The Flies refuse to lay on the sand, apparently for the same reasons.

The paper was considered an obstacle which the frail vermin would not be able to overcome.

With sand, the case is worse.

Its grittiness would hurt the new-born weaklings, its dryness would absorb the moisture indispensable to their movements.

Later, when preparing for the metamorphosis, when their strength has come to them, the grubs will dig the earth quite well and be able to descend: but, at the start, that would be very dangerous for them.