This habit of passing the dry season in the hardened mud
forming the bottom of exhausted pools and rivers is practised
by several species of tropical freshwater fishes, belonging principally
to the family Siluridae. The members of this group are
able to exist and thrive in moist mud, and can even support
life for a comparatively long time out of water altogether. The
instinct is exhibited by species occurring both in the eastern and
western hemispheres, as is shown by its record in the case of
species of Callicthys and Loricaria in Guiana and by Clarias
lazera in Senegambia. It is also met with, according to Tennent,
in a species of climbing perch (Anabas oligolepis) found in Ceylon
and belonging to the family Anabantidae, all the species of
which are able to live for a certain length of time out of water,
and may sometimes be found crawling across land in search of
fresh pools. The habit is also common to some species of mud
fishes of the order Dipneusti, in which the air bladder plays
the part of lungs. Protopterus, from tropical Africa, for instance,
burrows into the mud and remains for nearly half the year
coiled up at the bottom in a slightly enlarged chamber. The
walls of this are lined with a layer of slime secreted from the
fish’s skin, and the orifice is closed with a lid the centre of which
is perforated and forms an inturned tube by means of which
air is conducted to the fish’s mouth. The aestivating burrow
of the Brazilian mudfish (Lepidosiren) is similar, except that
the lid is perforated with several apertures. The Australian
mudfish (Ceratodus) is not known to hibernate or aestivate.
In countries where winter frosts arrest the growth of vegetation terrestrial mollusca seek hibernacula beneath stones or fallen tree trunks, in rock crannies, holes in walls, in heaps of dead leaves, in moss or under the soil, and remain quiescent until the coming of spring. Amongst pulmonate gastropods, most species of snails (Helix, Clausilia) close the mouth of the shell at this period with a membranous or calcified plate, the epiphragm. Slugs (Limax, Arion), on the contrary, lie buried in the earth encysted in a coating of slime. Similarly in the tropics members of this group, such as Achatina in tropical Africa and Orthalicus in Brazil, aestivate during the dry season, the epiphragm preserving them against desiccation; and examples of two species of Achatina from east and west Africa exhibited in the Zoological Gardens in London remained concealed in their shells during the winter, although kept in an artificially warmed house, and resumed their activity in the summer.
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.
Spiders, like nearly all insects, hibernate in cold temperate latitudes. Burrowing species like trap-door spiders of the family Ctenizidae and some species of Lycosidae seal the doors of their burrows with silk or close up the orifice with a sheet of that material. Other non-burrowing species, like some species of Clubionidae and Drassidae, lie up in silken cases attached to the underside of stones or of pieces of loose bark, or buried under dead leaves or concealed in the cracks of walls. Other species, on the contrary, pass the winter in an immature state protected from the cold by the silken cocoon spun by the mother for her eggs before she dies in the late autumn, as in the “garden spider” (Aranea diadema). Commonly, however, when the cocoons are later in the making, or the cold weather sets in early, the eggs of this and of allied species do not hatch until the spring; but in either case the young emerge in the warm weather, become adult during the summer and die in the autumn after pairing and oviposition. Some members of this family, nevertheless, like Zilla x-notata, which live in the corners of windows, or in outhouses where the habitat affords a certain degree of protection from the cold, may survive the winter in the adult stage and be roused from lethargy by breaks in the weather and tempted by the warmth to spin new webs. Typical members of the Opiliones or harvest spiders, belonging to the family Phalangiidae, do not hibernate in temperate and more northern latitudes in Europe and America, but perish in the autumn, leaving their eggs buried in the soil to hatch in the succeeding spring. During the early summer, therefore, only immature individuals are found. Other species of this order, belonging to the family Trogulidae, spend the winter in a dormant state under stones or buried in the soil. False scorpions (Pseudo-scorpiones) also hibernate in temperate latitudes, passing the cold months, like many spiders, enclosed in silken cases attached to the underside of stones or loosened pieces of bark. Centipedes and millipedes bury themselves in the earth, or lie up in some secluded shelter such as stones or fallen tree trunks afford during the winter; and in the tropics millipedes lie dormant during seasons of drought.
What is true of the dormant condition of arthropod life in the winter of the northern hemisphere is also true in a general way of that of the southern hemisphere at the same season of the year. This is proved—to mention no other cases—by the observations of Darwin on the hibernation of insects and spiders at Montevideo and Bahia Blanca in South America, and by Distant’s account of the paucity of insect life in the winter in South Africa; by his discovery under stones of hibernating semi-torpid Coleoptera and Hemiptera at the end of August in the Transvaal, and of the gradual increase in the numbers of individuals and species of insects in that country as the spring advanced and the dry season came to an end.
Bibliography.—T. Bell, A History of British Reptiles (and Amphibians) (1849); W. T. Blanford, Fauna of British India: Mammalia (1889–1891); G. A. Boulenger, Monograph of the Tailless Batrachians of Europe, edited by the Ray Society; “Teleostei” in Cambridge Natural History, vii. 541-727 (1904); T. W. Bridge, “Dipneustei” in Cambridge Natural History, vii. 505-520 (1904); A. H. Cooke, “Molluscs” in Cambridge Natural History, iii. 25-27 (1895); T. A. Coward, P.Z.S. pp. 849-855 (1906), and pp. 312-324 (1907); C. Darwin, A Naturalist’s
