20 



ECONOMIC ENTOMOLOGY 



bring about the contraction and expansion of the abdomen necessary 

 for respiration. 



In the thorax of larval insects and of the wingless forms the arrange- 

 ment of muscles follows the same general plan as in the abdomen, 

 but the presence of the leg muscles makes it more complicated (Fig. 

 27). In the winged insects, however, this primitive arrangement 

 cannot be readily recognized owing to the presence of large wing muscles 

 and also to the fact that the thoracic segments are very often fused 

 together. The muscles which move the head originate in the prothorax 

 and are inserted into the base of the head. Within the head are muscles 



Fig. 25. — Internal organs of the cricket {Gryllus pennsylvanicus). AN., Anten- 

 nal nerve; Oes., oesophagus; Br., brain; T., teutorium; Ao., aorta; S., suspensorium 

 of ovary; H., heart; Ov., right ovary; Col., colon; R., rectum; A., anus; MT., Mal- 

 pighian tubules; Od., oviduct; Op., ovipositor; BC, bursa copulatrix (copulatory 

 pouch); Vag., vagina; RS., receptaculum seminalis; Mes., mesenteron; VC, con- 

 nectives of ventral nerve chain; VG., ganglia of ventral nerve chain; S.GL, salivary 

 glands; Lb.N., labial nerve; SG., subcesophageal ganglion; Md.N., mandibular nerve; 

 SD., salivary duct; HP., hypopharynx; P., pharynx; Mx.N., maxillary nerve; LN., 

 labial nerve; ON., optic nerve; C, crop; P., proventriculus; GC., gastric cseca. 



for the control of the mouth-parts and antennae. In the mandibulate 

 insects the muscles which move the mandibles occupy the greater 

 part of the head cavity. 



Mention should also be made of the alary muscles of the heart, 

 the circular and longitudinal muscles of the digestive canal, and the 

 spiracular muscles which bring about the closure of the spiracles during 

 respiration. 



A muscle consists of a bundle of long fibres, each with several nuclei 

 and a sheath, the sarcolemma. In most cases the fibres present a 

 striated appearance due to the presence of alternate light and dark 

 bands. 



