Scorpionflies

Scorpionflies belong to the Phylum Arthropoda, Class Insecta, Superorder Endopterygota, and Order Mecoptera. There are about 605 species within thirty-four genera and nine families worldwide. There are also 400 known fossil species in about eighty-seven genera, which are more diverse than even the extant members of the order. Sixteen species representing three families of scorpionflies occur in Arkansas.

Most mecopterans live in moist environments, although adults of a few species are found in hotter semi-desert habitats and may be active and noticeable only for short intervals of the year. Those in the family Panorpidae generally inhabit broad-leaf woodlands with abundant damp leaf litter. Snow scorpionflies (family Boreidae) appear in winter and are often seen on snowfields and on moss; the larvae are able to jump in a similar manner to fleas. Hangingflies (family Bittacidae) occur in caves, forests, and grassland with increased moisture levels.

Mecopterans are cosmopolitan in distribution, with the greatest species diversity in the Afrotropical and Palearctic realms; however, there is greater diversity at the generic and family levels in the Australasian, Nearctic, and Neotropical realms. They are apparently absent from Madagascar and many islands and island groups. The Mecoptera are closely related to the Siphonaptera (fleas) and somewhat distantly related to the Diptera (true flies).

The families of Mecoptera are well accepted by taxonomists, but their relationships have been debated. In 1987, most authorities treated the Mecoptera as a clade, containing the Boreidae as sister to the Meropeidae, but in 2002, others proclaimed the Mecoptera so-defined as paraphyletic, with the Boreidae as sister to the fleas. For example, the Mecoptera and Siphonaptera share characteristics such as the production of resilin (a rubbery cuticle material) and possession of panoistic ovarioles that have germ cells forming a continuous tube in place of the usual nurse cells. In several recent molecular (DNA) analyses, the Boreidae are the sister clade to the Siphonaptera, so the Mecoptera, as traditionally understood, is paraphyletic, although the balance of the order forms a clade. Other recent analyses, based on morphology rather than molecular techniques, instead suggest that Diptera and Siphonaptera are sister to one another, either arising from within a paraphyletic Mecoptera or together comprising the sister lineage to a monophyletic Mecoptera. However, the taxonomic relationships between the families are a matter of debate. One impressive cladogram places the Nannochoristidae as a separate order, with the Boreidae, as the sister group to the Siphonaptera, also as its own order. The Eomeropidae is suggested to be the sister group to the rest of the Mecoptera, with the position of the Bittacidae unclear. Of those other families, the Meropeidae is the most basal, and the relationships of the remainder are nebulous.

The earliest mecopterans are of Upper Permian age. During the Cretaceous, fossil mecopterans become abundant and diverse. Even before other pollinating groups such as bees evolved, now-extinct Mecoptera species may have been important pollinators of early gymnosperm seed plants during the late Middle Jurassic to mid–Early Cretaceous periods.

Mecopterans are somewhat dipteran-like in appearance, being small to medium-sized insects (2 to 35 mm [0.1 to 1.4 in.] long) with long slender bodies, beaklike rostra, narrow membranous wings, and elongated bodies. The largest family (Panorpidae) contains about fifty-eight species, and males possesses a long beaklike rostra and enlarged genital bulbs that look similar to the stingers of scorpions, hence the common name. Another prominent family, the Bittacidae (hangingflies), is famous for its elegant mating rituals, in which females choose mates based on the quality of gift prey males offer them. A minor group is the snow scorpionflies, family Boreidae, adults of which are sometimes seen walking on snowfields. In contrast, the majority of species in the order inhabit moist tropical environments.

Mecoptera are small to medium-sized insects with membranous wings and slender, elongated bodies. They have relatively simple mouthparts, with a long labium and mandible and fleshy palps, compound eyes on the sides of their heads, and three ocelli on the top. Their antennae are filiform (thread-shaped) and contain multiple segments. The fore and hind wings are long and narrow with numerous cross-veins, to some extent resembling those of primitive insects such as Ephemeroptera (mayflies). A few genera, however, have reduced wings, or have lost them altogether. The abdomen is cylindrical, with eleven segments, the first of which is fused to the metathorax. The cerci consist of one or two segments. The abdomen typically curves upward in the male, superficially resembling the tail of a scorpion, the tip containing an enlarged structure called the genital bulb. The larvae are usually quite caterpillar-like, with short, clawed, true legs on the thorax, and a number of abdominal prolegs on the first eight abdominal segments. They have sclerotized heads with jaws (mandibulate) mouthparts. Their tenth (terminal) abdominal segment possesses either a suction disc, or, less commonly, a pair of hooks. The non-feeding pupae are exarate, meaning that their limbs are free of the body, and are able to move their mandibles, but are otherwise entirely non-motile rather than being secured within a cocoon. They may pass through a diapause until weather conditions are favorable.

Adult mecopterans are overwhelmingly predators or scavengers on moribund and dead organisms, including decaying vegetation and soft-bodied invertebrates. They also supplement their diet with vertebrate and snail carrion, living slugs, bird droppings, fruit juices, midge larvae, moss fragments, nectar, and pollen. Scorpionflies of the genus Panorpa are kleptoparasites known to invade spiderwebs to feed on trapped insects and even the resident spiders themselves. Hangingflies capture flies and moths with their specially modified legs. The caterpillar-like larvae generally feed on vegetable matter or scavenge dead insects, although some predatory larvae are known.

Scorpionflies are the initial insects to arrive on a cadaver, making them valuable to investigators in forensic entomology. Therefore, this scientific field makes use of scorpionflies’ habit of feeding on human corpses. In field experiments conducted on donated human cadavers, mecopterans arrived and remained on corpses for about one and a half days. Thus, their presence indicates that a person has not been dead for very long, and their body is typically fresh and not yet in a decaying condition.

Various courtship behaviors have been observed among mecopterans, with males commonly discharging pheromones to attract mates. In addition, males may provide an edible gift such as a dead insect or a brown salivary secretion to the female. Other examples of courtship include: (1) some male boreids using hook-like wings to pick up and place the female on the back while copulating; (2) male panorpids vibrating their wings or even stridulating while approaching a female; and (3) hangingflies (Bittacidae) providing a nuptial meal (gift) in the form of captured insect prey, such as a caterpillar, bug, or fly. In this latter example, a male also attracts females with a pheromone from vesicles on his abdomen. Once a female is nearby, he retracts the vesicles and presents her with the prey. During evaluation of the gift by the female, he locates her genitalia with his own. If she remains in place to eat the prey, his genitalia attaches to hers, and the female lowers herself into an upside-down hanging position and eats the prey while mating. Most scorpionflies breed in moist environments such as leaf litter or moss, and the eggs may not hatch until the wet season arrives. Hangingflies mostly breed among mosses, in leaf litter, and in other moist places, but little is known about their reproductive habits. In addition, at least one species, Nannochorista philpotti, has aquatic larvae.

Following copulation, females deposit their eggs in places with moisture, and their eggs typically absorb water and increase in size. In species that live in arid conditions, the eggs may not hatch for several months, the larvae only emerging when the dry season is over. On average, however, they hatch after a relatively short period of time. The larva crawls into the soil or decaying wood to pupate, and does not spin a cocoon. In drier environments, the exarate pupae may spend several months in diapause, before emerging as adults once the conditions are more suitable.

Sixteen species representing three families of scorpionflies occur in Arkansas, including a single species of Meropeidae, ten species of Panorpidae, and five species of Bittacidae. A panorpid scorpionfly, Panorpa braueri, is ranked G1 in Missouri and in Arkansas in Benton and Washington counties.

For additional information:
Bashkuev, A. S. “Nedubroviidae, a New Family of Mecoptera: The First Paleozoic Long-Proboscid Scorpionflies.” Zootaxa 2895 (2011): 47–57.

Bilinski, S. M., J. Büning, and B. Mimiczyjew. “The Ovaries of Mecoptera: Basic Similarities and One Exception to the Rule.” Folia Histochemica Cytobiologica 36 (1998): 189–195.

Bockwinkel, G., and K.-P. Sauer. “Panorpa Scorpionflies Foraging in Spider Webs – Kleptoparasitism at Low Risk.” Bulletin of the British Arachnological Society 9 (1993): 110‒112.

Byers, George W. “Autumnal Mecoptera of Southeastern United States.” University of Kansas Science Bulletin 55 (1993): 57‒96.

———. “The Life History of Panorpa nuptialis (Mecoptera: Panorpidae).” Annals of the Entomological Society of America 56 (1963): 142‒149.

Byers, George W., and R. Thornhill. “Biology of the Mecoptera.” Annual Review of Entomology 28 (1983): 203‒228.

Chen, Qing-Xiao, and Bao-Zhen Hua. “Ultrastructure and Morphology of Compound Eyes of the Scorpionfly Panorpa dubia (Insecta: Mecoptera: Panorpidae).” PLoS One 11 (2016): e0156970.

Grimaldi, D., and M. A. Johnston. “The Long-Tongued Cretaceous Scorpionfly Parapolycentropus Grimaldi and Rasnitsyn (Mecoptera: Pseudopolycentropodidae): New Data and Interpretations.” American Museum Novitates 3793 (2014): 1–24.

Penny, N. D. “Evolution of the Extant Mecoptera.” Journal of the Kansas Entomological Society 48 (1975): 331‒350.

Poole, R. W., and P. Gentili, eds. Nomina Insecta Nearctica: A Checklist of the Insects of North America. Volume 2 (Hymenoptera, Mecoptera, Megaloptera, Neuroptera, Raphidioptera, Trichoptera). Rockville, MD: Entomological Information Services, 1996.

Ren, D., C. C. Labandeira, J. A. Santiago-Blay, A. Rasnitsyn, C. K. Shih, A. Bashkuev, M. A. Logan, C. L. Hotton, and D. Dilcher. “Probable Pollination Mode before Angiosperms: Eurasian, Long-Proboscid Scorpionflies.” Science 326 (2009): 840–847.

Robison, Henry W., George W. Byers, and Christopher A. Carlton. “Annotated Checklist of the Mecoptera (Scorpionflies) of Arkansas.” Entomological News 108 (1997): 313‒317.

Thornhill, R. “Scorpionflies as Kleptoparasites of Web-Building Spiders.” Nature 258 (1975): 709‒711.

Webb, D. W., N. D. Penny, and J. C. Marlin. “The Mecoptera, or Scorpionflies, of Illinois.” Illinois Natural History Survey Bulletin 31 (1975): 250‒316.

Whiting, Michael F. “Mecoptera is Paraphyletic: Multiple Genes and Phylogeny of Mecoptera and Siphonaptera.” Zoologica Scripta 31 (2002): 93–104.

Whiting, Michael F., Alison S. Whiting, Michael W. Hastriter, Katharina Dittmar. “A Molecular Phylogeny of Fleas (Insecta: Siphonaptera): Origins and Host Associations.” Cladistics 24 (2008): 1–31.

Willman, R. “The Phylogenetic System of the Mecoptera.” Systematic Entomology 12 (1987): 519–524.

Chris T. McAllister
Eastern Oklahoma State College

Henry W. Robison
Sherwood, Arkansas