Particles may be defined as discrete units of organic, or inorganic, material which can be physically sorted and biologically selected. Studies of the economics of building behavior, and particle selection, are rare, but energy expenditure in building is implied by the observation that animals removed from their cases, and forced to construct a new one, exhibit a progressive decrease in the size of successive structures built. Instead, because their function is the subject of some controversy, and may be dependent upon the nature of the particles used in their construction, a short discussion of the importance of external structures in determining the distribution, and habitat occupancy, of Trichoptera and Chironomidae is given. An array of taxa exhibit building behavior; polychaetes as well as trichopteran and chironomid larvae are considered here, along with the special case of hermit crabs, which acquire an external structure after selecting only a single particle.