Insects living in salt water, such as brine flies, are generally hypoosmotic to their environment. These insects tend to lose water to the environment. Again, if left unattended, this would put the cells within the insect in a state of water loss and shrinkage. Equipped with a feeling for the nature of the problem, let us begin with a consideration of fresh water insects. The hemolymph, or extracellular compartment, of these insects is hyperosmotic to the water comprising their habitats.
Water tends to pass through the cuticle and into insects.
Some adult forms, such as the predatory beetle Dytiscus , have cuticles relatively impermeable to water. However, the majority of aquatic insects in their larval stages are quite permeable to water. Often respiratory gill structures are even more permeable to water, and additional water is taken in with food. We gather that fresh water insects have a problem of taking in too much water. The relatively large amounts of water invading fresh water insects are removed from their bodies by formation of copious amounts of dilute urine.