The snake is raising its sickle-shaped neck in a threat display.
The praying mantis's sickle-shaped forelegs are suited for catching prey.
Compound of 鎌 (kama, 'sickle') and 首 (kubi, 'neck'), literally 'sickle neck'. The exact historical derivation is uncertain, but the word transparently describes a neck curved like a sickle.