scorpion
蠍 refers specifically to a scorpion, the arachnid with a segmented tail and venomous sting.
蠍 combines the insect radical 虫 with 歇, which likely contributes the sound. The character was created to represent a scorpion as a type of insect-like creature.
The insect radical 虫 on the left signals a creepy-crawly creature. The right side 歇 looks like a scorpion's body with a raised tail ready to strike. Together they form 蠍, a scorpion.
For カツ, imagine a scorpion's sting causing a sharp 'katsu!' sound as it strikes. The sudden 'katsu' cues カツ while the scorpion attacks.