Scarlet Caterpillarclub Identification Guide

The moment you spot it

Almost everyone finds their first one by accident, on their knees looking at something else. The picture is a small orange finger standing bolt upright out of damp leaf litter, moss or the soft soil of a shaded bank, usually 2 to 5 cm tall and rarely thicker than a pencil. Colour is what carries at range: a clean orange to orange-red, much closer to satsuma peel than to the browns and ochres surrounding it, and it holds that colour even in flat winter-woodland light. The clubs often come in small groups, two or three within a hand span, so once you have found one, slow right down and search the surrounding half metre before you disturb anything.

Set that first impression against the single most important fact about the species: it is not growing out of wood, and it is not growing out of plain soil. It is growing out of a buried insect. Every step that follows is really about proving that.

Step one: read the shape of the club

Work from the top down. The upper half to two thirds of the club is the fertile head. It is slightly swollen compared with the part below it, cylindrical to narrowly club-shaped with a blunt rounded tip, and typically 1 to 4 cm long by 3 to 8 mm thick. Some fruitbodies are flattened, forked or fused at the tip; treat that as ordinary variation rather than evidence of a different species.

Below the head, the club narrows into a sterile stalk that is noticeably paler, often yellow-orange to apricot, smooth, sometimes faintly grooved or slightly twisted. It disappears down into the litter rather than sitting on top of it, and that is your cue for the excavation step later.

Step two: the fertile surface under a lens

This is the step that separates the species from everything that merely looks like an orange stick. Put a hand lens on the head. The surface is not smooth: it is finely pimpled, like coarse orange peel or a very fine sandpaper of raised dots. Each dot is a perithecium, a tiny flask embedded just below the surface with a minute pore at the top through which spores are fired. With good light you can often see those pores as darker specks at the centre of each pimple.

Now run the lens down onto the stalk. The pimpling stops fairly abruptly and the stalk is smooth. That contrast, a pimpled head above a smooth stalk, is the standard architecture for the group and rules out the smooth club and coral fungi immediately.

Step three: excavate the host

Do not pull. Push a knife blade or your fingers into the ground 3 to 4 cm out from the club and lift the whole plug of soil and litter, then tease the material apart gently, working inwards. Somewhere between 1 and 6 cm down, the stalk will be attached to the body of a moth or butterfly pupa, or less often a larva. The insect cuticle is usually still intact, with segments and wing cases visible, and the body cavity is packed solid with a firm white to cream fungal mass. There is often orange staining where the fungus has broken out through the cuticle.

If you cannot find an insect, you have not finished the identification. Sieve the soil, follow the stalk, and keep going.

Step four: flesh in section

Slice a club lengthways. The flesh is white to pale orange, firm and slightly fibrous or corky rather than brittle, and it does not snap cleanly the way a Clavulinopsis does. Nothing bleeds, nothing bruises blue, and there is no gelatinous layer anywhere. In section the perithecia show as a neat row of small flasks sitting in the outer rind of the head, angled outwards, over a paler solid core. That sectional view is worth photographing, because it is diagnostic in a way a surface photograph is not.

Step five: spores

This is an ascomycete, so the familiar routine of laying a cap on paper does not apply. Instead, stand a fresh club with the head over a glass slide or a piece of dark paper inside a covered box overnight. The ascospores are shot out sideways and downwards and leave a faint whitish bloom rather than a crisp print.

Under the microscope the ascospores are extraordinarily long and thread-like, filling the length of the ascus, and they break apart into short cylindrical part-spores of roughly 3 to 6 by 1 to 1.5 micrometres. That fragmentation into part-spores is characteristic of the genus and explains why the deposit looks like fine dust instead of a defined print.

Substrate and season

Late summer through autumn, occasionally lingering into early winter in mild years. Look on mossy woodland banks, along grassy rides and clearings, in old unimproved pasture, at the edges of conifer plantations and in dune slacks: anywhere moth pupae overwinter in litter or turf. It is widespread across Europe, northern Asia and North America. The substrate record is always the same, a buried lepidopteran pupa or larva, so note the surrounding trees for the record but do not treat them as a partner. The host is the insect, not the plant.

Telling it apart on morphology alone

• Snaketongue Truffleclub, Tolypocladium ophioglossoides: a similar club silhouette, but the head is olive-black to nearly black, the stalk is olive-yellow, and yellow mycelial cords run from the base to a hard warty underground Elaphomyces fruitbody rather than to an insect.
• Tolypocladium capitatum: an almost spherical ochre to brown head on a paler stalk, again anchored to Elaphomyces.
• Ophiocordyceps gracilis: dull ochre to pale orange-brown, the head much less sharply set off from the stalk, attached to buried larvae, and usually a spring fruiter.
• Golden Spindles and their relatives (Clavulinopsis, Clavaria): bright orange or yellow but completely smooth and often slightly shiny under a lens, with no pimpling at all; the flesh snaps cleanly; they rise in tufts straight from grassland soil with nothing buried beneath.
• Yellow Stagshorn, Calocera viscosa: gelatinous and rubbery-elastic, sticky to the touch, usually forked into antler-like branches, always attached to buried conifer wood.
• Bog Beacon, Mitrula paludosa: a smooth glossy yellow-orange head on a sharply contrasting translucent white stalk, rising from waterlogged leaves in shallow running water.

A working sequence

Colour and the upright club shape get you to a shortlist. The pimpled fertile head under a lens gets you to the group. The cleanly excavated moth pupa closes the identification. Photograph the host in place before you lift it, because that single image is worth more than a dozen pictures of the orange club on its own.