Freshwater Pulmonata do not appear to hibernate, such forms as _Limnaea_ and _Planorbis_ having been frequently seen crawling about beneath the ice of frozen ponds. During periods of drought in England, however, they commonly bury themselves in the mud, a habit which is also practised during the dry season in the tropics by species of Prosobranchiate Gastropods belonging to the genera _Ampullaria_, _Melania_ and others, which lie dormant until the first rains rouse them from their lethargy. Freshwater Pelecypoda (_Anodonta_, _Unio_) spend the European winter buried deep in the muddy bottom of ponds and streams. In cold and temperate latitudes a great majority of insects pass the winter in a dormant state, either in the larval, pupal or imaginal (reproductive) stages. In some the state of hibernation is complete in the sense that although the insects may be roused from their lethargy to the extent of movement by spells of warm weather, they do not leave their hibernacula to feed; in others it is incomplete in the sense that the insects emerge to feed, as in the case of the caterpillar of _Euprepia fuliginosa_, or to take the wing as in the case of the midge _Trichocera hiemalis_. Others again, like _Podura nivalis_ and _Boreus hiemalis_, never appear to hibernate, at least in England. The insects which hibernate as larvae belong to those species which pass more than one season in that stage, such as the goat-moth (_Cossus ligniperda_), cockchafers (_Melolontha_), stagbeetles (_Lucanus_) and dragon-flies (_Libellula_), &c.; and to some species which, although they only live a few months in this immature state, are hatched in the autumn or summer and only reach the final stage of growth in the following spring, like the butterflies of the genus _Argynnis_ (_paphia_, _aglaia_, &c.) in England. As an instance of species which survive the winter in the pupal or chrysalis stage may be cited the swallow-tailed butterfly of Europe (_Papilio machaon_); while to the category of species which hibernate as perfect insects belong many of the Coleoptera (Rhyncophora, _Coccinellidae_), &c., as well as some Hemiptera, Hymenoptera, Diptera and Lepidoptera (_Vanessa io_, _urticae_, &c.). In the case of the social Hymenoptera it is only the fertilized queen wasp out of the nest that survives the frost of winter, all the workers dying with the onset of cold in the autumn; the common hive bees (_Apis mellifica_), although they retire to the hive, do not hibernate, the numbers and activity of the individuals within the hive being sufficient to keep up the temperature above soporific point. Ants also remain actively at work underground unless the temperature falls several degrees below zero.