Green’s Dictionary of Slang

insect n.

(Aus./US) a horse.

1886		‘Ned Buntline’ Buffalo Bill 31: The horse [...] walked up to the porch [...] ‘Isn’t he a rare insect? He can run ten hours and never flag, swim any current this side of the big hills, and he knows as much as I do about hide and seek’.
1906		Sydney Morn. Herald 11 July 6/1: The everyday and all-day kind of quadruped is most often referred to as a ‘prad’, but he is almost certain to come also under such descriptions as ‘insect,’ ‘crocodile’ (condensed to ‘crock’) and ‘cuddy’.

In phrases

have the insects (v.)

(Aus.) to suffer delirium tremens.

1973

Tharunka (Sydney) 8 Nov. 28/1: ‘Shut up Sharkey, you’ve got the insects again...stop gibbering.’.