The _mesothorax_, or central division of the thorax, has inserted on each side near the centre the four wings, the anterior pair articulating beneath the _squamulæ_, or wing-scales, which cover their base like an epaulette, and this wing scale often yields a specific character. In repose the four wings lie, horizontally, along the body, over the abdomen, the superior above, the inferior beneath. The wings themselves are transparent membranes, intersected by threads darker than their own substance, called their nervures, which are supposed to be tubular. These nervures and the spaces they enclose, called cells, are used in the superior wing only, and only occasionally, as subsidiary generic characters, and their terminology it will be desirable to describe, as use will be made subsequently of it. At the same time, to facilitate the comprehension of the terms, an illustrative diagram is appended; but those parts only will be described which have positive generic application. I may, however, first observe that upon the expansion of the wings in flight, the insect has the voluntary power of making the inferior cling to the superior wing by a series of hooklets with which its anterior edge is furnished at about half the length of that wing, which gives to the thus consolidated combination of the two a greater force in beating the air to accelerate its progress. That the insect has a control over the operation of these hooklets is very evident, for, upon settling, it usually unlocks them, and the anterior are often seen separated and raised perpendicularly over the insect; but that this can be mechanically effected also is shown sometimes in pinning a bee for setting, when by a lucky accident the pin catches the muscles which act upon the wings, and they become distended, as in flight, closely linked together. Both the diagram and the description of this superior wing I borrow from an elaborate paper of my own in the first volume of the ‘Transactions of the Entomological Society of London,’ wherein I gave a tabulated view, in chronological order, of the nomenclature introduced by successive entomologists in the use they made of the anterior wing of the _Hymenoptera_ for generic subdivision, and which I subsequently applied to my own work upon the ‘Fossorial Hymenoptera of Great Britain.’ [Illustration: Fig. 12.—Superior wing. _a_, marginal cell; _b_, first cubital or submarginal cell; _c_, second ditto; _d_, third ditto; _e_ and _f_, first and second recurrent nervures.]