Complex sentence: It is difficult to tell red admiral and yellow admiral caterpillars apart: looking at the 4th and 6th body segments, the yellow admiral larva has a wider light coloured area than the red admiral.
Split into simpler sentences: It is difficult to tell red admiral and yellow admiral caterpillars apart. Looking at the 4th and 6th body segments, the yellow admiral larva has a wider light coloured area than the red admiral.