Zhenming,

I suspect your samples are being exposed way too long in the processing solutions.  Fly heads and brains are extremely small.  I would recommend cutting the times in each your processing solutions to 20-30 minutes maximum.

To section the ones you already have processed, try facing into the tissue and then letting them sit for about 5 minutes in a crushed ice/ice water bath.  Blot them and try sectioning again.  You may have to repeat this every several sections because you will eventually cut past the area that the water has penetrated into the sample.  You never mentioned how thick you are trying to section these as well.  If you are sectioning thicker than 7 microns, you might also try using room temperature or warmer water.

Teri Johnson
Managing Director Histology Facility
Stowers Institute for Medical Research
1000 E. 50th St.
Kansas City, Missouri  64110
tjj@stowers-institute.org