Among the Instars: Sialis aequalis
Hey bug folks! Last week began with International Women’s Day and in the Midwest, we were also celebrating Wisconsin Water Week which was formally proclaimed by the governor. It was a busy week for me, full of appreciation and education about the freshwater resource that we are blessed with here. This week I’m looking into a new order that I haven’t focused on before: Megaloptera, with a special emphasis on Sialis aequalis. This species is an alderfly in the family Sialidae. All alderfly species in North America are encompassed in the single genus Sialis. These critters are named alderflies because the adults often spend time on or around alder trees near waterbodies.
Sialis aequalis, an alderfly in the family Sialidae
Alderfly Life Cycle
The life cycle of alderflies typically lasts one year, but some require two years to fully mature. Eggs are laid around May, and larvae hatch and develop throughout the rest of the year. In late winter around March, larvae start to prepare for their next life stage. During the night, these critters will climb out of the water body and burrow into moist soil on the bank to pupate. After a week or two, the adults emerge from the pupal burrows. The adult alderflies are not strong fliers so they stay close to the water where they spent most of their life, and they are mostly active during the day.
Ecology of Sialis aequalis
Sialis aequalis is a fairly widespread species of alderfly, occurring across most of the eastern United States. They are found in small to medium streams consisting of a sandy or rocky bottom. Within those streams, they habit pockets of debris or leaf litter along the edges of streams. In the Upper Midwest, the genus Sialis has a pollution tolerance level of 4, indicating that they are moderately tolerant of impacts to their stream. However, S. aequalis has been known to survive and thrive in habitats with strongly acidic conditions. One study found them living in an acid mine drainage stream with a pH of 3.0. That’s the same acidity as grapefruit or orange juice!
Larval alderflies are mostly predaceous, eating detritus, plankton, and other invertebrates such as Chironomid midges, aquatic worms, and even other alderfly larvae. Studies from different streams find that they tend to eat whatever is most abundant in that habitat. For some, that is detritus, and for others it can be midges or worms, so they are opportunistic predators. I truly cannot imagine what it would be like to perform a gut content analysis on something as small as one of these larvae. If you’ve done something like that – please let me know what it is like!
Sialis aequalis Taxonomy
This is a fairly small order with only two families and 10 genera, but it contains some of the largest specimens. Alderflies are generally smaller, both as larvae and adults, than the fishflies and dobsonflies that are also included in Megaloptera. The larvae usually range from 12 mm to 18 mm in length. Sialidae are characterized by 7 pairs of gill filaments along the edge of their abdomen, each consisting of 4 to 5 segments. Additionally, the last segment of the abdomen is extended into a central “tail” or filament.
Identification is straightforward to this level with very few look-alikes. The dobsonflies and fishflies (family: Corydalidae) in Megaloptera also have gill filaments, but the end of their abdomen will have two proleg-like extensions with a pair of hooks on each. There are also three genera of beetle whose larvae contain lateral filaments that can look similar to Sialidae. Dineutus and Gyrinus (family: Gyrinidae, also known as whirligig beetles) each have 10 pairs of filaments and the abdomen ends in two pairs of hooks. Coptotomus larvae (family: Dytiscidae) each have 6 pairs of filaments and the abdomen ends in two long “tails” covered in little hairs.
The blue highlights point out some of the identifying characteristics for Sialis aequalis.
Once identification to Sialidae is established, the genus for specimens found in North America is Sialis. Determining the species of larvae can be tricky though. I use a key located in an unpublished PhD dissertation, so this area of invertebrates can definitely benefit from more study. For Sialis aequalis, identification uses characteristics like the dark abdomen with light chevron-shaped marks and the first thoracic segment (prothorax) with indistinguishable markings. Much of the key uses color characteristics so specimens must be undamaged and well preserved to try and identify them. Most keys are regionally based as well, so that adds further limitations to identifications.
Have you ever come upon one of these larva while out in your local waterways? Or perhaps spotted one of the adults? Let me know in the comments below or through the contact page!
Sources
Canterbury, L.E., 1978. Studies of the genus Sialis (Sialidae: Megaloptera) in eastern North America. Unpub. Ph.D. Dissertation, Univ. Louisville, Kentucky. x + 93 pp.
Hilsenhoff, W.L., 1995. Aquatic Insects of Wisconsin. Keys to Wisconsin genera and notes on biology, habitat, distribution, and species. Publication of the Natural History Council, University of Wisconsin-Madison 3:11-17.
Liu, X., F. Hayashi, and D. Yang, 2015. Phylogeny of the family Sialidae (Insecta: Megaloptera) inferred from morphological data, with implications for generic classification and historical biogeography. Cladistics 31:18-49. https://doi.org/10.1111/cla.12071
Rasmussen, A.K. and M.L. Pescador, 2002. A guide to the Megaloptera and aquatic Neuroptera of Florida. Department of Environmental Protection, State of Florida, Tallahassee, Florida.
Woodrum, J.E. and D.C. Tarter, 1973. The Life History of the Alderfly, Sialis Aequalis Banks, in an Acid Mine Stream. The American Midland Naturalist 89: 360-68. https://doi.org/10.2307/2424040
