[82] Entw. der Biene. Zeits. f. wiss. Zool. Bd. XX. Or, see Balfour’s Embryology, Vol. I., p. 338.
[83] From more recent observations it is probable that abdominal appendages are usually present in the embryos of Orthoptera, Coleoptera, Lepidoptera, and possibly Hymenoptera. The subject is rapidly advancing, and more will be known very shortly.
[84] See, for example, Klein’s Elements of Histology, chap. ix.
[85] The exceptions relate chiefly to the alary muscles of the pericardial septum. Lowne (Blow-fly, p. 5, and pl. v.) states that some of the thoracic muscles of that Insect are not striated.
[86] For example, Prof. Huxley, in his Anatomy of Invertebrated Animals (p. 254), says that “as the hard skeleton [of Arthropods] is hollow, and the muscles are inside it, it follows that the body, or a limb, is bent towards that side of its axis, which is opposite to that on which a contracting muscle is situated.” The flexor muscles of the tail of the Crayfish, which, according to the above rule, should be extensors, the muscles of the mandibles of an Insect, and the flexors and extensors of Crustacean pincers are among the many conspicuous exceptions to this rule.
[87] Haller. This and other examples are taken from Rennie’s Insect Transformations.
[88] Bull. Acad. Roy. de Belgique, 2e. Sér., Tom. xx. (1865), and Tom. xxii. (1866).
[89] Loc. cit. 3e. Sér., Tom. vii. (1884). Authorities for the various estimates are cited in the original memoir.
[90] Klassen und Ordnungen des Thierreichs, Bd. V., pp. 61–2.
[91] This change in the relation of weight to strength, according to the size of the structure, has long been familiar to engineers. (See, for example, “Comparisons of Similar Structures as to Elasticity, Strength, and Stability,” by Prof. James Thomson, Trans. Inst. Engineers, &c., Scotland, 1876.) The application to animal structures has been made by Herbert Spencer (Principles of Biology, Pt. II., ch. i.). The principle can be readily explained by models. Place a cubical block upon a square column. Double all the dimensions in a second model, which may be done by fitting together eight cubes like the first, and four columns, also the same as before except in length. Each column, though no stronger than before, has now to bear twice the weight.