They leave the tip of their penis in the female as a plug that stops the sperm falling out.
Most male squid don't have penises long enough to be useful. Instead, they propel sperm-containing capsules at females with one of their eight arms. When a sperm capsule attaches to a female squid, she releases a hormone that dissolves her skin, allowing the sperm to get into her bloodstream.
If a barnacle's penis isn't long enough, it can do something known as "spermcasting", in which it sends sperm out to sea for another barnacle to pick up.
A male damselfly's tiny penis contains an even tinier spoon to scoop out leftover sperm from other males in the female's vagina.
There's a tiny Australian mammal that has so much sex in a period of a few weeks that it basically disintegrates.
A male damselfly's tiny penis contains an even tinier spoon to scoop out leftover sperm from other males in the female's vagina.