There are so many things happening in Arachnid that I can't tell how they're even possible to pull off.
I mean yes, I really do understand that it's a manga, and that there's always the "suspension of disbelief" to consider when reading works of fiction. However, some things I seriously don't understand how they're possible to pull off within given circumstances (timeframe, spacial elements, body's condition, etc.), and also what's the general purpose of some ideas presented.
Maybe I've missed a couple of the explanations along the way while reading, but I really would like to know whether or not the author's just making way too many flaws along the way without realizing it.

1. As far as I can tell, Alice's thread comes from her weapon. Her weapon looks like it's roughly the size of a switchblade, so how is she able to put it all over the school rooms' floors and walls while still having enough left to always spare? Better question: WHEN does she have the time to set up her thread before/during unexpected encounters with the assassins popping up in school?
2. Just out of curiosity: What kind of assassinations would water strider be of any use for? I doubt she'd get around easily in a moving body of water, and I don't see her having any opportunities to pull off an assassination without it being in public, as lakes and pools (etc.) tend to have lots of people swimming in them, plus she's standing on the water itself...
3. The Beetle Girl gets exposed to nerve gas that's meant to dull the senses, and even make it hard for her to talk. She's somehow still able to move and talk, and the author goes on to explain how the beetle she's based off of is really strong. How does that explain her not being affected
4. Alice and the Cockroach Girl had guns before. Why are they acting like they've never had them before, even though more assassins have appeared and they're both desperately trying to survive?
5. Since when did Alice's thread take voice commands? Why should that be a thing at all? :/
6. I get that the organization leader wants to have more violence/"excitement" going on, but why the hell is she fond of the idea to have her organization's assassins go kill each other secretly like it's a good thing? Even if it's to see who could last the longest in the contest like some big game of survivor, she still has an organization to run after the contest is over. What, is membership at an all-time high or something?
7. Alice's thread is apparently hollow, and helps save her life from drowning in a pool. Her thread's strong as hell though, plus it's attached to her knife, so how was she able to get an end of the thread to breath out of? Also, when did she have the time to make that soccer ball-sized air sac secretly on her back?
8. Again out of curiosity: Under what real circumstance would water strider need to be kept afloat + expand full of air? It kept her alive until help conveniently came along when she was tied up in the pool, but if she didn't get help, she would have died. I'm sure she's a powerful enough swimmer to reach land if it's nearby, and I doubt she'd try and assassinate someone out so far in open water. So once again, why would she ever need to make herself into an inflatable human?

There's still other things I find puzzling, but I'll end it there for now. I'm sorry this post is really long, but I gotta say, there's a lot of things that really need some reconsidering...

thanks to you ive started reading this series so thank you.
and i agree that theres a ton of plot holes but i doubt you're gonna get any amazing answers so ill answer to the best of my ability.
1. the thread is extremely thin and only about the width of 6-10 spider web strands and if the container is hollow then that could hold a shit ton of length.
2. shes most likely a specialized situational killer. meaning that her methods are rarely applicable to most situations.
3-5.i haven't gotten that far.
6. i think she is just a cruel hearted sadist who believes in survival of the fittest and expendability of the weak.
7. we know 1 end of the thread is connected to the knife so the other end is probably at the back of the device where she pulls out the excess thread. how she made the air sac? no idea... maybe she already made miniatures hidden along the thread before she got in and then when she couldn't touch the bottom she made them slightly bigger.
8. no idea. again sadistic females are odd. she probably just does it to match her name and she likes drowning people from the safety above her victims as they slowly die from lack of oxygen.

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Fujoshis rule!

Huh, didn't think anyone would reply to this post. I'm glad to hear that I was able to get you into reading a series.
The only thing though is that I'll be honest with you: I'm on the fence with Arachnid. I like certain things such as a few of its character designs (Scorpion, Caterpillar, etc.), as well as it's creative ambitions to give humans insect/arachnid abilities. On the other hand though, I've come to find waaaay too many issues going on with the logic within the story. I didn't think about any of it as I read the beginning and onward, but now it's built up to a point of being really tough to ignore when I read it. It's partly why I asked those questions in the first place.
My biggest issue is the author's decision to have Alice return to school. I don't understand why she'd decide to go back when she's homeless, helpless, and especially broke. Even beyond her decision, it's gotten ridiculous for how much fighting's going on in the school when there's still other normal kids and teachers attending it. I at least wish the setting could've changed already... oh well...
It's not really a terrible series, given the action and its ambition. Still, I figure it could've been something more complex, rather than the constant shounen battles going on.

If you'd like, I could recommend you some other series that I think are definitely worth reading (just let me know what kind of genres/themes you're looking for).