Female indra swallowtails prefer to lay eggs on plants that have the best chance of providing healthy stalks long enough for their caterpillars to make it to pupa.  For many desert and/or otherwise xeric populations of Papilio indra, the plants with the healthiest roots are those growing between rocks where moisture is retained in those roots.  Oftentimes, the stalks of Lomatium and Cymopterus plants growing out in the open burn up much faster than those growing between rocks.