Ancistroides folus, the grass demon,[1] is a small but prominent butterfly found in India & Nepal that belongs to the skipper family, Hesperiidae.

Upperside black, the basal half of both wings covered with minute yellowish scales, markings pure white, semi-hyaline.

Hindwing with a very large medial white patch with its margin sinuous all round; abdominal area covered with yellowish hairs.

Hindwing with the white medial patch extending to the abdominal margin from near the costa to one-third above the anal angle, the extension suffused with grey, also extending narrowly to the outer margin, double blackish-brown lunular marks, within the white space at the bases of the sub-median nervules.

This butterfly is seen chiefly during the monsoon, perhaps due to its larval host plants growing at this time of the year.

The grass demon prefers forest edges or clearings where dappled light is present.

Its black and white colouration may have evolved to take advantage of the dappled light in these areas.

The pied pattern which seems prominent in the open is effectively disruptive in the shade and the butterfly is difficult to locate once it settles down.

The grass demon is a nectar lover and has a long proboscis compared to body size.

In gardens, the common periwinkle (Vinca rosea) and Lantana are its favourite flowers.

The female normally sits on the underside of the leaf of the host plant and lays a single egg.

[9] Soon after hatching, the caterpillar does not consume the eggshell completely and it makes itself a spacious cell and hides inside it.

The full grown larva has the habit of resting with the first three segments contracted, so as to give a humped appearance.

It always lies closely applied to the leaf, by turning over a triangular piece from the edge onto the upper surface over itself.