Aquatic insects have a rigid exoskeleton or cuticle composed of chitin, keratin which prevents them from continuing growth. Successive moults allow step-by-step growth. The cuticle is renewed at the level of the epidermis in four main stages, all guided by the hormonal system of the insect. During exuviation, the larvae or nymphs swallow air or water in large quantities to increase the pressure of hemolymph (equivalent to blood) Their cuticle tears on the lines of least resistance (head and thorax ) letting pass the new larva, a nymph or a winged and adult insect. The new cuticle is soft and supple. The insect remains very vulnerable at these times due to the density of its body but also due to the emission of molecules in the water during moulting which are recognized by aquatic predators. Fish recognize these odors carried in the water veins. They stay downstream waiting for aquatic insect outbreaks. According to hormonal odors, they can recognize the species of aquatic insects and see their life stages as well as the quality of production.
The new insect emerges while seeking refuge. He leaves behind him a permanent trace of his passage and his existence. The qualitative and quantitative presence of the stored exuviae makes it possible to study the ecotoxicity of the sediments. If a sediment is polluted, the insects disappear and so do their moults. The disappearance can be selective. It helps to understand the impacts and their identification. The pollutants are transferred by contact of the sediments with the skins of the insects: the exuviae. By analyzing the moults, we identify the pollutants present in the sediments.
If the exuviae play a role of bio indication, they above all make it possible to evaluate and situate the nutritional production of watercourses. The quality, the quantity of exuviae on a course defines its biological richness. Exuviae trace the production of insects over time. They tell the story of the river and its projection into the future while conveying information about its health, its biodiversity through the development of populations of aquatic insects. The exuvia source of nutritional information for fish and health information for humans integrates all the events taking place in a river to better understand the scenario.