Fly Fishers' Republic


February 12 2009

by Raif Killips



Order: Megaloptera; Family: Sialidae; Subfamily: Sialinae; Genus: Sialis.


All alderflies have a lifecycle that includes a complete metamorphosis with only the larval stage being aquatic. Commonly, in late spring and early summer (May/June for species Sialis lutaria and S. fuliginosaS. mohri and S. vegans) adults can be found amongst, or otherwise close to, bank side vegetation of ponds, lakes, reservoirs and slower reaches of streams and rivers. Here the female lays her eggs on plants overhanging the water or on emergent vegetation. A single female will lay many hundreds of eggs in a neat cluster. S. lutaria and S. fuliginosa lay up to two thousand eggs.

Upon hatching, usually a week or two after the eggs are laid, the young larvae crawl or drop into the water where they seek out a substrate of decaying vegetation and other detritus. Concealed in their lentic habitat, many species living in burrows, the larvae develop through as many as 11 instars reaching the end of their final instar after a period of between 9 months and just under 3 years. S. lutaria and S. fuliginosa mature in around 23 months. In spring, mature larvae crawl from the water onto dry land where they pupate in damp earth, under rocks and plant litter. Pupation takes a few weeks, after which the next generation of alderflies emerge.


While alderflies are distributed across all the continents they are most common in regions with temperate climates. Species in Euope include Sialis lutaria and S. fuliginosa both of which are very common. In the UK these species populate typical habitat throughout. There are 23 species of alderfly in North America, two are S. vegans and S. mohri. North American alderflies are most densely distributed in the central and eastern states. Australian species include Stenosialis australiensis, andAustrosialis ignicollis; distribution is concentrated in the east.


The adult alderfly averages 10-20mm in length (head to tip of abdomen) with a wingspan of 22-34mm. Its overall colouration is black or dark brown. It has four translucent dark grey or brown hairless wings with distinct dark veins; at rest these are held over the body tent fashion, not unlike a caddis. The alderfly has a broad head similar in diameter to its thorax and abdomen. Either side of the head is one small eye. The alderfly lacks any ocelli a characteristic that distinguishes it from the dobsonflies (Order: Megaloptera, Family: Corydalidae, Genus: Corydalinae) which have three. The antennae of the alderfly extend from the front of the head just above the mouth parts; they are segmented and similar in length to the abdomen. Adult alderflies may be distinguished from the stoneflies by complete absence of tail elements.


Alderfly larvae average 17mm in length though commonly reach 25mm. They are more or less dark brown in colour on their dorsal surface, and cream coloured on their ventral surface. The larvae are more or less cigar shaped, with a head as broad as the thorax, and with pronounced pincer-like jaws. The thorax and abdomen are segmented. The head and thorax are heavily sclerotized while the abdomen is relatively soft. Abdominal sections 1-7 carry two segmented lateral breathing filaments (one at either side), while the caudal section carries a single tapered axial filament. The breathing filaments are fringed with small hairs. Alderfly larvae may be distinguished from the dobsonfly and caddisfly larvae by the absence of anal hooks.

Fishing opportunities

At all times alderfly larvae may be vulnerable to predators as they leave their burrows in search of prey. However, in the few weeks prior to migration they become more active, crawling amongst the detritus, preying upon other insects. Before they are secure in their pupal chambers, this activity and the final migratory passage from water to dry land, a journey sometimes spanning hundreds of metres, especially exposes them to predation. In the UK, throat pump and stomach samples show that trout are amongst the predators that take advantage of this stage in the alderfly’s lifecycle and some fish feed selectively on alderfly larvae at this time. From early spring, fishing an artificial larvae in the slower reaches of rivers and streams can prove very effective. Fishing the margins of ponds, lakes and reservoirs can also prove very effective, though if fishing while afloat don’t neglect the water beyond the bank angler’s casting range.

Sexually mature alderflies often emerge from their pupal cases under cover of darkness, whereupon males and females seek one another out. Mating takes place with the insects clinging to foliage and other plant structure, or on the ground away from the water. Aided by their characteristic weak flying and with the help of a strong breeze, during their nuptial activities some flies may fall onto the water. Where numbers of swamped flies are large enough, fish will key in on them as prey items. Though this scenario is considered uncommon in the UK, in other countries, including the US, falls of alderflies that attract the fishes attention occur more frequently, making the presentation of a suitable artificial a worthwhile tactic. And hey, never say never UK anglers, keep your eyes open late spring and if you do come across that odd fish that’s put alderflies on the menu, a dark elk hair caddis or a spent caddis pattern should deceive your quarry.