Muscles analogous to those just noticed exist in insects whose prothorax is connate, that is, intimately united to the succeeding segment; but, in these the mesothorax being most highly developed, it is there that they acquire their greatest dimensions, and one pair is generally enlarged at the expense of another. Thus, the dorsal pair is most voluminous in the Hymenoptera and Lepidoptera, while it is the lateral pair in the Diptera.
Many of the muscles already noticed contribute, in a greater or less degree, to promote the act of flight, by contracting or dilating the walls of the thoracic cavity, but there are a few to which the office of moving the wings is exclusively assigned. These originate from the lateral parts of the sternum, and are attached by pointed tendons to the principal nervures of the wing. Their development is always in proportion to that of the wing which they are destined to move. If the anterior wings be largest, as stated by Burmeister, the dorsal muscle of the anterior wing is likewise the largest; if the posterior wings are wanting, their extensor is also wanting, and if both are of equal size, their extensors also are of equal size; but, if the posterior wings are the largest, this is likewise the case with their extensors, as may be seen in the Coleoptera, while the extensor of the elytra in that order is very small. A small extensor, flexor muscles, and a series of smaller ones, which, when in action, cause the relaxation of the extensors, are the other motive instruments of the wings.
