ANATOMY AND PHYSIOLOGY 



101 



brane; between these, in the heart, is a layer of fine muscle fibers, circular 

 or spiral in direction, which effect the contractions of the organ. 



Ventral Sinus. In many if not most insects a pulsatory septum 

 (Fig. 178, v) extends across the floor of the body cavity to form a sinus, 



FIG. 161. Blood corpuscles of a grasshopper, Stenobothrus. a-f, corpuscles covered with fat- 

 globules; g, corpuscle after treatment with glycerine, showing nucleus. After GRABER. 



in which the blood flows backward, bathing the ventral nerve cord as it 

 goes. This ventral sinus supplements the heart in a minor way, as do 

 also the local pulsatory sacs which have been discovered in the legs 

 of aquatic Hemiptera and the head of Orthoptera. 



Blood. The blood, or hcemolymph, 

 of an insect consists chiefly of a watery 

 fluid, or plasma, which contains cor- 

 puscles, or leucocytes. Though usually 

 colorless, the plasma is sometimes yellow 

 (Coccinellidae, Meloidae), often greenish 

 in herbivorous insects from the pres- 

 ence of chlorophyll, and sometimes of 

 other colors; often the blood owes its 

 hue to yellow or red drops of fat on the 

 surface of the blood corpuscles (Fig. 

 161). 



Leucocytes. The corpuscles, or 

 leucocytes, are minute nucleated cells, 

 6 to 30 IJL in diameter, variable in form 

 even in the same species, but commonly 

 (Fig. 161) round, oval or ovate in pro- 

 file, though often disk-shaped, elongate 

 or amoeboid in form. 



Function of the Blood. The blood 

 of insects contains many substances, 

 including egg albumin, globulin, fibrin, iron, potassium and sodium 

 (Mayer), and especially such a large amount of fatty material that its 

 principal function is probably one of nutrition; the blood of an insect 

 contains no red corpuscles and has little or nothing to do with the 



FIG. 162. Diagram to indicate the 

 course of the blood in the nymph of a 

 dragon fly, Epitheca. a, aorta; ^\h, 

 heart; the arrows show directions taken 

 by currents of blood. After KOLBE. 



