All in all you're doing a pretty good job of applying the principles and concepts from the lesson, though there are a number of things I'll point out that should keep you on the right track.

Starting with your organic forms with contour curves, you're definitely trying to stick to simpler sausage forms, but one thing that stood out was that the ends of your sausages tend to be more stretched out, so they're not properly spherical. The degree of your contour curves also seems to be somewhat overly consistent, rather than shifting naturally over the course of the form (getting wider or narrower) as shown here.

So the biggest strength I'm seeing is that over the course of the lesson, you definitely explore and cement your grasp of how these forms exist as three dimensional organic masses, and how they can be fitted into one another and combined in a way that reinforces the illusion that they're all 3D, instead of flattening things out. One thing I am noticing however is that you tend to draw really confidently, but lightly at first.

It's as though you're creating a more explorative under-drawing, and then you come back to cement your lines, or replace the ones you'd already drawn. Now, this is something I staunchly discourage in students, but it's for reasons that are not really present here. Usually when dealing with underdrawings and a clean-up pass, students will draw more hesitantly on the second round, tracing over their lines and in turn flattening things out. You continue to draw with a lot of confidence, so it's not as much of an issue here. Still, I'd rather you not get into the habit of separating into an underdrawing within my lessons. You can apply line weight after you've put marks down, but this is primarily to strengthen and clarify specific overlaps between forms - not to outline the entirety of a form's silhouette.

Something I don't want to see, for example, is what happened with the scorpion's tail - you drew smaller forms for each section, and then ended up enveloping each one in a larger form with a darker stroke. Once you put a form down in the world, you have to deal with it. Sometimes that means working with forms smaller than you intended, or things that don't match the reference as closely as you'd like, but you roll with the punches anyway. In certain situations, we can wrap forms around existing structures to help add bulk and mass where it's needed, but there are definitely cases where it'd be better to just leave things as they are (like the scorpion's tail).

You mentioned that you were struggling with cast shadows - speciifcally the shadows on the ground plane, and how they'd relate to the actual object casting them. I noticed that you actually did a pretty good job with this in a number of places, but that when looked at alongside the smaller cast shadows within the construction itself, I notice that they're not always entirely consistent. The thing with cast shadows is that they're all originating from a consistent light source (or series of light sources), and therefore those shadows need to be cast in the same direction. So if we look at this ant, you've got some really long shadows being cast by the segmentation (honestly, those shadows are way too big), and shadows that are being cast from the legs across the body. If you compare them to the shadow on the ground, they don't quite suggest a consistent lighting scheme. The shadow on the ground aligns better to a light source directly above the ant, whereas the shadows being cast from the legs across the body suggest a light source off to the side.

The thing about cast shadows is that they help describe the relationship between a form and another surface. The closer the form is to the surface, the closer the shadow will fall. The further the form is, the farther away it will fall. This sounds pretty easy, but it's actually quite powerful. If you think about how the shadow works on the ground where an ant's foot is, that shadow is going to barely be visible because it's mostly covered up by the foot itself. Therefore the distance between the shadow and the object is very small. Then as we go further along the leg, which is held above the ground, the distance between the leg and the part of the shadow it casts gets larger. This tells us that the leg is not resting along the ground, but rather that it's being held up. This continues to the body, where its shadow will be further as well, unlike the foot.

The shadow's size is still going to depend on the object casting it though - so if you look at your treehopper's front legs, you've got that leg casting a massive shadow, when in truth it should continue to be quite skinny until we reach the portion of the shadow being cast by the body. Here's what I mean. Now, the shadow obviously doesn't have to be accurate - it's just an approximation, and that's enough to establish the relationship between the form and the surface.

Looking at your cicada, I'm generally very pleased with the sausage structures for the legs, and the way you've constructed much of the body but I have two complaints. One's minor - just, don't fill in things that appear as black, just because they're black. Save your filled shapes for cast shadows only. The second however is just that the segmentation along the abdomen became visibly lopsided. One thing that can help avoid that sort of thing is to draw a line along the center of the earlier masses. Having a line running along the physical center will help remind you of how and where your added features (like contour lines, segmentation, etc.) need to be symmetrical.

Aside from that, you've definitely learned a lot from this lesson, and it shows with the confidence you've applied, especially in your later constructions. You're definitely getting a better handle on those skinnier sausages, and are also showing a lot of competence with breaking constructions down into smaller components, building up from more than just the simplest structures to capture more of the nuance in your individual insects.

So! I'll go ahead and mark this lesson as complete.