Some of the tendons to which the muscles are attached are very elaborate structures, and are as hard as the chitinous skeleton, so as to be like small bones in their nature. A very elaborate tendon of this kind is connected with the prothoracic trochantin in Coleoptera, and may be readily examined in _Hydrophilus_. It has been suggested that the entothorax is tendinous in its origin, but other morphologists treat it, with more reason, as an elaborate fold inwards of the integument. [Illustration: FIG. 64.—Cephalic and ventral chain of ganglia: A, larva of _Chironomus_; B, imago of _Hippobosca_. (After Brandt.)] NERVOUS SYSTEM. Insects are provided with a very complex nervous system, which may be treated as consisting of three divisions:—(1) The cephalic system; (2) the ventral, or ganglionic chain; (3) an accessory sympathetic system, or systems. All these divisions are intimately connected. We will consider first the most extensive, viz. the ventral chain. This consists of a series of small masses of nervous matter called ganglia which extend in the longitudinal direction of the body along the median line of the lower aspect, and are connected by longitudinal commissures, each ganglion being joined to that following it by two threads of nervous matter. Each of the ganglia of the ventral chain really consists of two ganglia placed side by side and connected by commissures as well as cellular matter. In larvae some of the ganglia may be contiguous, so that the commissures do not exist. From the ganglia motor nerves proceed to the various parts of the {117}body for the purpose of stimulating and co-ordinating the contractions of the muscles. The number of the ganglia in the ventral chain differs greatly in different Insects, and even in the different stages of metamorphosis of the same species, but never exceeds thirteen. As this number is that of the segments of the body, it has been considered that each segment had primitively a single ganglion. Thirteen ganglia for the ventral chain can, however, be only demonstrated in the embryonic state; in the later stages of life eleven appears to be the largest number that can be distinguished, and so many as this are found but rarely, and then chiefly in the larval stage. The diminution in number takes place by the amalgamation or coalescence of some of the ganglia, and hence those Insects in which the ganglia are few are said to have a highly concentrated nervous system. The modes in which these ganglia combine are very various; the most usual is perhaps that of the combination of the three terminal ganglia into one body. As a rule it may be said that concentration is the concomitant of a more forward position of the ganglia. As a result of this it is found that in some cases, as in Lamellicorn beetles, there are no ganglia situate in the abdomen. In the perfect state of the higher Diptera, the thoracic and abdominal ganglia are so completely concentrated in the thorax as to