Unknown substance found on wall

Our Attacus Atlas butterflies are hatching and we are keeping them in the orangery of the house. One was resting on a climbing plant and next morning when we went looking we saw large stripe on the wall of a previous liquid substance that almost certainly must have come from the butterfly itself.

In my understanding Attacus Atlas butterflies don't have mouthparts and therefore don't have an anus. But could this be poo or is it another liquid that is excreted by the butterfly.

See pictures below

Ah... Thanks for your quick response. Learnt something new today.

Meconium as in what baby's produce. Do I have to expect another dropping anytime soon, or is it one time only?