You select the shutter speed by rotating the dial.

The numbers represent fractions of a second. For instance 60 indicates 1/60 sec.

The mark next to 60 indicates that flash exposures must be taken at 1/60 sec. or less (30 - 1). Otherwise you do not get a full image. This may be 125 on some cameras.

Exposures taken at 125 will freeze slowly moving subjects, and it is a good general shutter speed.

Higher settings can be used to freeze action for more rapidly moving subjects like bees and lizards!

I use either Ektachrome 100 or Fujichrome 100 slide film for well illuminated outdoor photos and for flash photography. They yield similar results.

Both can be processed locally. This is important for research photos because they are less likely to be lost. 

The number 100 indicates its ASA rating of sensitivity.

The higher the number, the greater the sensitivity.

ASA 400 film is more sensitive than ASA 100. Thus, ASA 400 is better for low light photography.

Image quality declines as film sensitivity increases. However ASA 400 films are still very good.

Print film does not produce the same level of resolution as slide film but it is more forgiving with incorrect exposures. Furthermore, you can scan the prints and their negatives. Print film is probably good for field situations while slide film is better for photomicroscopy.

Print film comes with ASA ratings up to 3600 and is consequently better for low light situations.

However, it is important to take some test exposures under conditions like those you expect to encounter at your research site to avoid any unpleasant surprises.