Following a tweet I saw today on Plecoptera slide collection, I became nostalgic of scale insect collections.
— Ashleigh Whiffin (@AshWhiffin) April 16, 2018
Scale insect descriptions and identification are done using mounted material of stained insect cuticle. Because both males and females don’t have very sclerotized cuticle, they become transparent and outer structures are very easy to observe with a light compound microscope.
An example here, we have wax scale insects on a leaf (top left). After removing the inside to keep only cuticle for staining and mounting, we can have a better view of micro structures (top right). This helps making descriptions illustrated by diagrams (bottom).
In the past, male samples were not mounted or if they were, the quality of mounts was not as good as female samples. This is mostly because males being ephemeral, they were not used for identification or description. There are however quite a few old slide mounted males in collections and especially for species that are difficult to find, they are in my opinion, quite valuable to study for phylogenetic purposes.
When I was working on a phylogenetic study of scale insects, I spent a lot of time examining male samples in old collections. Knowing that males are so rare compared to females, finding an old slide mounted male of a rare species was like finding a treasure.
For a couple of years, I focused on fossils and male morphology of the ensign scale insects (Ortheziidae). So I found slide mounts for Ortheziidae males but some of them couldn’t be observed clearly because the cuticle was not cleared properly. Using a traditional light compound microscope, it is just impossible to look at details of setae and secretion pores.
During my master’s degree, I worked in the Natural History Museum in Paris, and my supervisor showed how to unmount samples, and reprocess them for clearing and staining. But these Ortheziidae males were rare so I tried another alternative: confocal scanning laser microscopy.
So here it is:
A lot of surface structure were easily revealed at 633 nm. This is not surprising as normally, staining is made with fuchsin acid, its emission wavelength is 630 nm.
So how much detail can we actually see?
These are abdomens of mounted samples. Setae and secretion pores are easy to see.
When you look more closely, it is actually quite incredible how much detail can be captured!
I think confocal imaging is a great non-invasive option to work on old collection samples, especially ones that are slide mounted, obscured and rare. Although not every place has a confocal microscope (especially museums), the American Museum of Natural History is equipped with a Zeiss LSM710 which was really handy!
Do you know any confocal images of old collection slides? I would love to see them!