The musculature of the mesothorax and metathorax is similar in those insects both of whose pairs of wings are like each other, and act independently during flight, viz. in the Libellulidæ. On the other hand, in the second category, where the fore and hind wings act as a single pair and the fore wings are mostly larger than the hinder (except in most of the Trichoptera), the musculature of the mesothorax is more developed than that of the metathorax. To neither category belong the beetles, whose wing-covers are peculiar organs of flight, and not for direct use, and the Diptera, which possess but a single pair of wings. In the beetles the hind wings, in the Diptera the fore wings, serve especially as organs of flight. It may be observed that the Diptera are the best fliers, and that those insects which use both pairs of wings as a single pair fly better than those insects whose two pairs of wings work independently of each other. An exception are the swift-flying Libellulidæ, whose specially formed muscles of flight explain their unusual capabilities for flying (Kolbe). LITERATURE ON FLIGHT =Marey, E. J.= La machine animale. Locomotion terrestre et aërienne. Paris, 1874. —— Mémoire sur le vol des insectes et des oiseaux. (Annal. Scienc. natur., 5 sér., Zool. xii, 1869, pp. 49–150; 5 sér., Zool. xv, 1872, 42 Figs.) —— Note sur le vol des insectes. (Compt. rend. et Mém. Soc. d. Biol. Paris, 4 sér., v, 1869, C. R. pp. 136–139.) —— Recherches sur le mécanisme du vol des insectes. (Journal de l’Anatomie et de la Physiologie, 6 Année, 1869, pp. 19–36, 337–348.) —— Animal mechanism. New York, 1879, pp. 180–209. —— Movement. New York, 1895, pp. 239–274. =Hartings.= Ueber den Flug. (Niederland. Archiv f. Zoologie, iv, Leiden, 1877–78.) =Lucy.= Le vol des oiseaux, chauvesouris et insectes. Paris. =Tatin, V.= Expériences physiologiques et synthétiques sur le mécanisme du vol. (Ecole prat. d. haut. étud. Physiol. expérim. Trav. du laborat. de Marey, 1877, pp. 293–302.) —— Expériences sur le vol mécanique. (Ibid., 1876, pp. 87–108.) =Bellesme, Jousset de.= Recherches expérimentales sur les fonctions du balancier chez les insectes Diptères, Paris, 1878, 96 pp., Figs. —— Sur une fonction de direction dans le vol des insectes. (Compt. rend., lxxxix, 1879, pp. 980–983.) =Pettigrew, J. Bell.= On the mechanical appliances by which flight is attained in the animal kingdom. (Trans. Linn. Soc., 1868, xxvi, Pt. I, pp. 197–277, 4 Pl.) —— On the physiology of wings. (Trans. Roy. Soc. Edinburgh, 1871, xxvi, pp. 321–446.) =Krarup-hansen, C. J. L.= Beitrag zu einer Theorie des Fluges der Vogel, Insekten und Fledermause. (Copenhagen u. Leipzig, Fritsch, 1869, 48 pp.)