I have not done so with isopods, but you might try a method that's been used for manipulating small biological organisms and drawing hemolymph from small shrimp. This works best with pipettes that have a long tip. Try drawing out the tip of a glass transfer pipette into a very fine point.  Gently pull the tip away from the body of the pipette while carefully holding it over the flame of an alcohol lamp. The idea is to create a long, slender, very thin tube out of the tip.  It works with a bunsen burner too, but the glass melts very quickly and gets quite hot, so you have to be very careful. You can also do this with capillary tubes then use a micropipette bulb for the capillary tube. Wearing safety glasses and latex gloves, gently score the extended tip with a diamond-tipped marking pen, then wrap it in tissue and snap it off at the break. You may need to make a few dozen before you get a couple that work well.

If you use a micromanipulator, you might try supergluing the isopod to a small wooden dowel, then using the micromanipulator to control your sampling. Don't expect to get more than 10-15 microliters of hemolymph. Not sure where to try on an isopod. On decapods I take hemolymph from the juncture of the basis and the ischium of the last walking leg. I've also had some success with the fleshy cuticle of the limb apodemes. 

Thank you very much for your answer and useful tips, Dr. Shields. I was trying with a small syringe, but I believe that the tip was rather too thick. I will try to reproduce the pipette as you suggested. Hopefully, I will be able to make it work! 

Hi Paula

Have a look at Marianne Donker's ResearchGate page.  She has exactly that in an article called:

Metal binding compounds in hepatopancreas and haemolymph of porcellio scaber (isopoda) from contaminated and reference areas

https://www.researchgate.net/publication/223922551_Metal_binding_compounds_in_hepatopancreas_and_haemolymph_of_porcellio_scaber_%28isopoda%29_from_contaminated_and_reference_areas?ev=prf_pub

Article Metal-binding compounds in hepatopancreas and hemolymph of P...

Hi Dr. Reinier, 

thank you for suggesting the article. It is interesting how they do it, I will definitely give it a try! The species I work with are a little smaller, but that shouldn't be a problem. Thanks once again.