The lecture Hall – Entomology

Fly fishing is such a broad discipline, and I never want to loose touch with scientific side. So tonight, I’m writing a piece on entomology, just for you! Lucky people… When you are ‘matching the hatch’ and trying to dissect your targets feeding and behaviour habits, would you know a Medium Olive from a Caddis? Or a Stone fly nymph from a March Brown? As I sit here typing away, in enough entomology textbooks to even make Attenborough fall asleep, it brings a full circle to my fly fishing. Time invested off the water is just as valuable as time spent on the water, learning, discovering and feasting your eyes of endless diagrams and pictures. Insects are the basis of our sport, the foundations on which fly fishing was built, so I think they deserve five or ten minutes.

Watercraft is an important part of all angling, but especially when you’re chucking flies. It becomes a craft, a skill, an art, and will quickly consume you. I’ve always admired Paul Procter’s writings and work on the entomology side of fly fishing and really look up to his skill and knowledge, hopefully one day i'll be an expert on aquatic insects too!

We’ll start with the humble yet beautiful, Medium Olive – Baetis vernus. A small (no more than 25mm) nymph, they swim in the water column occasionally holding on to debris. They are of the order Ephemeroptera, which classifies all Mayflies, and can be readily seen on many waters across the UK.

Baetis vernus 

Baetis bernus during pupation

They undertake a risky transformation from aquatic to terrestrial life via pupation, which is where the metamorphosis happens. They loose their gills and respire as a terrestrial insect through spiracles, which lead to a network of trachea inside the body. The adults live for a day at most (some have been known to depart after just 37 minutes of terrestrial life) and spend their time mating and then laying eggs.

Next on stage is the Blue Winged Olive – Baetis tricaudatus. Another common species, also under the order of Ephemeroptera, they are very similar to B.vernus although they do sport a darker wing case and the terrestrials differ in appearance slightly. When I studied this species last spring, my specimens showed a drastic case of sexual dimorphism. This is where different genders of the same species develop different attributes. In the case of B.tricaudatus males have four compound eyes and females only have two, which is why some specimens appear to have larger and red eyes (males). Pretty amazing stuff!

Baetis tricaudatus

The sexual dimorphism shown by Ephemeroptera
A) Female - 2 compound eyes
B) Male - 4 compound eyes

Staying within the Emphemeroptera order, this time moving into the genus Mccaffertium. The March Brown (Mccaffertium vicarium) is a ‘clinger’ nymph, and prefers to use its low profile to hold onto the river bed or under the substrate. The dorso-ventral compression (back and belly) creates a slim and aerodynamic body shape which is able to hold on in the fastest of runs.

Mccaffertium vicarium

Dorso - ventral compression displayed by
 Ephemeroptera's 'clinger' nymphs

Now for the humble Caddis, everyone loves a Caddis! From the charismatic case building to the angler-swarming hatches they create, they’re an essential part of the river ecosystem and are a reliable bet for nearly all still and moving waters. When in aquatic nymph form, they are best identified by the material their case is built from. All Caddis’ come under the order of Trichoptera. Take Microptera sequax for example, which is know for building its case from sand grains which increase in size towards the case opening. The actual Caddis inside used barbs on its body to hold itself inside the case.

Microptera sequax

Limnephilus flavicornis is another interesting example. Identified by its case, which is made from plant debris arranged in a criss-cross fashion. On incredibly rare cases L.flavicornis has been known to build its case entirely from tiny shells!

Limnephilus flavicornis

There are also caseless Caddis species. The ‘net spinning’ Caddis’ are covered by the two genera Hydropsyche and Cheumatopsyche. These specimens build silk nets and externally attach materials for camouflage. They carry out a rather unusual behavior known as behavioral drift, which is where they leave the safety of their nets and drift to a new location, perfect Trout food!

Net spinning Caddis - I have not been able to
ID this specimen to species level 

Next I’ve decided to write about the Freshwater Shrimp. Not an insect I know, but an important food item on the piscatorial menu nonetheless! The only species found I England is Gammarus pulex. English anglers maybe noticing more and more of these small crustacea displaying an orange colouration, and this is due to a parasite. Parasitism is pretty disgusting, but an incredible feat of evolution and should be respected. The parasite affecting our little G.pulex is called Pomphorhynchus laevis, and makes its host less photophobic, forcing the host to spend more time out in the open and not in safe shelter under debris. This makes G.pulex an easy prey item and consequently is easily taken by a fish, exactly what the parasite wants as it then resides within its final host, the fish. So maybe you’ll consider adding some hot spots to your shrimp patterns now!

Gammerus pulex

Back to the insects for our final subject, the feisty Alder Fly (Sialis lutaria). I say feisty, but only the nymphs are aggressive, the adult flies tend to keep themselves to themselves. The nymphs are mainly active at night, and crawl ashore to pupate. The aduts then mate, and eggs are laid on the underside of bankside vegetation that overhangs the waters edge. When the eggs hatch the nymphs fall straight into the water. 

Sialis lutaria

So there you have it, a little entomology for you. Hope you enjoyed it, and I hope it has enhanced your watercraft and bankside knowledge because remember – catching isn’t everything. Please share your knowledge in return using the comments! Whatever you’re up to this weekend - fishing, tying, drinking.. make sure you smash it!