounding foliage. Also, the form of the food encoun- 

 tered affects, through the food "handling" or "process- 

 ing" time (feeding and resting), subsequent time avail- 

 able to flies for further foraging or other activates. Dry 



Table 3. Effect of droplet size of constant concentration of 

 protein (1% yeast), on apple maggot fly discovery or 

 leaving time on apple twigs. 



Droplet 

 size 



Discovering 

 food droplet 



(%) 



Average time 

 to discover 

 food (min.) 



Average time* 



to leave twig 



(min.) 



0.0^1 

 1.0^1 

 10.0/<1 



30 

 52 



6.4 

 6.5 



9.7 

 9.5 

 8.2 



Includes only flies not discovering a food droplet. 



8 



Fruit Notes, Summer, 1990 



