Biology of Blephariceridae

By Gregory W. Courtney

Net-winged midges (Diptera: Blephariceridae) are one of the most distinctive and specialized insect families. The immature stages of these flies are highly specialized for life in the cascades, rapids, and waterfalls of mountain streams. The four larval instars show many adaptations to a life in flowing water, including a cephalothorax (fused head, thorax and first abdominal segment) and six ventral suctorial discs.

Prepupal larvae often migrate to specific areas of the rock or stream, which in some groups (e.g., many Blepharicera) includes depressions on the upper, downstream faces of rocks; other groups (e.g., some Apistomyia, Philorus) prefer madicolous- or splash zones on emergent rocks. Rarely does pupation occur on organic substrates. The emerging pupa sheds the larval skin, usually intact, and simultaneously attaches itself to the rock by 3-4 pairs of ventrolateral adhesive discs. The entire process usually requires approximately 5-10 minutes. Blepharicerid pupae are well adapted to life in high current velocities, being dorsoventrally compressed and streamlined. Some rocks may harbor hundreds of pupae, most oriented in the same direction. Pupal orientation apparently promotes formation of a downstream vortex across the respiratory organs (Pommen and Craig 1995), which may enhance delivery of oxygen to the pupa’s respiratory surfaces.

Suctorial discs function as true hydraulic suckers and help the larva attached to smooth substrata (e.g., they easily attach to glass). The pattern of suctorial-disc attachment and general movement of larvae are complex, behavior dependent, and predictable. Larval blepharicerids are grazers (scrapers), using their highly specialized mouthparts to feed on the thin film of algae, bacteria and other organic matter (= periphyton) on submerged rocks. Diatoms appear to be the major component of their diet.

The duration of the pupal stage varies with species and stream temperature, but usually lasts between 2-3 weeks. Emergence of the adult requires that the pupa is attached firmly to the substrate. Thoracic sutures are ruptured by downward pressure from legs and wings, and the emerging adult usually reaches the stream surface in an air bubble. Emergence can be quite brief, requiring from 3-5 minutes. The wings develop to full size within the pupal case and merely unfold during emergence; therefore, adults can fly immediately upon reaching the water surface. Some species emerge at night or during dawn or dusk, but many species emerge primarily during the day.

Adult blepharicerids show a diversity of habits. Females of many species are predators of other insects, especially soft-bodied aquatic insects (e.g., mayflies, stoneflies). Additional prey items include other Diptera: e.g., Chironomidae, Dixidae, Tipulidae, and smaller Blephariceridae. The food of males and nonmandibulate females is unknown, but nectarivory is likely in some species. Adults of most species are short-lived (1-2 weeks), with male longevity somewhat less than that of the female. Few species venture much beyond the riparian zone. Adults of some groups (e.g., Blepharicera) are seen frequently resting on the undersides of leaves of riparian trees. Other species (e.g., many Agathon, Philorus) prefer to rest on wetted, overhanging rock faces. The resting position is distinctive, with the wings held at a moderate angle to the body and the hindlegs angled at the tibiofemoral joint (i.e., knock-kneed). Knowledge of adult activity is poor for most species, even some that are widespread and common. For some species (e.g., many Apistomyia), the habits of males are a complete mystery and those of females are based primarily on observations of ovipositing individuals. Mating typically occurs soon after emergence and oviposition shortly after copulation. Small clusters of eggs are cemented to wetted or emergent rocks. In some species, the female crawls beneath the water and oviposits on submerged rocks.

Larval blepharicerids constitute a guild of highly specialized grazers in streams. In spite of their seemingly "narrow" habitat requirements, sympatry among congeners is common and frequently involves co-occurrence of several species. Studies of eastern North American blepharicerids have shown coexistence of 5-6 species at many sites and even more (up to ten species) at some streams. Temporal separation, microhabitat partitioning, differences in larval diet, and variable adult emergence times are among possible mechanisms of reproductive and ecological isolation. Phenological information is lacking for most blepharicerids. Available data suggest that many blepharicerid populations are univoltine and exhibit rapid growth after a long period of dormancy (e.g., egg diapause). In most species, post-diapause growth and development (i.e., egg hatching) begins when stream temperatures increase during the spring. Development time (i.e., egg-hatch to emergence) varies substantially among and sometimes within species, depending on season / thermal regime.

Although often considered rare, blepharicerids can be an important component of stream ecosystems. In some streams, densities of immature stages can exceed 1000/m², making blepharicerids not only the dominant grazer but one of the most abundant insects. Perhaps because of their specialized and demanding habitats, the larvae and pupae of blepharicerids are preyed upon by few organisms. Among the most important predators of pupae are larval rhyacophilid caddisflies. These flies are sometimes an important food for trout. Because blepharicerids inhabit clean, cool, well-oxygenated streams, the group is a potentially valuable bioindicator of water quality.

Additional ecological information is available from several general treatments of the family (Alexander 1963, Zwick 1977, Hogue 1981, Courtney 2000).

References

Alexander, C. P. 1963. Family Blepharoceridae. in Guide to the insects of Connecticut. Part VI. The Diptera or true flies of Connecticut. Eighth fascicle. Bull. Conn. St. geol. Nat. Hist. Surv. 93: 39-71.

Courtney, G. W. 2000. A.1. Family Blephariceridae. pp. 7-30 in L. Papp & B. Darvas (editors). Contributions to a Manual of Palaearctic Diptera. Appendix. Science Herald, Budapest.

Hogue, C. L. 1981. Blephariceridae. pages 191-197, in McAlpine, J. F. et al. (eds.): Manual of Nearctic Diptera. Volume 1. Research Branch, Agricultural Canada, Ottawa. Agric. Can. Monogr. 27.

Lenat, D. R. 1993. A biotic index for the southeastern United States: derivation and list of tolerance values, with criteria for assigning water quality ratings. J. N. Am. Benthol. Soc. 12: 279-290

Pommen, G.D.W. & D.A. Craig 1995. Flow patterns around gills of pupal net-winged midges (Diptera: Blephariceridae): possible implications for respiration. Can. J. Zool. 73: 373-382.

Zwick, P. 1977. Australian Blephariceridae (Diptera). Aust. J. Zool. Suppl. 46: 1-121.