Strepsiptera

Strepsiptera are a specialized order of insects known as twisted-wing parasites.

Their name comes from Ancient Greek words meaning "turning around" and "wing."

These insects are endoparasites, which means they live inside the bodies of other living hosts. They target many different types of insects, such as bees, wasps, cockroaches, and silverfish. Because most of their lives are spent hidden inside a host, they are not well known to most people. Some people use the common name "stylops" in reference to the genus Stylops.

The life cycle of a strepsipteran begins with a tiny, highly active larva called a planidium.

These first-instar larvae are incredibly small, sometimes even smaller than single-celled organisms. They possess stemmata, which are simple eyes that allow them to distinguish colors. To move, they use minute hair-like structures called microtrichia to stick to wet surfaces via capillary action. They also have large, muscular bristles called cerci that allow them to jump. These larvae must find a host quickly before they run out of energy. Once they locate a host, they secrete enzymes to soften the host's cuticle, or outer skin. They then enter the host, often through the abdomen or even the host's feet.

Inside the host, the larvae undergo a process called hypermetamorphosis. They transform from active movers into immobile forms that live inside the host's body. The larvae induce the host to create a protective, bag-like structure made of host tissue. This structure helps protect the parasite from the host's immune defenses. As the larvae grow through four more instars, they undergo a unique type of molting. During this process, the old cuticle separates but is not discarded, creating multiple layers around the larvae. The offspring eventually consume their mother from the inside in a process known as haemocoelous viviparity. This occurs because the female lacks complex organs like ovaries, so her eggs hatch into larvae within her body fluid, or hemolymph.

Adult males and females of the order look remarkably different from one another. Adult males possess wings, legs, eyes, and antennae, and they often resemble flies. Their forewings are modified into small, club-shaped structures called halteres, which act like gyroscopes to sense movement. Their eyes are unique among insects, consisting of a few dozen "eyelets" that each produce a complete image. These eyelets give the eyes a blackberry-like appearance. Males use flabellate antennae covered in chemoreceptors to detect female pheromones over long distances. However, males are short-lived and do not feed, often surviving for less than five hours.

Most female strepsipterans belong to the group Stylopidia, which includes 97% of all described species. These females are neotenic, meaning they retain juvenile characteristics into adulthood. They lack wings, legs, and eyes, and they remain inside the host for their entire lives. In many species, the female's anterior region protrudes through the segments of the host's abdomen. These females can occupy up to 90% of the total abdominal volume of their host. Mating occurs through a process called traumatic insemination. The male ruptures the female's cuticle to pass sperm directly into her body. This specific method of mating has also evolved independently in other insects like bed bugs.

Scientists have a long and complex history of studying the taxonomy of Strepsiptera. The order was named by William Kirby in 1813. For a long time, researchers struggled to find the insect's true place in the tree of life. Some early studies suggested they were related to flies, while others thought they were related to beetles. Most modern molecular studies now suggest that strepsipterans are the sister group to beetles, a group known as Coleoptera. They are believed to have diverged from beetles approximately 300 to 350 million years ago.

Fossil evidence provides a window into the ancient history of these parasites. While they diverged from beetles long ago, they do not appear in the fossil record until the mid-Cretaceous period. Fossils found in Burmese amber are approximately 99 million years old. These fossils, such as those from the family Phthanoxenidae, show that the parasitic lifestyle of Strepsiptera has likely remained unchanged for nearly 100 million years. This long history shows how successful their specialized way of life has been. Even today, different families target specific hosts, such as the Stylopidae, which target bees exclusively.