Caenis

Stephens, 1835

Common Square-gills

Species Guides

Caenis hilaris
Caenis latipennis(small squaregilled mayfly)
Caenis tardata(small squaregilled mayfly)

Caenis is a of small mayflies in the Caenidae, commonly known as "square-gilled mayflies." are among the smallest mayflies, with some measuring only 3–4 mm in body length. The genus is globally distributed across multiple continents and serves as an indicator for aquatic health. Nymphs inhabit various freshwater environments, and adults emerge in synchronized events.

Caenis hilaris by (c) Joseph Aubert, some rights reserved (CC BY), uploaded by Joseph Aubert. Used under a CC-BY license.

Aquatic insects of California, with keys to North American genera and California species (1956) (19559655168) by Internet Archive Book Images. Used under a No restrictions license.

Pronunciation

How to pronounce Caenis: //ˈsiː.nɪs//

These audio files are automatically generated. While they are not always 100% accurate, they are a good starting point.

Identification

Caenis are distinguished from other by their small size and the characteristic square or rectangular shape of the gills on the nymphal . lack hindwings, possessing only two pairs of forewings. The genus is separated from other Caenidae by specific wing venation patterns and genitalic structures that require microscopic examination for definitive identification.

Images

Habitat

Freshwater aquatic environments including rivers, streams, and lakes. Nymphs occupy diverse substrates ranging from silt and sand to rocky surfaces.

Distribution

Global distribution spanning North America, South America, Europe, Africa, and Asia. Documented from Argentina, Bolivia, Brazil, Uruguay, Colombia, Denmark, Norway, Thailand, Indonesia (Java, Sumatra), India, and the United States.

Life Cycle

Multivoltine with overlapping observed in studied . Nymphal development requires accumulated (1709 degree days documented for C. latipennis). development requires substantial thermal accumulation (132 degree days for C. latipennis). has been documented in at least one South American species (C. cuniana). emerge in extended periods with multiple peaks; females of C. latipennis have been observed to emerge, , mate, and oviposit within approximately 37 minutes. Adults molt from subimago to stage, making Caenidae one of the few insect groups with post-adult molting.

Behavior

exhibit synchronous mass events ('hatches') that can produce overwhelming abundance.

Ecological Role

Member of the EPT (Ephemeroptera-Plecoptera-Trichoptera) biological index used for water quality assessment. Nymphs serve as indicators of health, with sensitivity to pollutants, turbidity, and thermal pollution varying by . Contributes to secondary production in aquatic (6,052.57 mg/m²/yr documented for C. latipennis).

Human Relevance

are significant to fly fishing as "spinners" and "duns," though their small size makes them less commonly targeted than larger . Mass emergences can create road hazards requiring removal.

Similar Taxa

CloeonSimilar small size and reduced wing structure; distinguished by differences in gill and wing venation
BrachycercusOverlapping and distribution; separated by genitalic characters and nymphal gill structure