If you cannot find this structure in the literature
then describe it the very best you can and name it as a vacuole with definite electron staining bodies.
For example this is obviously a vacuole (measure it and provide measurements).
I do not study the Drosophila larvae so I do not know if it is been seen before or not or named or not?
Does this structure appear in any other of your images?
If so it may be important and gone unnoticed so describe and name it or at least describe and place this structure in perspective of its location - sometimes I get a better perspective at low and then high mag . Check the light micro literature as well. What cellular structure does this vacuole reside in and then look at the literature for described vacuoles that may be within this same cell that has been described...
The best thing is to adequately describe this structure within the context of where you found it.
These are just some thoughts as I examine this large vacuole.
Also attempt to describe its contents and share with those who have more knowledge regarding TEM findings in the Drosophila lavae ... They may have also noticed similar findings in their studies.
You may actually found something new and novel especially if it is recurrent... what kind of larvae is it a genetic or normal conrol Drosophila etc ..
If it is recurrent it may be of great importance and even novel.