Here is one of them caught in the act. You can see her clutch of eggs as the light colored lumps just above the end of her abdomen.
After she left, I harvested the clutch and moved it under a microscope just to take a look.
At low magnification, shown as a crossed-eye stereo pair, what I found looked like this:
Zooming in, I was expecting to see smooth shiny eggs. But instead they were covered with what looked like dewdrops.
The tiny drops made me curious, and I dived down the rabbit hole of trying to figure out what the liquid was.
I tried treating the eggs with distilled water and with alcohol; both treatments caused the small droplets to disappear, leaving a smooth surface to the eggs. Under microscopic examination it seemed that surrounding the eggs with water left a relatively thick "shell" around each egg, while surrounding them with alcohol left a much thinner shell. In neither case were there any small droplets remaining.
A couple of days exposure to room air at 80 degrees F, 50% RH, made no obvious change in either the droplets or the weak adhesion of the eggs to each other and to the substrate. In all cases the eggs could be separated without apparent damage.
After some searching, I found in https://doi.org/10.3390/insects13010017 , "A Comprehensive Method for the Evaluation of Hermetia illucens Egg Quality Parameters: Implications and Influence Factors", that
But that simply added to my confusion. All the mucus that I'm familiar with is water-based and dries hard. In contrast, this stuff -- whatever forms the tiny drops -- did not dry out. I was thinking "maybe it's hygroscopic". So I placed some in a small container with lots of dry silica gel, and after 2+ days of that treatment, they still had not changed. It seemed very unlikely that whatever forms the drops could be more hygroscopic than dry silica gel, so I shifted gears and went back to considering oils.Females lay eggs in sheltered, hidden spaces on a dry substrate [10,15]. The eggs are laid in clusters in the form of a package composed of overlapping layers of eggs (clutch) that adhere to each other due to a mucus that facilitates adhesion to the oviposition material [18].
After considerable manipulation, I was eventually able to separate some of the material into a large enough droplet that I could see it was not miscible with water. It was, however, miscible with denatured alcohol and with lighter fluid. This all seems like oil, which is my current thinking.
Someplace in the journey, I found a third clutch of eggs, this one laid in a small space under a rotting beet. The others had been laid on more exposed vertical surfaces. When I put that third clutch under the microscope, I was surprised to see that it did not have tiny droplets, but instead the eggs had smooth surfaces with liquid apparent only as fillets where two eggs touched each other. In close view, as crossed-eye stereo, that looked like this:
Thinking back to my earlier experiments, I'm guessing that touching the eggs with anything including a drop of water was enough to prompt the oil to spread out across the surface of the eggs in a smooth layer. Of course that leaves the mystery of how the small drops form in the first place, during or shortly after the egg-laying process.
One final amusement... In the last image, many of the eggs can be seen to have two small brown spots near one end. I had not noticed those until preparing this post, but I'll bet that those are developing eyespots of the 1st instar larvae, shown in more detail at viewtopic.php?f=27&t=46416 .
Photographic technique was a mixed bag. The first image was with cell phone alone, first stereo was a cha-cha pair shot with Canon R7 and MP-E 65 on tripod and 2-axis rail, the "dew drops" were shot with cell phone through eyepiece of a dissecting scope, and the final stereo pair was synthetic stereo from a stack shot with Mitutoyo M Plan Apo 5X NA 0.14 with 20 micron steps. All of the indoor shots were using Jansjö LED illumination.
--Rik
