Description
A small bright yellow butterfly with a wingspan of 35-45 mm. In the dwarf forms, the wingspan may be as small as 25mm. It is variable in colour and markings, the dry season form being much lighter in colour with fewer and less pronounced markings than the wet season form. The sexes are similar, though the males are often brighter in colour.

The upper side forewing is distinctly two toned, black towards the termen and yellow towards the base. The line traced between the black and yellow sections is quite variable, but always resembles the outline of a dog's face. The lower extremity of the black border of the forewing is never subtended by yellow. It is continued on vein 1A for a distance of 2-6 mm. The narrower band on the termen of the hind wing is wider in the female.

The underside of both wings is a uniform yellow with fine rusty brown speckles or irregularly shaped rings of varying sizes. In some females, there is a narrow reddish-brown patch below the forewing apex. The forewing cell almost always has 2 ringed spots. When only one is present, it is usually small and indistinct. Individuals without any spots in the cell are rare.

Similar species
Three Spot Grass Yellow and One Spot Grass Yellow. More information.

Status, distribution and habitat
A very common widely distributed butterfly, and by far the commonest of all Grass Yellows in the island. It occurs from the coastal plains to the highest hills and is found everywhere, including urban areas. The populations peak towards the end of the south-west and north-east monsoons, with fair numbers present in between. Both wet and dry season forms fly together. Sometimes it is a minor pest of leguminous plants in nurseries. 

Habits
It flies a foot or two feet above ground, rather weakly, and  moves about in its habitat with a continuous fluttering of its wings. Females fly higher up in the canopy when in search of larval host plants. It frequently settles on small flowers to nectar and obtain nourishment, but very rarely displays the upper side of its wings. This makes field identification without netting very difficult. It usually flies in the sun but is not averse to partial shade. In the dry zone, during the hottest part of the day, it seeks shelter in the shady thickets and may be seen resting on the underside of leaves. It congregates in large numbers on wet soil, particularly during hot weather.

Early stages

Larva of Common Grass Yellow on a
Cassia nodosa  seedling in a nursery

The eggs are laid singly on the leaves of a very wide range of common leguminous plants. These include many species of Cassia, Albizzia, Pithecellobium, Caesalpinia, Sesbania and Acacia. Its favorite larval food plant in the dry zone is Cassia thora, a plant of the open spaces that springs up in large numbers with the onset of the first monsoonal rains. The adult larva is green, and it usually sits, and often feeds, along the midrib of the leaf or leaflet. In this position, it is well camouflaged from its many predators. Where possible, it will consume the entire leaflet to avoid leaving any clues for avian predators. Pupation occurs on or near the host plant, usually lower down the plant where it is more concealed.

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