ANATOMY AND PHYSIOLOGY 1 2/ 



ity to form a sinus, in which the blood flows backward, bathing 

 the ventral nerve cord as it goes. This ventral sinus supple- 

 ments the heart in a minor way, as do also the local pulsatory 

 sacs which have been discovered in the legs of aquatic Hemip- 

 tera and the head of Orthoptera. 



Blood. The blood, or hamolymph, of an insect consists 

 chiefly of a watery fluid, or plasma, which contains corpuscles, 

 or leucocytes. Though usually colorless, the plasma is some- 

 times yellow (Coccinellidae, Meloidae), often greenish in her- 

 bivorous insects from the presence of chlorophyll, and some- 

 times 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 /n in diameter, variable in form even 

 in the same species but commonly (Fig. 161) round, oval or 

 ovate in profile, though often disk-shaped, elongate or amce- 

 boid 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 cor- 

 puscles and has little or nothing to do with the aeration 

 of tissues, that function being relegated to the tracheal 

 system. 



Circulation. The course of the circulation is evident in 

 transparent aquatic nymphs or larvae. In odonate or ephe- 

 merid nymphs, currents of blood may be seen (Fig. 162) flow- 

 ing through the spaces between muscles, tracheae, nerves, etc., 

 and bathing all the tissues; separate outgoing and incoming 

 streams may be distinguished in the antennae and legs; the 

 returning blood flows along the sides of the body and through 

 the ventral sinus and the pericardial chamber, eventually to 

 enter the lateral ostia of the dorsal vessel. A circulation of 

 blood occurs in the wrings of freshly emerged Odonata, Ephe- 



