The duration of the period of larval existence depends on the temperature of the water and on the abundance or scarcity of food. When first hatched out from the egg, the larvæ are barely visible to the naked eye, whereas when full grown they contain within themselves the entire material of the adult flying insect. Once full-grown, the weather being favourable, they change into “nymphs” or pupæ, not unlike small tadpoles in form, the head and thorax being enclosed in an almost spherical envelope, to which is appended a sort of tail, formed by the abdomen, which is usually kept folded under the body, as shown in the figure below. In this stage the animal breathes through a pair of horns springing from the back of the thorax. The mouth is completely closed, so that the pupa is incapable of feeding, but is in no sense quiescent, as it is capable of lively movements and tries to elude capture almost as briskly as the larva. After a sufficient time has elapsed for the completion of the extraordinary anatomical changes that convert the larva into the adult insect, the pupa-case bursts along the back, and the perfect gnat gradually disentangles itself from its temporary home, and flies away, very shortly to reinitiate the cycle of events by laying a fresh batch of eggs. As has already been remarked, the time required to complete these changes varies greatly in different species and under varying environments, of which climate is the most important factor, but under moderately favourable conditions the time required is a fortnight or three weeks, the greater part of which is passed as a larva. In countries where water freezes during winter for any length of time, all larvæ that have not completed their metamorphosis by the end of autumn, must necessarily perish; and the continuity of the species is maintained entirely by the survival of pregnant females that hide themselves in warm corners and pass the winter in a truly hybernating or dormant condition. Further south, both males and females hybernate, and the dormancy is so much less pronounced that the insects are often tempted to issue from their hiding places on exceptionally warm days. In such climates, which include Italy and most of the sub-tropical zone, the species finds an additional string for its bow in the survival of larvæ, which, though they cannot be said to hybernate in the strict sense of the term, being always lively and alert in their movements, are yet incapable of growing, and appear to remain at whatever size they may have previously attained until the return of warmer weather. Quite recently Dr. Bancroft, of Queensland, has discovered that in some species these wintering larvæ can give birth to small broods of young larvæ, so that in such cases the bow has no less than three strings. Further south again, in truly tropical climates, breeding goes on all the year round.