Providing the work load isn't too heavy, it would probably be best to just get the measurements over with.  Maybe weigh them now and photograph them in the planes that will be most relevant to the features you are going to measure, those photos may also be useful for geometric morphometric analysis. Alcohol or fixative would both probably affect size and shape as you suggest.  If necessary, you could store them in 30% sucrose in PBS for a short period.

Thank you 

I agree with Justin to use 30% sucrose in PBS BUT for short time. 

What about Propylene glycol?

We use it without problems.

Thank you Dr. Galko and Dr. Mukhtar

I usually kill them in hot water and then keep preserved in 70% alcohol. This way you keep the color and other characteristics.

Dear Dr. Gonçalves, that's easy to try! i will try to do it this way..

Thank you

I agree with Dr. Igor. That's the method I use for my larvae

Dear Zeiri, the method suggested by Igor is surely the best way to preserve larvae for long time. My larvae, so killed and preserved, are perfect from more than twentyfive years. Just remind to boil larvae from a few seconds to about a minute (according to the size), and only for living larvae.

Regards,

Alessandro

my larvae are betwenn 0.5 and 1 mm so for how much i have to boil them

I believed that the size of larvae do not impede you of do this, if you put it directly in alcohol it will can stay dark.

Dear Zeiri, for this size 1-2 seconds will be ok, then put them in alcohol 70°

Thank you all 

Kind regards

Asma