The fascinating sex lives of insects

Some insects have detachable penises, others produce sperm that is 20 times the size of their own body. Others have evolved with special equipment to help them tear rivals off potential mates. Insects can be creepy, promiscuous or murderous – but they are rarely dull.

The stag beetle male – Europe’s largest beetle has huge jaws known as antlers designed for prising mating pairs apart. This behaviour is seen in numerous beetles, with horns of various shapes horns that have evolved to lever males off females. The Japanese rhinocerous beetle has a horn resembling a fork. Antlers are also used in battle to wrestle other males over access to females.

In many of these species, smaller males don’t stand a chance of winning a fight, so instead have evolved sneaky mating tactics. They wait for males to fight, then slink in and copulate with the female while the males are distracted. Small male dung beetles slip past large males that guard entrances to tunnels containing females, and dig secret passages to find the underground females while the larger males’ backs are turned.

Sperm competition

In addition to the physical contests between males, competition to fertilise an egg also happens between sperm. In the animal kingdom, females are rarely faithful to their mates, so there is probably sperm from a number of males inside a female reproductive tract. Males have evolved several ways to counter this, such as producing large sperm. Fruit fly sperm, is almost 6cm long when uncoiled, around 20 times the size of the fly.

But, perhaps the most extraordinary method to win the sperm competition is seen in the odonata (dragonflies and damselflies) who have evolved ornate penises. These come complete with hooks and whips, to dislodge the sperm of rival males and pack the male’s own sperm into the far corners of the female reproductive tract, away from other male penises.

And it’s not just the males that have elaborate penises. The female cave insects of Brazil compete for access to males. The insects have sex-reversed genitalia, where the males have an opening and the females have a spiky erectile organ. The female uses her “penis” to suck sperm out of the male, and she can even decide which of two chambers in her body to store the sperm in. It is thought that this behaviour evolved as an adaptation to a limited food supply as the females gain energy from feasting on the seminal fluid acquired during copulation, which can last up to 70 hours.

Butterflies only live for a few weeks, so if males are going to father any offspring, they can’t hang around. Except, some do. Many butterflies are sexually mature as soon as they emerge from their chrysalis. So, in some species the males emerge a few days before the females, then sit and wait, copulating with the females as soon as possible.

More disturbing behaviour is seen in the bed bug. Males simply pierce a female’s abdomen then inject sperm through the wound into her abdominal cavity. As insects have an open circulatory system without arteries and veins, the sperm can easily migrate from the abdominal cavity to the ovaries for fertilisation.

Sexual cannibalism

Probably the most famous of the insect sexual behaviour is that of the praying mantis, where the female bites the head off her partner during or after sex, gaining nutrients for herself and her offspring. This behaviour increases the number of eggs that males fertilise. Recently, scientists found that males also attack females. They don’t eat the females although they sometimes seriously injure them. Males that won fights with females were more likely to go on to mate rather than just get eaten.

Chastity belts

Many male insects only get to mate once, even when they aren’t eaten by their partners. For example, male bees ejaculate with such explosive force that it is loud enough for humans to hear. This ensures the sperm is passed to the female, but it results in paralysis of the male, which kills him. So, males need to make the most of their exploits.

One way of preventing other males from mating with a female is to produce a copulatory plug – something that will prevent a different male from inserting his sperm inside a female to fertilise her eggs. The European dwarf spider produces a plug by secreting a liquid during copulation that hardens over time. Researchers have found that longer copulations result in larger plugs which are more difficult for other males to remove.

In order to ensure that nobody else mates with his female once he is dead, the male orb-web spider has evolved an extreme copulatory plug. He has a detachable penis that remains inside the female once copulation is over. Although it is common for the tip of a spider’s penis to break off inside a female, preventing other males from entering, the orb-web spider’s detachable penis has an additional function as it continues to transfer sperm on its own – for over 20 minutes – increasing mating success.

So you see, insects are, in fact, amazing.