Strepsiptera, commonly known as twisted-wing parasites, are a unique and enigmatic order of insects. Their name, derived from Greek words meaning “twisted” and “wings,” refers to the distinctive forewings of the males. These insects are obligate endoparasites, living inside and depending entirely on other insects for survival. Their unusual morphology and highly specialized life cycle are a subject of ongoing study.
Unveiling Strepsiptera
Strepsiptera are a small insect order, with around 600 described species across nine extant families. They exhibit extreme sexual dimorphism. Adult male Strepsiptera are free-living and possess large, fan-shaped hindwings and small, club-shaped forewings, which are modified into halteres. Their eyes are unique among insects, composed of multiple “eyelets” that each produce a complete image.
Most female Strepsiptera are neotenic, remaining as legless, wingless, and sac-like forms permanently embedded within their host’s body. Only their cephalothorax, a fused head and thorax, protrudes from the host’s abdomen.
The Parasitic Journey
The life cycle of Strepsiptera begins with the emergence of minute, highly mobile first-instar larvae, called triungulins, from the female’s brood canal. These larvae possess six legs and actively seek out new hosts. For species parasitizing bees or wasps, a triungulin may climb onto a flower and wait for a pollinator to arrive, using its adhesive tarsi and caudal filaments to attach securely. They may also be found partially packed into pollen loads or within the crop of their phoretic host.
Once a triungulin finds a suitable host, it burrows through the host’s cuticle in the abdominal region, by secreting enzymes that soften the outer layer. Inside the host’s body cavity, the larva molts into a less mobile, legless, grub-like form. The parasite then induces the host to form a protective, bag-like structure from its own tissues, within which the strepsipteran feeds and grows, shielded from the host’s immune defenses. The parasite undergoes several molts, and the old cuticle separates but is not discarded, forming multiple layers around the larva.
Strepsiptera parasitize a wide range of insect orders, including:
- Blattodea (cockroaches)
- Diptera (flies)
- Hemiptera (true bugs)
- Hymenoptera (ants, bees, wasps)
- Mantodea (mantises)
- Orthoptera (grasshoppers, crickets)
- Zygentoma (silverfish)
In the Myrmecolacidae family, males parasitize ants, while females infect Orthoptera and Mantodea, demonstrating a complex dual-host relationship.
Impact on Their Hosts
The presence of Strepsiptera within a host leads to a phenomenon known as “stylopization,” which causes various morphological, physiological, and behavioral changes. The most common effect is the sterilization of the host, as the parasite often redirects the host’s energy and resources towards its own development, hindering the host’s reproductive organ development.
Stylopization can also alter the host’s physical characteristics, such as changes in body color, size, or even the development of secondary sexual characteristics that blur the lines between male and female appearances. For instance, in some stylopized delphacid planthoppers, external genitalia become significantly reduced or absent. The parasite’s feeding on the host’s hemolymph, or insect blood, disrupts the host’s nutritional balance, contributing to these changes.
Beyond physical alterations, Strepsiptera can manipulate host behavior to their own advantage. Myrmecolacid strepsipterans, for example, can cause their ant hosts to climb and linger on grass blades, potentially increasing the likelihood of males finding females for mating or facilitating male emergence. Another observed behavioral change involves parasitized bees altering their flower-visiting patterns, pressing their abdomens against flowers in a way that may promote the release of the strepsipteran larvae.
Strepsiptera’s Place in Nature
Strepsiptera are found globally across various habitats where their insect hosts reside. With around 600 described species, their distribution is necessarily restricted to the ranges of their hosts. They play a role in regulating insect populations as obligate endoparasites of other insects.
Despite their parasitic nature, Strepsiptera are generally not considered harmful to humans, livestock, or crops. Their hosts are primarily other insects, and they do not transmit diseases to humans. While some species parasitize agricultural pests like grasshoppers or leafhoppers, others parasitize beneficial insects such as pollinators, creating a balanced ecological impact. These unusual insects serve as a reminder of the intricate and hidden relationships within natural ecosystems.