TWISTED-WING PARASITES: Strepsiptera

PHYSICAL CHARACTERISTICS

Adult males look like insects and measure 0.04 to 0.3 inches (1 to 7 millimeters) in length. The surfaces of their compound eyes are rough and resemble blackberries because each lens is slightly separate and clearly distinct from the surrounding lenses. Their antennae (an-TEH-nee), or sense organs, are branched and resemble tiny antlers. The mandibles, or biting mouthparts, if they have them at all, are cone-shaped and pointed downward. The first segment of the three-segmented thorax, or midsection, is short and saddle-shaped, while the last segment is much larger and houses most of the flight muscles. The front wings are twisted and knob-like. In flight they are used as balancing organs. The hind wings are clear and fan-shaped, with few supporting veins. Some of the leg segments are fused together, and the feet sometimes lack claws. The abdomen is distinctly segmented.

Most adult females strongly resemble larvae (LAR-vee), or young animals, and measure 0.08 to 1.18 inches (2 to 30 millimeters) in length. They lack wings and legs and have greatly reduced mouthparts, antennae, and eyes. The head and thorax are covered by a thick exoskeleton, or hard outer covering, and are joined together in a single body region. This part of the body sticks out from between the abdominal segments of the host's body. The abdomen is large and barely shows any traces of segmentation. The abdominal exoskeleton is very thin. The abdomen swells up like a balloon when it is filled with eggs.

The larvae have two distinct forms. Those hatching from eggs move freely in the environment and lack antennae, but have simple eyes with one lens each, mouthparts, and legs. The abdomen is tipped with two long, threadlike projections. Once they find a host they transform into a grub without legs. The pupa forms inside the hollow exoskeleton of the mature larvae. In most species male and female characteristics are clearly visible in the pupa.

GEOGRAPHIC RANGE

Twisted-wing parasites are found on all continents except Antarctica. There are about 550 species worldwide, of which 109 live in the United States and Canada.

HABITAT

Twisted-wing parasites are found in a wide variety of habitats wherever their hosts live, especially in tropical habitats. Both females and larvae are endoparasites (EN-doh-PAE-rih-saits). Endoparasites are organisms that live inside the bodies of their hosts. They invade the bodies of silverfish, cockroaches, mantids, grasshoppers, crickets and katydids, true bugs and hoppers, some flies, and ants, bees, and wasps. Males are sometimes found under stones or are attracted to lights at night.

DIET

The parasitic larvae and adult females absorb nutrients directly from the blood of their insect hosts. Free-living adult males and females do not feed.

BEHAVIOR AND REPRODUCTION

The life cycles of twisted-wing parasites are complex. Adult males move about freely in the environment, while most females live their entire lives as parasites inside the bodies of other insects. However, in one group of twisted-wing parasites the females are free-living and go about their lives outside the bodies of their host insects.

Emerging adult males use an inflatable sac on their heads to burst out of their pupal case. They have only a few hours to live and spend all their time looking for a mate. Adult females are surrounded by their old larval exoskeleton inside their host. With only their head and part of their thorax sticking out of the host's body, they release pheromones (FEH-re-moans). Pheromones are chemicals that are especially attractive to males of their own species. There is no courtship. Males deposit sperm directly into the female's body by injecting it between her head and thorax. In some species the females are able to reproduce without being fertilized by a male.

The eggs remain inside the female's body and are nourished directly by the host's blood. The hatching larvae emerge from their mother and the host to search actively for a new host on nearby vegetation. Some will hitch a ride on a wasp and settle in on an egg or larva in its nest. Others simply attack the developing larvae of other insects. Once they enter the body of their new host, the larva will molt, or shed its exoskeleton, and become a legless grub. This type of development, where the larvae alternate between active, legged forms and legless grubs is called hypermetamorphosis (HAI-purh-MEH-te-MOR-fe-sihs). Hypermetamorphosis is typical of many parasitic insects. The larvae of twisted-wing parasites molt four to seven times before reaching the pupal stage. Pupation usually takes place inside the last larval exoskeleton, with just the head and thorax sticking out from between the fourth and fifth abdominal segments of the host.

TWISTED-WING PARASITES AND PEOPLE

Twisted-wing parasites are not very common, and few people ever see them. Although they do not kill their hosts, they do keep them from getting enough food and prevent them from reproducing. This means that they might be considered beneficial if their hosts happen to be considered pests.

A LESSON IN ADAPTATION

Adaptations (ae-dep-TAY-shuns) include physical features or behaviors that help an organism survive and reproduce. Most female twisted-wing parasites are surrounded by food and have no need to leave their hosts. Developing wings and legs would only be a waste of energy for them. But males need plenty of mobility so they can search for females. For them, having legs and wings to get around in the environment is an absolute necessity!

CONSERVATION STATUS

No twisted-wing parasites are listed as endangered or threatened.

NO COMMON NAME (Halictophagus naulti): SPECIES ACCOUNT

Physical characteristics: Adult males are about 0.04 inches (1 millimeter) in length. They have three-segmented feet and seven-segmented antennae. The females are pale yellow and brown and resemble the larvae.

Geographic range: This species is known only from the state of Morelos, Mexico.

Habitat: This species parasitizes the corn leafhopper.

Diet: The larvae and adult females are parasites that live inside the bodies of corn leafhoppers. They feed on the body fluids of their hosts. Adult males are free-living and do not eat.

Behavior and reproduction: Nothing is known about the behavior or reproduction of this species.

Halictophagus naulti and people: The corn leafhopper is the most destructive pest for corn crops in Latin America. Halictophagus naulti parasitizes the corn leafhopper and may prove to be useful for controlling this pest.

Conservation status: This species is not listed as endangered or threatened. ∎

FOR MORE INFORMATION

Books:

Carvalho, E. L., and M. Kogan. Order Strepsiptera. Immature Insects. Vol. 2, ed. by F. W. Stehr. Dubuque, IA: Kendall/Hunt Publishing, 1994.

Tavolacci, J., ed. Insects and Spiders of the World. Vol. 9: Stonefly-Velvet Worm. New York: Marshall Cavendish, 2003.

Periodicals:

Kathirithamby, J. "Review of the Order Strepsiptera." Systematic Entomology 14 (1989): 41–92.

Web sites:

Strepsiptera. http://www.strepsiptera.uni-rostock.de/e/strepsiptera.html (accessed on October 19, 2004).

Strepsiptera. Stylopids. http://www.ento.csiro.au/Ecowatch/Insects_Invertebrates/strepsiptera.htm (accessed on October 19, 2004).

"Strepsiptera: Twisted-Wing Parasites." Tree of Life Web Project. http://tolweb.org/tree?group=Strepsiptera&contgroup=Endopterygota (accessed on October 19, 2004).