50 THE BIOLOGY OF INSECTS 



seen, surrounded by blood which brings nutrient substances 

 to the active tissue and removes waste products from it, 

 and supplied with numerous fine branches (tracheoles) of 

 the air-tube system through whose delicate walls oxygen 

 passes into the muscle for support of the combustion 

 processes that Hberate the energy needed for contraction, 

 while the carbon dioxide resulting from the combustion is 

 diffused out. In outer contact with the sarcolemma are 

 numerous discoid nerve-endings (Fig. i6, A, N) associated 

 with nerve fibres along which travel the impulses which 

 excite the muscle-fibres to contract, as will be elucidated 

 in the next chapter. 



The muscles of insects are not surrounded by tough 

 connective-tissue sheaths like those of vertebrates ; the 

 bundles of fibres can be readily separated by teasing out, 

 wherever a muscle is exposed by removal of some part of 

 the exoskeleton. In many cases these fibres are directly 

 connected with the portion of the exoskeleton which is 

 moved by their contraction, but generally the muscle works 

 on the skeletal sclerite through a tendon or a number of 

 tendinous cords. These are cuticular structures secreted by 

 inpushed portions of the skin which grow towards and find 

 contact with the muscles. The insertion of a muscle by 

 means of a tendon results in the pull due to its contraction 

 being exerted on a certain special small area of sclerite ; 

 thus precision of action is ensured. In studying the 

 mechanism of movement among insects, the arrangement 

 of the firm regions of the cuticle which make up the 

 exoskeleton is as important as the form and nature of the 

 muscles themselves ; the admirable accuracy of an insect's 

 observable actions depends on the correlation of these two 

 sets of organs. As an introductory illustration of such 

 action the exoskeleton and muscles of the legs, already 

 briefly mentioned, may be profitably studied. 



The three pairs of legs of an insect are articulated respec- 

 tively to the three thoracic segments. Each leg is seen to 

 consist of a series of hard sclerites segmentally arranged with 

 flexible tracts of cuticle at the joints between adjacent 



