Hi Raven -
Nice Job!! If I might offer some advice to help you...
As a professional biomedical illustrator we learned over and over that you can get so hung up on the details (like say, painting snake scales) that one tends to lose form. Always try to think form before detail. Look at your pictures from a slight distance and squint your eyes... I'll notice, 'whoa, my whole painting looks flat!' Often the only place you truly see details are in the area between the highlight and dark areas (saves time too). Another trick is to look at your art in a mirror once in a while to get a fresh perspective while working on it.
On your piece in particular it would be nice to see a core shadow on the pitcher (as if you were doing a cylinder study drawing) and also some shadow under the peristome to make those teeth pop.
The Guild Handbook of Scientific Illustration is a fantastic helpful reference book you may want to check out. See if you can get it through inter library loan.
http://http://www.amazon.com/Guild-Handbook-Scientific-Illustration/dp/0471360112/ref=sr_1_1?s=books&ie=UTF8&qid=1291648201&sr=1-1
When the second edition came out, my wife and volunteered to write a new chapter on "From 2D to 3D."
Her's a piece I did years ago. It was in pre computer-art days and was created with watercolor, gouache, colored pencil, and airbrush. Notice you how you can see details and the core shadow/shading on the wasp too:
Detail:
Here's something more recent, a frame from a 3D animation we did on Lyme Disease in dogs. It was created with Maya and composited with Adobe After Effects. The image shows canine red blood cells, antibodies, and the spirochete that causes the disease:
Hope this info helps and inspires you.
WildBill