The text describes the freshwater insect that is called by the author “niveau d’eau douce” and is drawn just below the title.
Looking like a hammer shark, its T shape evokes a “niveau”. It also has three legs on each side.

In this text, the only reference to an inclinometer is the word “niveau”. It is used to give a name to the insect because their shapes are similar.

As indicated by @SimonMourier, there is no indication of the level technology (spirit or plumb bob level).

 The insect is a dragonfly larvae (larve de libellule)