Most of the nests of the Poecilocyttares have only a single layer of comb. The wasps of the genera _Synoeca_ and _Polybia_ have the habit of spreading a layer of cells on a leaf, or on the bark of a tree, and of covering this with an envelope that is pierced by a single orifice only, but that does not rest on the cells, and so allows circulation of the Insects between the cells and the envelope. This appears to be the arrangement in a nest of _Synoeca cyanea_ preserved in the British Museum; in this construction a large layer of cells is moulded on the branch of a tree, whose contour, for a length of two or three feet, it consequently follows; while outside the mass there is placed a continuous envelope, leaving a considerable distance between it and the cells. It would be impossible in the space at our disposal to give a satisfactory account of all the forms of wasp-nests, and we must therefore refer the student to de Saussure's work, confining ourselves to a brief notice of some specially interesting forms. The habitation of the Brazilian _Polybia_ (_Myrapetra_) _scutellaris_ is a very solid, closed structure, covered externally with rough knobs or angular projections. Although of very large size—it may be upwards of two feet in length—it is suspended from a branch, and has but one orifice; the arrangement of the combs in the interior is that of the Phragmocyttares, they being firmly attached to the outer envelope, and so placed as to form a curved surface, the convexity of which is downwards: the number of wasps in a well-developed nest of this kind must be very great. This species is said to be a honey-gathering wasp. One of the best known of the South American wasps' nests is the construction (Fig. 33) of _Chartergus chartarius_; these nests are so regularly shaped, and formed of papier-maché so compact and solid, as to look like stone: this edifice is attached in a very firm manner to the branch of a tree, and has a single portal of entry beneath; its interior arrangement is much like that of _Myrapetra scutellaris_. {83}A very remarkable wasp's nest is preserved in the British Museum of Natural History; it is considered to be the work of _Montezumia dimidiata_ Sauss. an Eumenid wasp; it is a large mass of cells encircling the branch of a tree, which therefore projects somewhat after the manner of an axle through the middle: the cells are very numerous, and are quite as regular as those of the most perfect of the combs of bees: the mass is covered with a very thick layer of paper, the nest having somewhat the external appearance of half a cocoa-nut of twice the usual size. [Illustration: Fig. 33—Section of nest of _Chartergus chartarius_. South America, _o_, Entrance. (After de Saussure.)]