Under the name Musca/Calliphora erythrocephala it was probably the most intensively studied of all diptera in Europe in the early twentieth century, later to be overtaken by the similar Black Blowfly: Phormia regina in North America, then the much smaller and ubiquitous Drosophila which has many anatomical features in common with the two blowflies.
The labella is surrounded by chemosensory ["taste"] hairs springing from its aboral [back] surface. Each hair is hollow with a pore at the tip which allows tastants to enter and make contact with the chemosensory neurons inside. All labellar taste hairs house a single mechanosensory neuron, but the number of receptor sites, each of which responds to a different chemical structure, varies from two to four..
The hairs can be divided into four groups by length, and it turns out that these groups are related to the chemical structures to which the hairs respond. This is also true of similar hairs on the feet. There have been many experiments in which the electrical activity of individual hairs has been recorded in response to a variety of chemical stimulants.
The arrangement of the chemosensory hairs around the labella is roughly symmetrical. Maps of the positions of the base of the hairs on the aboral surfaces of the labellar lobes show that they are almost - but not quite perfectly, mirror images. There are also variations between individuals so the arrangement can't be used to identify species.
Gustatory neurons on the leading edges of the wings and chemosensory cells on blowly's palps and antennae are used for foraging and can detect traces of odours over long distances [maybe more than a kilometer].
On landing on potential food the hairs on the feet, which curve downwards, test the surface for attractive compounds [e.g. water and sugars] or noxious [e.g. alcohols] chemicals. An attractive response stimulates extension of the proboscis to explore the substrate. With the labella closed the hairs on the aboral surface point forwards. If the response continues to be attractive the labella open to sample the surface.
The response is sophisticated. If the sensilla detect water, the proboscis will only extend if the fly is thirsty. If sugar or other nutrient is detected, the proboscis will only extend if the fly is hungry. It is possible for only a single neuron to induce a response because each hair contains only one water detector and one salt detector [plus up to two others].
The surfaces of the labella are covered by lines of pseudotrachea, which are tubes formed around a skeleton of thousands of forked, chitinous rings, resembling curved pitchforks, which can collect fluid and conduct it by capillary action towards the oral opening leading to the food canal. There are more chemosensors in small pegs (papillae) between the pseudotrachea, and cells inside the alimentary tract to keep testing the fluid as it is ingested.
Surrounding the oral channel in the centre of the labella are the "prestomal teeth". These vary between species, but where they exist they can rasp the surface of some foods to expose liquids for ingestion. [The scrape marks have been recorded after flies have been feeding on sugar or baits laced with harmless dyes.] In C. vicina there are three rows of prestomal teeth on each side of the oral opening but the outermost row is usually difficult to see.
In C. vicina the pseudotrachea are covered by an almost transparent and presumably semi-permeable membrane.
As a specimen dries out the well-known collapse of delicate tissues due to surface tension effects means that the labella becomes dark and shrivelled. The membrane "shrink wraps" the crumpled tissue making it difficult to photograph because of the specular reflections: as if wrapped in cling film.
This shape of the labella are similar, but it not identical to the appearance fully distended labella have in life. The main difference is that the living fly extends the proboscis and labella using a set of muscles [not by inflation as was once thought], whilst in these images different post-mortem methods have been used to get the labella to distend and remain stable.
This specimen has been macerated, of course. 3 hours in 9% aqueous KOH. I think that this may be close to the limit at which the structure becomes too fragile to be handled.
Fluorescence induced by staining with citrazinic acid.
The first image is one tenth of its original scale, and the second a detail of the same image.
These include the instability of many of the reagents used when they are in solution, pH [which always seems to be close to an inflection in the titration curve making it difficult to repeat accurately], the purity of the dye [often not that good], the concentration [planar dyes form aggregates at increasing concentrations which have different properties] and the fact that the concentrations used may be very low [of the order of 5 to low hundreds of parts per million] which makes them vulnerable to accidental contamination.
One of the biggest problems is photodegradation. These images were made with Mitutoyo 10x and 20x objectives on a full frame DSLR. For the fluorescent signal to be bright enough to be captured at a low ISO to avoid shadow noise, the intensity of the excitation causes rapid photodegradation within a single run. A succession of runs may produce quite different results - and the first is not necessarily the best. Adding an oxygen scavenger helps and does not fluoresce so allows deep background shadows. An anti-fade reagent helps even more - but commercial products are egregiously costly and my improvised formula fluoresces blue. That does not affect the foreground of opaque subject but does produce a blue background [not always bad].
Illumination was a Nichia 6W 365 nm. LED driven by benchtop PSU in current-limiting mode, through a pulse-width modulator to control the output. A pair of N-BK7 glass collecting lenses projected a 7 mm square image of the dye onto the subject, slightly defocused to blur the image of the die electrodes. No diffuser was used because the images are of light emitted by the subject, not reflected by it.
And finally, at least for now here are two images of the prestomal teeth.
These subjects have not been macerated.
One was made with white light, and the other captures autofluorescence in 365 nm light.
