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8. Index
The following two indices of relative size are often used for ant classification. They are particularly important when distinguishing between different species of the genus Lasius.
Cephalic index (CI): This is the ratio of the head width (usually excluding the eye) to the head length (from the posterior end of the head to the anterior end of the clypeus). It is calculated using the following equation: CI = (head width / head length) x 100. The greater the CI, the wider the head.
Scape index (SI): This is the ratio of the antennal scape length (excluding the globular segment at the base) to the head width. It is calculated as follows: SI = (scape length x head width) x 100. The greater the SI, the longer the antennal scape.
(Comments by Kazuo Ogata)