THE BLOOD CORPUSCLES OR LEUCOCYTES 407 



directions, and when the motion ceases, the blood-currents in the legs stop ; the 

 rate of the pulsating organ is always faster than that of the heart, and the action 

 is automatic. 



b. The blood 



The blood of insects, as in other invertebrates, differs from that of 

 the higher animals in having no red corpuscles. It is a thin fluid, a 

 mixture of blood (serum) and chyle, usually colorless, but sometimes 

 yellowish or reddish, which contains pale amoeboid corpuscles corre- 

 sponding to the white corpuscles (leucocytes) of the vertebrates, 

 though they are relatively less numerous in the blood of insects. 

 The yellow fluid expelled from the joints of certain beetles (Cocci- 

 nella, Timarcha, and the Meloidae) is, according to Leydig, only the 

 serum of the blood. In phytophagous insects the blood is colored 

 greenish by the chlorophyll set free during digestion. The blood of 

 Deilephila euphorbia is colored an intense olive-green, and that of 

 Cossus ligniperda is pale yellow. (Urech.) The blood of case-worms 

 (Trichoptera) is greenish. In some insects it is brownish or violet. 

 The serum is the principal bearer of the coloring material, yet 

 Graber has shown that in certain insects the corpuscles are more or 

 less beset with bright yellow or red fat-globules, so as to give the 

 same hue to the blood. 



The leucocytes. - - The corpuscles are usually elongated, oval, or 

 flattened oat-shaped, with a rounded nucleus, or are often amcebi- 

 form ; and they are occasionally seen undergoing self-division. 

 When about to die the corpuscles become amoebiform or star-shaped. 

 (Cattaneo.) Their number varies with the developmental stage of 

 the insect, and in larvae increases as they grow, becoming most 

 abundant shortly before pupation. The blood diminishes in quantity 

 in the pupal stage, and becomes still less abundant in the imago. 

 (Landois.) The quantity also varies with the nutrition of the 

 insect, and after a few days' starvation nearly all the blood is 

 absorbed. Crystals may be obtained by evaporating a drop of the 

 blood without pressure ; they form radiating clusters of pointed 

 needles. The freshly drawn blood is slightly alkaline. (Miall and 

 Denny.) 



The size of the corpuscles has been ascertained by Graber, who found that the 

 diameter of the circular blood-disks of the leaf-beetle, Linn popitli, is 0.006 mm. ; 

 of Cetonia aurata and Zahrns gibbus, 0.008 to 0.01 mm. ; and those of certain 

 Orthoptera (Decticus i'errncivoriis, Ephippirjer vithim and fEdip<nJ<t ra'rtdescens, 

 0.011 to 0.014 mm. The longest diameter of the elongated corpuscles of Carabus 

 canceUatus is 0.008 mm.; of Gryllus campestris, Locusta viridissima, Cossus 

 liyniperda, Sphinx liytistri (pupa), and others, 0.008 to 0.01 mm.; of Cdloptenus 



