II] INTERNAL ANATOMY 



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has drawn up an elaborate system of nomenclature for the 

 arrangement of the bristles, which is of considerable value for 

 purposes of classification. 



Internal Anatomy. 



The alimentary canal is provided with a muscular 

 pharynx. The oeosphagus usually gives off a large diverti- 

 culum known as the crop or sucking stomach, but in some 

 insects, e.g. Culicidse, its place is taken by two or three long 

 thin-walled sacs, the oesophageal diverticula. The stomach 

 is large and consists of an anterior portion, the proventriculus, 

 and a true stomach behind it ; the latter usually gives off 

 caeca. The intestine is coiled and ends in a rectum ; at its 

 junction with the stomach open the long Malpighian tubules, 

 generally four in number. The salivary glands are large and 

 their common duct opens at the top of the hypopharynx. 



The heart consists of a thin-walled tube running along 

 beneath the dorsal surface of the insect. Usually it is divided 

 into several chambers but in the more specialized families 

 there are only two. 



The respiratory system consists of two main tracheae 

 running longitudinally one on each side of the body and open- 

 ing to the exterior by means of the spiracles. The latter are 

 arranged on each side of the body, two large pairs in the thorax 

 and one pair to each abdominal segment. The two main 

 tracheal trunks expand at the base of the abdomen into con- 

 spicuous air-sacs, which are probably of use when the insect 

 is flying. 



The nervous system is very variable. In the elongate 

 Nematocera there are five or six abdominal ganglia and three 

 distinct thoracic ganglia, but in the more specialized forms, 

 such as the Muscidse, all the thoracic and abdominal ganglia 

 are fused into a single mass. Various gradations may be 

 found between these two extremes and it is interesting to trace 

 the gradual concentration of the nervous system side by side 

 with the higher speciahzation of the insects. 



The reproductive organs present some interesting 

 peculiarities. The ovaries consist of a very large number of 



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