"Simultaneously with the formation of these cells, an umbrella-shaped covering is prepared above them. More cells are added, an egg being deposited in each of them as soon as formed, while constant additions are made to the covering until it has assumed a globular form, with only an aperture sufficiently large for the insect to pass in and out. Before the completion of the first covering, a second, just large enough to enclose it, is begun, and while this is in progress a third is commenced, and then a fourth, and so on. When young wasps have been produced in sufficient numbers to carry on the work without the assistance of the parent, an event which usually takes place in about six weeks from the commencement of the nest, she does not again leave home, but occupies herself solely in the task of depositing eggs as fast as cells can be formed by the workers for their reception.
"There are two methods by which the nests are enlarged by the workers after the queen has given up the task of building; some species choosing one, some adopting the other. One consists in forming a series of regular sheets or layers, which are made to overlap each other like the slates or tiles on the roof of a building, in the same way as is pursued by the queen of every species so long as she continues to be the architect. When a few of these sheets have been completed, that is, when they have been made to assume a spherical form, with only a small aperture for ingress and egress, each internal sheet is cut away, nearly but not quite, as fast as additional ones are formed externally, the shell or covering therefore slightly increasing in thickness as the nest increases in size. Thus architects among the human race are careful to proportion the thickness, and consequently the strength of the walls to the magnitude of the building designed to be erected.
"The other method consists in forming hollow pieces, or raising, as it were, blisters all over the plain surface which the queen has left; and upon these other blisters, and so on continually; cutting away, as in the former case, the under skin on the formation of the outer one. The latter method is adopted by the workers of _V. crabro_, _V. vulgaris_, and _V. germanica_; the former by _V. Norvegica_, _V. sylvestris_, _V. rufa_, and probably by _V. arborea_. Cutting away the inner portions of the coverings is a necessary process in order to make room for the increased size of the comb or combs. The material cut away is not thrown by as useless, but is worked up afresh; indeed this is effected in, and by, the very act of removing it; it is then either used in enlarging the combs or it is brought out and employed in making additions to the outside.