The Strepsiptera are an order of insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...

s with ten families making up about 600 species. The early stage larvae and the short-lived adult males are free-living, but most of their lives are spent as endoparasites in other insects such as bee
Bees are flying insects closely related to wasps and ants, and are known for their role in pollination and for producing honey and beeswax. Bees are a monophyletic lineage within the superfamily Apoidea, presently classified by the unranked taxon name Anthophila...

s, wasp
The term wasp is typically defined as any insect of the order Hymenoptera and suborder Apocrita that is neither a bee nor an ant. Almost every pest insect species has at least one wasp species that preys upon it or parasitizes it, making wasps critically important in natural control of their...

s, leafhopper
Leafhopper is a common name applied to any species from the family Cicadellidae. Leafhoppers, colloquially known as hoppers, are minute plant-feeding insects in the superfamily Membracoidea in the order Hemiptera...

s, silverfish
Lepisma saccharina, frequently called silverfish, fishmoths, carpet sharks or paramites, are small, wingless insects in the order Thysanura...

, and cockroach
Cockroaches are insects of the order Blattaria or Blattodea, of which about 30 species out of 4,500 total are associated with human habitations...


Appearance and biology

Male Strepsiptera have wings
Insect wing
Insects are the only group of invertebrates known to have evolved flight. Insects possess some remarkable flight characteristics and abilities, still far superior to attempts by humans to replicate their capabilities. Even our understanding of the aerodynamics of flexible, flapping wings and how...

, legs
Arthropod leg
The arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments are of Latin origin, and may be confused with terms for bones: coxa , trochanter , femur, tibia, tarsus, ischium, metatarsus, carpus, dactylus ,...

, eyes, and antennae
Antenna (biology)
Antennae in biology have historically been paired appendages used for sensing in arthropods. More recently, the term has also been applied to cilium structures present in most cell types of eukaryotes....

, and look like flies, though they generally have no useful mouthparts. Many of their mouth parts are modified into sensory structures. Adult males are very short-lived (usually less than five hours) and do not feed. Females, in all families except the Mengenillidae, are not known to leave their hosts and are neotenic
Neoteny , also called juvenilization , is one of the two ways by which paedomorphism can arise. Paedomorphism is the retention by adults of traits previously seen only in juveniles, and is a subject studied in the field of developmental biology. In neoteny, the physiological development of an...

 in form, lacking wings and legs. Virgin females release a pheromone for which the males search. In the Stylopidia, the female has its anterior region extruding out of the host body and the male mates by rupturing the female's brood canal opening, which lies between the head and prothorax. Sperm passes through the opening in a process termed hypodermic insemination. Each female produces many thousands of triungulin larvae that emerge from the brood opening on the head, which protrudes outside the host body. These larvae have legs (which lack a trochanter
Łęg may refer to the following places in Poland:*A former name for the town of Ełk *Part of the Czyżyny district of Kraków*Łęg, Pleszew County in Greater Poland Voivodeship...

, the leg segment
Łęg may refer to the following places in Poland:*A former name for the town of Ełk *Part of the Czyżyny district of Kraków*Łęg, Pleszew County in Greater Poland Voivodeship...

 that forms the articulation between the basal coxa
Coxa may refer to:* Theta Leonis, a star* Nickname of Brazilian soccer team Coritiba Foot Ball Club* Hip, below the lateral side of the abdomen* The short most-proximal base jointed segment of the arthropod leg...

 and the femur
The femur , or thigh bone, is the most proximal bone of the leg in tetrapod vertebrates capable of walking or jumping, such as most land mammals, birds, many reptiles such as lizards, and amphibians such as frogs. In vertebrates with four legs such as dogs and horses, the femur is found only in...

), and actively search out new hosts. Their hosts include members belonging to the orders Zygentoma, Orthoptera
Orthoptera is an order of insects with paurometabolous or incomplete metamorphosis, including the grasshoppers, crickets and locusts.Many insects in this order produce sound by rubbing their wings against each other or their legs, the wings or legs containing rows of corrugated bumps...

, Blattodea, Mantodea, Heteroptera
Heteroptera is a group of about 40,000 species of insects in the Hemiptera. Sometimes called "true bugs", that name more commonly refers to Hemiptera as a whole, and "typical bugs" might be used as a more unequivocal alternative since among the Hemiptera the heteropterans are most consistently and...

, Hymenoptera
Hymenoptera is one of the largest orders of insects, comprising the sawflies, wasps, bees and ants. There are over 130,000 recognized species, with many more remaining to be described. The name refers to the heavy wings of the insects, and is derived from the Ancient Greek ὑμήν : membrane and...

, and Diptera
Diptera , or true flies, is the order of insects possessing only a single pair of wings on the mesothorax; the metathorax bears a pair of drumstick like structures called the halteres, the remnants of the hind wings. It is a large order, containing an estimated 240,000 species, although under half...

. In the Strepsipteran family Myrmecolacidae
Myrmecolacidae are an insect family of the order Strepsiptera. There are four genera and about 98 species in this family. Like all strepsipterans, they have a parasitic mode of development with males parasitizing ants while the females develop inside Orthoptera...

, the males parasitize ants while the females parasitize Orthoptera.
Suggested phylogenetic position of the Strepsiptera.
Strepsiptera eggs hatch inside the female, and the planidium larvae
A planidium is a specialized type of first-instar insect larva, seen in groups that are parasitoids; they are generally flattened, highly sclerotized, have legs, are quite mobile, and sometimes have eyes...

 can move around freely within the female's haemocoel, which is unique to these animals. The female has a brood canal that communicates with the outside world, and the larvae escape through this. The larvae are very active, as they only have a limited amount of time to find a host before they exhaust their food reserves. These first-instar
An instar is a developmental stage of arthropods, such as insects, between each molt , until sexual maturity is reached. Arthropods must shed the exoskeleton in order to grow or assume a new form. Differences between instars can often be seen in altered body proportions, colors, patterns, or...

 larvae have stemmata (simple, single-lens eyes), and once they latch onto a host, they enter it by secreting enzymes that soften the cuticle, usually in the abdominal region of the host. Some species have been reported to enter the eggs of hosts. Larvae of Stichotrema dallatorreanurn Hofeneder from Papua New Guinea
Papua New Guinea
Papua New Guinea , officially the Independent State of Papua New Guinea, is a country in Oceania, occupying the eastern half of the island of New Guinea and numerous offshore islands...

 were found to enter their orthoptera
Orthoptera is an order of insects with paurometabolous or incomplete metamorphosis, including the grasshoppers, crickets and locusts.Many insects in this order produce sound by rubbing their wings against each other or their legs, the wings or legs containing rows of corrugated bumps...

n host's tarsus (foot). Once inside the host, they undergo hypermetamorphosis
Hypermetamorphosis is a kind of complete insect metamorphosis in which the different larval instars represent two or more different forms of larva. As the larva molts its morphology can change from that of a campodeiform to scarabaeiform or to vermiform...

 and become a less-mobile, legless larval form. They induce the host to produce a bag-like structure inside which they feed and grow. This structure, made from host tissue, protects them from the immune defences of the host. Larvae go through four more instars, and in each moult there is separation of the older cuticle but no discarding ("apolysis
Apolysis is the separation of the cuticula from the epidermis in arthropods and related groups . Since the cuticula of these animals is also the skeletal support of the body and is inelastic, it is shed during growth and a new covering of larger dimensions is formed. During this process, an...

 without ecdysis
Ecdysis is the moulting of the cuticula in many invertebrates. This process of moulting is the defining feature of the clade Ecdysozoa, comprising the arthropods, nematodes, velvet worms, horsehair worms, rotifers, tardigrades and Cephalorhyncha...

"), leading to multiple layers being formed around the larvae. Male larvae produce pupae after the last moult, but females directly become neotenous adults. The colour and shape of the host's abdomen may be changed and the host usually becomes sterile. The parasites then undergo holometabolous metamorphosis to become adults. Adult males emerge from the host bodies, while females stay inside. Females may occupy up to 90% of the abdominal volume of their hosts.

Adult male Strepsiptera have eyes unlike those of any other insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...

, resembling the schizochroal eyes found in the trilobite
Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...

 group known as Phacopida
Phacopida is an order of trilobite that lived from the Ordovician to the Devonian. It is made up of a morphologically diverse group of related suborders....

. Instead of a compound eye consisting of hundreds to thousands of ommatidia, each with a single lens and capable of producing a picture element (pixel), the strepsipteran eyes consist of only a few dozen ommatidia separated by cuticle and/or setae, giving the eye a blackberry-like appearance.

Multiple females may live within a stylopized host. Males are rarely observed, although they can be collected at light traps or lured using cages containing virgin females.

Strepsiptera may alter the behaviour of their hosts. Myrmecolacids may cause their ant hosts to climb up the tips of grass leaves, possibly to increase the spread of female pheromones to increase the chances of being located by males.


The order, named by William Kirby in 1813, is named for the hind wings (strepsi=twisted + ptera=wing), which are held at a twisted angle when at rest. The forewings are reduced to halteres
Halteres are small knobbed structures modified from the hind wings in some two-winged insects. They are flapped rapidly and function as gyroscopes, informing the insect about rotation of the body during flight....

 (and initially were thought to be dried and twisted).

Strepsiptera are an enigma to taxonomists. Originally, they were believed to be the sister group to the beetle families Meloidae and Ripiphoridae
The family Ripiphoridae is a cosmopolitan group of beetles commonly known as wedge-shaped beetles containing some 450 species...

, which have similar parasitic development and forewing reduction; early molecular research suggested their inclusion as a sister group to the flies
Fließ is a municipality in the Landeck district and is located5 km south of Landeck on the upper course of the Inn River. It has 9 hamlets and was already populated at the roman age; the village itself was founded around the 6th century. After a conflagration in 1933 Fließ was restored more...

, in a clade called the halteria, which have one pair of the wings modified into halteres, and failed to support their relationship to the beetles. More recent molecular studies, however, suggest they are outside the clade Mecopterida (containing the Diptera and Lepidoptera), yet there is no strong evidence for affinity with any other extant group. Study of their evolutionary position has been problematic due to difficulties in phylogenetic analysis arising from long branch attraction
Long branch attraction
Long branch attraction is a phenomenon in phylogenetic analyses when rapidly evolving lineages are inferred to be closely related, regardless of their true evolutionary relationships. For example, in DNA sequence-based analyses, the problem arises when sequences from two lineages evolve rapidly...

. The most basal strepsipteran is the fossil Protoxenos janzeni discovered in Baltic amber
Baltic amber
The Baltic region is home to the largest known deposit of amber, called Baltic amber or succinite, with about 80% of the world's known amber found there. It dates from 44 million years ago...

, while the most basal living strepsipteran is Bahiaxenos relictus
Bahiaxenos relictus
Bahiaxenos relictus is the sole member of the family Bahiaxenidae, a type of winged insect. It was only discovered and described in 2009 from relictual sand dunes associated with the Rio São Francisco in Bahia, Brazil. It is considered to be the most basal living member of the order Strepsiptera,...

, the sole member of the family Bahiaxenidae. The earliest known strepsipteran fossil is that of Cretostylops engeli, discovered in middle Cretaceous amber from Myanmar
Burma , officially the Republic of the Union of Myanmar , is a country in Southeast Asia. Burma is bordered by China on the northeast, Laos on the east, Thailand on the southeast, Bangladesh on the west, India on the northwest, the Bay of Bengal to the southwest, and the Andaman Sea on the south....



The Strepsiptera have two major groups: Stylopidia and Mengenillidia. The Mengenillidia include three extinct families (Cretostylopidae, Protoxenidae, and Mengeidae) plus two extant families (Bahiaxenidae and Mengenillidae; the latter is not monophyletic, however.) They are considered more primitive, and the known females (Mengenillidae only) are free-living, with rudimentary legs and antennae. The females have a single genital opening. The males have strong mandibles, a distinct labrum, and more than five antennal segments.

The other group, Stylopidia, includes seven families: Corioxenidae, Halictophagidae, Callipharixenidae, Bohartillidae, Elenchidae, Myrmecolacidae, and Stylopidae. All Stylopidia have endoparasitic females having multiple genital openings.

Stylopidae have four-segmented tarsi and four to six-segmented antennae, with the third segment having a lateral process. The family Stylopidae may be paraphyletic. The Elenchidae have two-segmented tarsi and four- segmented antennae, with the third segment having a lateral process. The Halictophagidae have three-segmented tarsi and seven-segmented antennae, with lateral processes from the third and fourth segments.
The Stylopidae mostly parasitize wasps and bees, the Elenchidae are known to parasitize Fulgoroidea, while the Halictophagidae are found on leafhoppers, treehoppers and mole cricket hosts.

External links

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