Midden Inkcap Identification Guide

The Setting and the First Impression

You rarely find Coprinus sterquilinus by wandering through woodland. You find it by paying attention to the parts of the landscape most walkers step around: the crumbling straw-and-manure midden behind a stable block, the rain-slumped heap at the corner of a paddock, a scatter of well-rotted horse droppings in a bare gateway or on a bridleway verge. This is a fungus of nitrogen-rich waste, and the substrate is half of the identification before you have even knelt down.

The first impression is of a small white shaggy egg balanced on an improbably long, thin white stalk. Fruit bodies often appear overnight and have begun to dissolve by mid-afternoon, so morning visits after a warm, wet night are far more productive than afternoon ones. A group may number two or three, or a dozen scattered across a heap, each at a slightly different stage, which is a gift: you can examine a button, a half-open bell and a dissolving veteran in a single sitting.

Working Through the Cap

Start with the youngest specimen. In button stage the cap is ovoid to short-cylindrical, roughly 2 to 4 cm tall and 1.5 to 2.5 cm across, chalk white, and covered over its whole surface with a soft woolly to floccose coat. That coat is the broken remains of a universal veil, and as the cap expands it tears into discrete upturned white flakes and patches with smoother greyish surface showing between them. The flakes are woolly rather than pointed, and they wipe off easily with a fingertip.

As expansion continues the cap becomes bell-shaped, develops radial grooves that run from the margin toward the disc, and greys from the edge inward. Then the classic inkcap process takes over: the margin turns up, blackens and liquefies, and the whole cap digests itself into a dripping black fluid over a few hours. A fully flat, intact cap is essentially never seen. If you arrive late in the day, look for the tell-tale black drips down the stipe and on the substrate below.

Reading the Gills

Slice a young cap vertically from apex to stipe. This single cut is the most informative thing you can do with this species. It shows the gills to be free from the stipe, packed extremely close together, narrow and thin, and it shows the veil tissue and the ring zone in cross-section at the same time.

The gill colour sequence is a clock. Pure white in the button, then a pale pinkish grey, then dark grey-brown as the spores mature, then jet black and inky. Blackening starts at the gill edge nearest the cap margin and works upward and inward, so a mid-aged specimen shows a neat gradient from black outer edge to still-pale inner portion.

Stipe, Base and Flesh

The stipe is white, 5 to 15 cm tall and only 3 to 8 mm thick, tapering slightly upward, hollow, and finely silky-fibrillose. It snaps rather than tears. Trace it downward and clear the substrate away carefully with a knife: the base is slightly swollen and often tapers into a short rooting portion buried in the dung, sometimes with white mycelial threads binding straw fragments around it.

The feature to look for on the lower stipe is a thin, white, membranous ring. It is fragile and movable, and it frequently slides down to sit as a loose collar near the base, or collapses into a narrow flaring rim. Many specimens have lost it altogether, so its absence proves nothing, but its presence on a dung inkcap is a strong pointer. Flesh throughout is thin, white and soft. There is no latex, no blueing and no bruising reaction; the only colour change is the age-driven blackening of the gills and cap, which is autodigestion rather than a response to injury.

Taking and Reading the Spore Print

Time the print carefully. Choose a cap whose gills are grey-brown but not yet dripping. Cut the stipe flush with the cap, set the cap gills-down on white paper with a strip of glass or foil under one half for contrast, cover it with a bowl or beaker to hold humidity, and leave it. Because the species is racing toward deliquescence, a good deposit usually appears within thirty to sixty minutes rather than the several hours a firmer agaric needs.

The print is dense black. If the cap has already begun to dissolve, you can simply collect the black liquid, which is a concentrated spore suspension, and put a drop on a slide. Under magnification the spores are strikingly large for an inkcap at roughly 17 to 24 by 10 to 13 micrometres, smooth, thick-walled, dark reddish-black to opaque, ellipsoid in face view, and with a distinct central germ pore at the apex. That spore size is the single cleanest confirmation available.

Substrate, Season and Ecology

This species is a dung and manure specialist. Horse dung is the classic substrate, with cow dung and heavily manured straw, compost and midden crusts also serving. It fruits in flushes after rain from spring through autumn, favouring warm, humid spells, and populations are boosted wherever livestock are kept and bedding is stacked.

Ecologically it sits fairly late in the coprophilous succession. Fast, small ascomycetes and the spore-flinging Pilobolus colonise fresh droppings first; the larger agarics, including this inkcap, arrive once the substrate has weathered, aggregated and been partly broken down. The mycelium exploits the nitrogen-rich residue and the straw or fibre mixed into it, and the whole reproductive strategy of the fungus is built around getting spores away fast from an ephemeral, competitive resource, which is why the fruit body lasts hours rather than days.

Separating It from the Look-alikes

Coprinus comatus, the Shaggy Inkcap, is the closest relative and shares the movable ring, the free crowded gills and the self-digesting cap. It is far larger, with a cylindrical cap 5 to 15 cm tall, tiered shaggy scales, and a habit of growing on disturbed grassy ground, verges, lawns and made-up soil rather than on dung. Its spores are much smaller, around 11 to 15 by 6.5 to 8 micrometres.

Coprinopsis cinerea and its relatives grow on compost and manured straw and can look superficially similar, but the veil is greyish and hair-like rather than white and woolly, there is no true membranous movable ring, the cap is greyer overall, and the spores are considerably smaller.

Coprinopsis lagopus and C. radiata are greyer still, clothed in fine silvery hairs rather than flakes, ringless, and with proportionally more slender stipes.

Panaeolus semiovatus is the trap. It shares the dung habitat and it also has a ring on the stipe. Separate it on the spore surface and cap: its gills become mottled black-and-grey and do not dissolve, its cap is smooth, tan to pale ochre and often shiny or slightly tacky rather than woolly-scaly, and the fruit body persists and dries intact instead of turning to ink.

Bolbitius and Conocybe species on dung are eliminated instantly by their rusty-brown spore prints and their non-deliquescing gills.

Quick Field Checklist

• On dung, manured straw or a midden, not in woodland or plain grass
• Small white cap clothed in soft woolly flakes, becoming grooved and then inky
• Free, very crowded gills running white to pink-grey to black
• Slender white hollow stipe with a thin movable ring, often slipped low
• Black spore print with unusually large, pore-bearing spores