Insects and millipedes were among the very first terrestrial animals. hypothesize what characteristics of their aquatic ancestors among animals related to crustaceans (think lobsters, crabs, etc.) may have acted as a preadaptation that could be co-opted for use in terrestrial habitats.

Preadaptations like tegument and body segmentation could have been used by insects and millipedes to survive in terrestrial habitats.

Explanation:

As crustaceans, millipedes and insects have a protective, hard tegument, which surface is frequently smooth. Some groups can also present spines, bristles, or tubercles.  The tegument protects the animals against harmful stimuli of different nature, such as mechanic, thermal, chemical, etc. It is also an effective barrier against pathogen microorganisms and excessive dehydration. In groups like crustaceans and insects, as the external layer gets hard, the tegument constitutes an important support organ. It is composed of a simple, cubic epithelium called hypodermis, and secretes exoskeleton, which is composed of chitin and can also be shaped by calcareous impregnations. Chitin is an impermeable, light y hard substance, that allows life on land without dehydration.   Segmentation (repetition of identical anatomical units) is another characteristic in these groups that have helped to domain terrestrial habitats. It seems like segmentation is highly advantageous from an evolution point of view because it is much easier to adapt each segment separately to environmental pressures, responding to different necessities, than adapting the whole body to it, developing a new organ for each necessity. Also, it is a strategy to defend themselves from predators by rolling up.

when a reversible reaction is occurring in both directions at the same rate; the products and reactants do not change in concentration. a chemical reaction where the compound is broken into its constituent parts.