Scientific drawing is a technique, not art. It is both representation of the object and scientific interpretation. It may show structures that a photo would not show, like a dark element in front of a dark background. It may emphasize structures considered important by the scientist. At the same time, it always remains as close to the object as possible.
 

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This series shows four steps, beginning with the original (1: a male spider palp as seen under the microscope), then the tracing of contours using a camera lucida on a compound microscope (2), adding of shadows and degree of sclerotization, using a dissecting microcope (3), and final illustration after editing in Photoshop (4).
I use simple copy paper, a hard pencil for the contours and a soft one for the shadowing. This saves a lot of time in comparison to stippling with ink, but it is not done quickly either: I spend about one hour on a drawing like the one shown here.

Click here to download a Powerpoint presentation containing this sequence in higher resolution.