Gary Borger wrote an article called "Film Flies, Five Stages Of Insect Emergence And The Best Flies To Imitate" for Fly Fisherman Magazine. It used to be on the FFM archive but is no longer available. The start of the article is available on the Web archive:
Film flies are flies that are fished just under, in, and on the film. You have asked about flies "in the film".
As Rip Tide has noted, these are flies that imitate emergers. The use of the single term, “emerger,” to describe these flies and insect stage implies that they have a specific form or shape, but this is far from true.
Emergence is a seamless
process. To better understand this process we have divided nymphs from emergers but that division is made so that it is easier for us to understand. For the organism, emergence is a seamless transition.
Most mayflies and caddis emerge in the same fashion. The nymph or pupa plasters it’s back against the underside of the film.
For the mayfly, the floating nymph happens to be the earliest phase of change from nymph to emerger. I find it easiest to think of a floating nymph as an emerger because we fish it as an emerger and the trout take it just like an emerger.
The floating nymph is an emerger and the nymph in or under the film is just a bit earlier before the actual emergence begins.
nymph in my opinion is a misnomer because the nymph is not actually floating on the surface, but in the film. The thorax of the nymph is against the underside of surface film. When the thorax breaks open, the dun begins to emerges into the air. This is the next stage in emergence.
The nymph never actually floats before, during, or after emergence. Only the emerging dun is on the surface using the nymphal body as an underwater platform.
The open thorax of the nymph is analogous to a window from the under water world of the nymph to the surface world of the dun. The dun pulls and craws out of this "window" and leaves the nymphal husk behind under the surface film.
As the dun of the mayfly or the caddis adult, pulls itself out of the thorax of the nymphal/pupal stage, it gradually changes shape from a formless lump emerging from thorax to a fully formed adult.
To better fish this process, Gary split emergence into 5 stages in his FFM article, each of which is imitated by a fly. The floating nymph would be stage 1. Stage 2 would be the early formless lump of the back of the adult protruding from the floating nymph.
See a stage 2 floating nymph pattern here
. Notice that it is a nymph pattern with the thorax spit and the dun in the early stage of emergence.
A frequently imitated stage is Stage 3, which is after the head and legs of the insect have emerged. This stage is imitated by the Klinkhammer and parachute adams.
Stage 4 is when the adult has emerged, but the wings of the insect are still not fully extended and the nymphal shuck is still attached.
At stage 5 the insect expands its wings and they start to harden and dry.
So if you are fishing a parachute adams, a sparkle dun, or an X-caddis, you are fishing in the film. You may think of these as adult patterns on the film, but they actually imitate a low riding emerger stage of the adult.
See the discussion below of Gary's book, Fishing the Film
, which discusses the parachute adams as an emerger. The supporting parachute hackle is ABOVE the flies body and the body is IN THE FILM.
Reading Fishing the Film by Gary Borger
Rip is correct in that naturals can get "stuck" in the film. These are often called stillborns or cripples.
Cripples or stillborn mayflies are insects that are unable to complete the emergence process. As noted above, the process of emergence can stop at any stage and most of the time these are "stuck in the shuck" insects. So the crippling process can result in a fly that looks just like a normal emerger.
Often times, there is confusion about these flies and the term cripple and emerger are used interchangeably as in the case of the Quigley Cripple.
Just to backslide for a moment, there is a fly called the Quigley emerger.
The Quigley Emerger
Fishing Pictures - Quigley Emerger
There is also a Quigley Cripple that is at a stuck at a later stage of emergence.
Quigley Green Drake Cripple
Somehow this identical fly has morphed on the internet to the Quigley emerger, displacing the earlier stage emerger known also fly by the identical name.
Quigley Green Drake emerger
Quigley’s Green Drake Emerger fly tying video | The Caddis Fly: Oregon Fly Fishing Blog
Calling the same fly by two different names:
Here's an actual picture of a stillborn or cripple mayfly
Crippling can occur at even a later stage when the insect has fully emerged from the shuck but has a deformity so that it is unable to unfold it's wings and it lies on its side, unable to fly off. This stage can also be imitated and it looks different than the above two flies.
Kelly Galloup's version
Utah's Fly Corner: Galloup's cripple Tutorial
Galloup's Slide Inn Fly Shop - Galloup's Cripple
If you want to understand the process of emergence and film flies, get a copy of Gary Borger's Fishing the Film
. I was fortunate to be a proofreader (along with John Beth and Jason Borger) for the manuscript. It will expand your knowledge of how to fish that thin film that separates the trout's world from ours.