TERGAL MUSCLES OF THORAX.--The _longitudinal tergal_ muscles are much reduced in width when compared with those of the abdomen. Sets of obliquely placed muscles, which may be called the _lateral thoracic_ muscles, arise from near the middle of each tergum, and converge to tendinous insertions on the fore edge of each succeeding tergum, close to the lateral wall of the body. The principal muscles of the legs are figured and named, and their action can readily be inferred from the names assigned to them. _Insect Mechanics._ The mechanics of Insect movements require exposition and illustration far beyond what is possible in a book like this. Even the elaborate dissections of Lyonnet and Straus-Dürckheim are not a sufficient basis for a thorough treatment of the subject, and until we possess many careful dissections, made by anatomists who are bent upon mastering the action of the parts, our views must needs be vague and of doubtful value. Zoologists of great eminence have been led into erroneous statements when they have attempted to characterise shortly a complex animal mechanism which they did not think it worth while to analyse completely.[86] [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. The action of flight and the muscles attached to the wings are best studied in Insects of powerful flight. The female Cockroach cannot fly at all, and the male is by no means a good flier. Both sexes are, however, admirably fitted for running. In running, two sets, each consisting of three legs, move simultaneously. A set includes a fore and hind limb of the same side and the opposite middle leg. Numbering them from before backwards, and distinguishing the right and left sides by their initial letters, we can represent the legs which work together as-- ┏━━━━━━━━━┷━━━━━━━━━┓ R_{1} L_{2} R_{3} ┏━━━━━━━━━┷━━━━━━━━━┓ L_{1} R_{2} L_{3} The different legs have different modes of action. The fore-leg may be compared to a grappling-iron; it is extended, seizes the ground with its claws, and drags the body towards its point of attachment. The middle leg is chiefly used to support and steady the body, but has some pushing power. The hind leg, the largest of the three, is effective in shoving, and chiefly propels the body. _Muscular Force of Insects._