Unfortunately for the well-being of collections, dried insects are liable to the attacks of various museum pests, the most troublesome of which are themselves insects, but altogether out of their proper place and role in the general collection. Unless constant precautions are taken, the collector will discover after a few months that instead of the rare specimens with the preparations of which he has taken no little pains there remains only a series of fragmentary specimens, which a few years' neglect will reduce to little more than a mass of dust or powder. The price, then, of a good collection is eternal vigilance. Most insects, when exposed for any length of time to strong light, fade or lose color, and the only way to prevent such achromatism is to exclude the light. Insect pests affecting collections include Psocidae, Mites, Tineidae, Coleoptera of the families Ptinidae and Dermestidae, these last being the most injurious. [Illustration: FIG. 121.--_Tineola biselliella_: _a_, adult; _b_, larva; _c_, cocoon and empty pupa--skin enlarged.] The Psocidae--degraded wingless insects already referred to in the classification (p. 24)--will find their way into the tightest boxes, but ordinarily do little if any damage, except in the case of delicate insects, such as Ephemerids, Microlepidoptera, and Microdiptera. The common forms found in collections are _Atropos divinatorius_ and _Clothilla pulsatoria_. Mites or Acari are rarely troublesome in collections, though Dr. H. A. Hagen reports having found a species (probably of Tyroglyphus) with imported insects, and considers them as liable to become dangerous enemies. Tineid larvae are rarely found in collections, and only affect the larger moths. They are not easily discovered, since they make no dust, as do most other pests. Some persons have been considerably annoyed by one of the common clothes moths, _Tineola biselliella_ (Fig. 121). Dr. Hagen found that it attacked freshly collected or newly spread insects, where the spreading-boards were left uncovered, but Mr. F. M. Webster has found it injurious to the general collections at Columbus, Ohio. Of beetles, the Ptinidae are sometimes found in collections but are not common. Two species are known to attack entomological specimens, namely, _Ptinus fur_, which is quite rare, in this country, but much more abundant in Europe, and _Tribolium ferrugineum_, a cosmopolitan species which, however, has several times been associated in injurious numbers with large collections of insects imported from the East Indies.