BIOCHEMISTRY AND TAXONOMY 331 



Thromboplastin + prothrombin — lateie^acclierator "*' thrombin 



Plasma Ac-globulin ''''°'"^'" — > serum Ac-globulin 

 Thromboplastin + prothrombin ^"""^ — - — >• thrombin 



platelet accelerator 

 serum Ac-globulin 



Fibrinogen _HE£E1^_ fibri 



rm 



Fig. 90a (Ware, Fahey and Seegers). Events in clotting of vertebrate blood plasnr<a 



is the only case studied to date, this protein is quite different from the 

 fibrinogen of Vertebrates. It is more soluble in salt solutions and moves 

 faster in an electric field. An extract of Arthropod tissue coagulates an 

 electrophorectically homogeneous preparation of lobster fibrinogen in the 

 presence of calcium. During coagulation in Arthropods under physiological 

 conditions, coagulin does not arise, as has long been believed, by an aggluti- 

 nation and disintegration of all blood cells, but from a special type of cell, 

 the coagulocyte, Coagulocytes are homologues of the explosive cells 

 described by Hardy (1892) in Crustacea. 



The exocuticle of Arthropods is the result of the hardening of part of 

 the endocuticle containing chitin in association with a water soluble protein 

 to w^hich Fraenkel and Rudall gave the name arthropodin. This hardening 

 arises through the tanning of the arthropodin vv^hich is converted into 

 sclerotin which is not soluble in water but is soluble in 5% sodium hy- 

 droxide at 50°. The tanning of arthropodin to sclerotin results from the 

 action of a phenolase on a phenol, the resulting quinone combining with 

 the amino-groups of the arthropodin. The classes Insecta and Crustacea 

 although both conforming to the general biochemical plan of Arthropods, 

 differ in the method of hardening of the exocuticle. In the Insecta this 

 hardening is due to a tanning action, while in Crustacea it is due to cal- 

 cification. 



The Insecta definitely differ from the Crustacea as well as from other 

 groups of animals in the composition of their blood plasma and particularly 

 by having a high concentration of free amino acids in this plasma. While, 

 in other groups, the plasma non-protein nitrogen does not exceed 10 mg 

 per 100 ml, it may be as high as 300 mg per 100 ml in insects. Other 

 characteristics of insect blood plasma are the high concentrations of 

 trehalose, of uric acid and of non-fermentable reducing substances. 



In the class Insecta, special biochemical characters of various sub- 

 divisions may be distinguished. As we have already stated, blood coagula- 

 tion in insects is of the general type found in Arthropods depending on 

 changes in special cells called coagulocytes, and the action of their coagulins 



