Hives found to be queenless may be supplied either with matured queens or with queen-cells. If the latter are sufficiently numerous, their introduction may easily be effected by exchanging a comb in each hive; if they have to be cut out and place loosely in the new hive, a triangular piece of comb should then be removed with them, to be used as a block in preventing any pressure coming on them. A space must be cut out of the middle in the centre combs of the hive into which they are to be introduced. Special care must be taken not to bruise the royal embryos, as they are particularly sensitive to pressure. It is sometimes best to introduce royal brood into queenless hives in preference to matured _unimpregnated_ queens, because, as mentioned at page 8, bees are reluctant to receive virgin queens, whilst they will tolerate one hatched in the hive, who will speedily depart to seek a drone. Bee-masters mostly use small hives for queen-rearing, consisting of, say, four combs. By extracting from a populous hive four such frames of combs (with the bees on them), and having eggs in the first stage (see page 9), or better still a royal embryo, the bees will rear a queen or queens therefrom. Care will have to be exercised to ensure that there is a sufficient number of bees to mature the brood brought from the hive.[21] [Footnote 21: These weak little colonies should have small entrances, so as to be better able to defend themselves from the attacks of robber-bees, and they will require to be assisted and strengthened by feeding.] A colony of this character is technically called a "nucleus." Such operations as queen-rearing should only be attempted in warm summer weather, and when drones are abundant. A very great advantage that the Woodbury bar and frame hive affords, is the safety and convenience with which a stock of bees can in it be transported to any part of the kingdom: by a few additional arrangements, stocks have even been sent in it to distant countries. In many districts hives are removed to moors, and heaths in autumn, for the purpose of gathering heather honey. In this operation, the frames are a great support to the combs, very much lessening the risk of a break down and consequent loss.