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TEXT-BOOK OF ENTOMOLOGY 



of the wings. In dragon-flies the muscles are arranged in two rows and 

 in such a way that the flexors or depressors (s, 1 bis) cling directly to 

 the thoracic wall (compare also the muscle dk in Fig. 172 and se in Fig. 174), 

 while the raiser or extensor (hi, to h 2, Fig. 172, hi and Fig. 174 he) lie farther 

 in. The form of the wing-muscles is sometimes cylindrical, sometimes like a 

 prism, or even ribbon-like. However, the contracted bundles of fibres do not 

 come directly upon the joint-process we have described, but pass over often 

 indeed at a very considerable distance from them, into peculiar chitinous ten- 

 dons. These have the form of a cap-like plate, often serrate on the edge, which 

 is prolonged into a thread, which should be considered as the direct continua- 

 tion of the base of the wings. The wings, therefore, sink down into the tho- 



bg gi 



tihm 



FIG. 174. Transverse section through the 

 thorax of a locust (Stenobothrus) : & lt leg; A, 

 heart: ga, ventral cord; se, depressor, he, 

 elevator, of the wing (.7?) ; b-r, lateral muscles 

 which expand the thoracic walls; 1m, longitu- 

 dinal muscles which contract them; xtim. ///////, 

 muscles to the legs; bg, apodeines. After 

 Graber. 



FIG. 1T5. Inner view of a portion of the 

 left side of body of Libellula depressa, showing 

 a part of the mechanism of flight, viz., some of 

 the chitinous ridges at base of the upper wing, 

 and some of the insertions of the tendons of 

 muscles : A, line of section through the base of 

 the upper wing, the wing being supposed to be 

 directed backwards. C, upper portion of mech- 

 anism of the lower wing; b, lever extending 

 between the pieces connected with the two 

 wings. After von Lendenfeld, from Sharp. 



racic cavity as if they were a row of cords ending in handles where the strain 

 of the muscles is applied. 



As may be seen in Fig. 173, the contractile section of several of the muscles 

 of the wing (s 5 ) is extraordinarily reduced, while its thread-like tendon is pro- 

 portionately longer. This gradation being almost like that of the pipes of an 

 organ in the length of the wing-muscles, as may so easily be observed in the 

 large dragon-flies, plainly indicates that the strain of the individual muscles is 

 quite different in strength, since, as the phenomenon of flight demands it, 

 the different parts of the base of the wing become respectively relaxed in very 

 dissimilar measure. 



We have thus far discussed only the elevator and depressor muscles. Other 

 groups (s a s 3 ) are yet to be added, however, crossing under the first at acute 



