The bird but seeks his proper food —
And Providence, whose power endu'd
That fly with life, when it thinks good,
May justly take it.
But you have no excuses for't —
A life by Nature made so short,
Less reason is that you for sport
Should shorter make it.
A fly a little thing you rate —
But, Robert do not estimate
A creature's pain by small or great;
The greatest being
Can have but fibres, nerves, and flesh,
And these the smallest ones possess,
Although their frame and structure less
Escape our seeing.
Even the smallest of all creatures have fibres, nerves, and flesh just like humans. Just because the fly is small and hard to see does not mean that it is different from any other creature. 

Thoughtless Cruelty