The thickness of sclerenchyma around the veins of lime leaves varies with both size of leaf and size of vein and may provide a barrier to stylet penetration. Although small veins confer greater nutritional benefits and have no lignified barriers, there are advantages to feeding on the larger veins for those more mature aphids which can penetrate the sclerenchyma. Feeding on the large veins of the upper surface, which are situated in grooves, may provide protection against dislodgement. On the undersurfaces of small leaves, feeding on large veins promotes orientation towards the petiole and early warning of approaching predators. On the corresponding surfaces of large leaves the sclerenchyma is thicker but aphids can achieve orientation by feeding on the smallest veins.